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Publication Date: 2/1/84
Pages: 20 Date Entered: 1/1/84 Title: IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP (5/74) Revision 1(*) February 1984 U.S. NUCLEAR REGULATORY COMMISSION REGULATORY GUIDE OFFICE OF NUCLEAR REGULATORY RESEARCH REGULATORY GUIDE 5.23 (Task SG 045-4) IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP A. INTRODUCTION Part 70, "Domestic Licensing of Special Nuclear Material," of Title 10 of the Code of Federal Regulations requires licensees authorized to possess more than 1 kilogram of plutonium to calculate a material balance based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70. Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inventories." Plutonium residual holdup is defined as the plutonium inventory component remaining in and about process equipment and handling areas after these collection areas have been prepared for inventory. Whenever possible, process equipment should be designed(1) and operated so as to minimize the amount of holdup. In this guide, procedures acceptable to the NRC staff for the in situ assay of the plutonium residual holdup are described. Assay information may be used in one of two ways: 1. When the standard error (estimator) of plutonium holdup is compatible(2) with constraints on the overall standard error of the inventory difference (SEID), the material balance can be computed using the measured contents of plutonium holdup. Additional cleanout and recovery for accountability will then not be necessary. 2. When the standard error of plutonium holdup is not compatible with constraints on the overall SEID, the information obtained in the holdup survey can be used to locate principal plutonium accumulations and to ensure that other areas of the process contain less than the detectable amount of plutonium. Once located, substantial accumulations can be recovered, transforming the plutonium to a more accurately measurable inventory component. Having reduced the amount of plutonium holdup, the standard error on the remeasurement of the remaining holdup may be sufficiently reduced to be compatible with overall SEID requirements. ---------- (1) Design features to minimize holdup in process equipment are the subject of a series of regulatory guides (5.8, 5.25, and 5.42). (2) Compatibility exists when the contribution of the standard error of the holdup to the total plant SEID is not large enough to cause the overall SEID to exceed allowed limits. If the plant SEID exceeds allowed limits because of an excessive contribution from the holdup standard error, compatibility does not exist and the remedial steps of paragraph 2 need to be taken. ---------- Any guidance in this document related to information collection activities has been cleared under OMB Clearance No. 3150-0009. B. DISCUSSION Plutonium accumulates in cracks, pores, and zones of poor circulation within process equipment. The walls of process vessels and associated plumbing often become coated with plutonium during solution processing. Surfaces internal and adjacent to process equipment, especially glovebox walls and floors, accumulate deposits of plutonium that can become appreciable. Plutonium also accumulates in air filters and associated ductwork. The absolute amounts of plutonium holdup must be small for efficient processing and proper hazards control. However, the total holdup can be large relative to the plant inventory difference (ID) but have no significant impact on the ID if it remains reasonably constant. It is the change in the holdup between beginning inventory and ending inventory that may impact the ID. The measurement procedures described in this guide involve the detection of gamma rays and neutrons that are spontaneously emitted by the plutonium isotopes. Because the gamma rays of interest are emitted by the major isotope, (239)Pu, gamma ray assay is the preferred method whenever its acceptance criteria are satisfied. The amount of (239)Pu holdup in a piece of equipment is proportional to the measured intensity of the emitted gamma rays after suitable corrections are made for attenuation by intervening materials, for self-attenuation by plutonium, for scattering, for geometrical factors, and for background radiation. ---------- (*) The substantial number of changes in this revision has made it impractical to indicate the changes with lines in the margin. ---------- If plutonium is held up in furnaces, grinders, or other heavy equipment that is too dense to permit the escape of gamma rays, an assay based on spontaneous fission neutrons from the even isotopes of plutonium may be possible. This technique requires knowledge of the isotopic composition of the plutonium, some knowledge of its chemical form, and knowledge of the presence of other radionuclide impurities. Thermoluninescent dosimetry is a third technique that can be used to measure holdup from the inside of large pieces of equipment. This technique is also useful for carrying out measurements in an unobtrusive manner outside normal plant operating hours. For all three techniques, the proportionally factors between amount of holdup and detector response are best determined prior to the holdup measurement by assays of known quantities of plutonium distributed in well-defined and representative geometries, as discussed below. 1. DELINEATION OF COLLECTION ZONES Typical plutonium process facilities comprise a number of interconnected gloveboxes that contain work areas and most process equipment, in-process storage areas, and self-contained process equipment. Also, solution processing requires tanks, plumbing, and pumping equipment, which are often located in close proximity to the glovebox lines. Finally, storage areas for feed, scrap and waste, and final product are often located in close proximity to the plutonium process area. To accomplish the holdup measurements, it is essential to consider the facility in terms of a series of zones that can be independently assayed. Such zones are designated "collection zones." Each plutonium-processing facility can be conceptually divided into a series of contiguous collection zones on the basis of process activities and collection geometries. Individual machines, filters, pipes, tanks, gloveboxes, or surface areas that can be isolated from one another may be suitable discrete collection zones. Great care is needed to define all collection zones so that (1) the assay of the zone can be performed with a minimum of interference from nearby zones, (2) the detector can be positioned reproducibly and in such a way that the radiation being measured experiences a minimum, or easily predicted, attenuation in the apparatus being measured, and (3) the distribution of material in the zone can be represented by one of the distribution geometries used in the calibration procedure described below. Gamma ray assay for plutonium holdup is practical when the collection zone consists of a single structure of relatively uniform cross section. When a collection zone contains a complex item of equipment with significant self-shielding properties, the uncertainty in the holdup measurement may be primarily due to attenuation of radiation in the internal structure. In such cases, neutron assay from the outside and thermoluminescent dosimeter assay from the inside may be applicable. If delineation of collection zones is not possible, two alternatives are experiments with mockup geometries or complex numerical calculations. 2. APPLICABLE METHODS AND INSTRUMENTS Two considerations are critical to the selection of methods and instruments. First, to perform an assay, one must ensure that the plutonium radiations reach the detector and are detected. Second, the observed response must be attributable to the collection zone being assayed. Therefore, the assay scheme is developed around penetrating radiations, and the detector is collimated to provide for sufficient directionality in the response to resolve a collection zone from its neighboring zones and from the background. Finally, some effort may be necessary to employ external "shadow shielding" to block radiation being produced in adjacent collection zones from the field of view of the collimated detector. 2.1 Gamma Ray Assay Under closely controlled conditions, the measured plutonium gamma ray spectrum can be interpreted in terms of the abundance of each gamma ray emitter present in the sample. Because of the large number of gamma rays (Refs. 1 and 2) present, many regions of the observed spectrum are characterized by overlapping lines. To accomplish the assay, it is necessary to select an appropriate spectral region and provide a detection system with sufficient resolution to measure the activity from the isotopes of interest. Gamma ray assay has an advantage over neutron assay in that the emissions are primarily from the principal isotopes of interest. Because of the high emission rate of gamma rays, a detection sensitivity of less than 1 gram is generally attainable. The most useful portion of the spectrum for holdup assay is the (239)Pu gamma ray complex in the 375- to 450-keV range.(3) The yields of these lines are given in Table 1. ---------- (3) In typical NaI detectors (with energy resolution of 10 percent at 414 keV), the 414-keV photopeak will produce counts in the approximate energy range of 373 to 455 keV. Thus, an energy window from 375 to 450 keV will include most of the 414-keV full-energy counts for a variety of detector resolutions. Further-more, such a window setting will include a significant fraction of the 375-keV (239)Pu gamma rays (see Table 1), but will also exclude most of the potentially interfering 332-keV gamma rays from (241)Am or (241)Pu. ---------- (Due to database constraints, Table 1 is not included. Please contact LIS to obtain a copy.) 2.1.1 Gamma Ray Detection Instruments Gamma ray detection systems consist of a scintillation or semiconductor gamma ray detector and appropriate electronics (Refs. 3 and 4). Required electronics include at least a single-channel analyzer and a timer-scaler unit. A second single-channel analyzer viewing the same detector pulses used to determine the background radiation correction is a timesaving feature. A number of portable battery-powered systems are commercially available for this application. The detection efficiency and resolution (10 to 15 percent) of NaI(T1) is generally adequate for holdup measurements. CdTe, Ge(Li), and high-purity germanium (also known as intrinsic germanium) detectors have better resolution than NaI(T1) but are more costly and more difficult to operate. For more information on Ge(Li) and intrinsic germanium detectors, see Regulatory Guide 5.9, "Guidelines for Germanium Spectroscopy Systems for Measurement of Special Nuclear Material," and the references cited therein. The 332.3-keV gamma ray from (237)U, a short-lived (6.75 days) daughter of (241)Pu, is usually the principal interference for (239)Pu assay by NaI detection of the 375- to 450-keV complex. If the (237)U is in equilibrium with (241)Pu, the intensity of this gamma ray is 1.15 x 10(6) gamma/sec-g (241)Pu. Since this gamma ray is also emitted in the decay of (241)Am, the interference from this decay branch may also be important in case of preferential americium holdups. To avoid this interference when using NaI detectors, the assay energy window is adjusted to span the range from 390 to 450 keV for plutonium holdup with high americium content. Detector crystal dimensions are selected to provide a high probability of detecting gamma rays from the 375- to 450-keV complex and a low probability of detecting high-energy radiation. For NaI, a crystal diameter of 2 inches (5 cm) with a thickness of 2 inches is recommended. For germanium detectors, a moderate-volume coaxial detector is recommended. 2.1.2Collimators and Absorbers for Gamma Rays A shaped shield constructed of any heavy-element material is appropriate for gamma ray collimation. For cost, availability, and ease of fabrication, lead is recommended. Less than 2 percent of all 400-keV gamma rays striking a 5-cm-thick sheet of lead will pass through without suffering an energy loss. The collimator will be most effective when it is concentric about the crystal and photomultiplier and completely covers the photomultiplier base. Extending the collimator forward of the crystal at least a distance equal to half the diameter of the crystal, and preferably the full diameter, is recommended (Ref. 5). Making this distance variable to reproducible settings will permit adjustment over a range of collection zone sizes. However, it is highly desirable to select collection zones and counting geometries so that one collimator setting will suffice for all measurements. This will simplify the calibration procedures because the calibration constants depend strongly on the dimensions and placement of the collimator aperture. The collimator not only defines the effective field of view but also shields the detector from unwanted radiation. To effectively accomplish this latter purpose, the collimator material must also cover the rear of the detector. This is usually easy to achieve with portable NaI detectors but requires more effort when germanium detectors are used. Intensive 50- to 100-keV X-ray radiation and 60-keV (241)Am gamma ray radiation are often emitted by process equipment, and this radiation can tie up the detector electronics unnecessarily. A 1.5-mm-thick layer of lead (on the outside) and a 0.75-mm-thick layer of cadmium (on the inside) may be placed against the front face of the detector to alleviate this problem. This graded energy shield will absorb most of the low-energy photons incident on the detector without substantially reducing the number of gamma rays detected in the 375- to 450-keV range. 2.1.3Check Source for Gamma Ray Assay It is important to check the operation of the detection system each time the instrumentation is moved or otherwise disturbed (e.g., power outage) during the course of each inventory sequence. Either recalibrating one or more collection zones and comparing the results to previous analyses or testing the instrument with a check source is appropriate. When the response remains within the expected value, the previous calibration data are assumed to be valid. If not, the energy window may have shifted, or the unit may be in need of repair and recalibration. An appropriate check source enables the stability of the assay instrument to be tested at any location. Such a source can be prepared by implanting a small encapsulated plutonium sample (containing ~0.5 gram of plutonium) in the face of a plug of shielding material. The plug is shaped to fit and close the collimator channel, and the source is positioned to be adjacent to the crystal when the plug is in place. The check source is fabricated in a manner that will ensure its internal stability. Other than radiations increasing from the ingrowth of (241)Am, the emission rate of the check source should remain constant. 2.2 Neutron Assay Neutrons are emitted in the spontaneous fission of (238)Pu, (240)Pu, and (242)Pu and through the interaction of emitted alpha particles with certain light nuclei. These neutrons suffer little attenuation in passing through uranium or plutonium or through most structural and containment materials. Glovebox windows may reduce the energy of emerging neutrons, but, because of their regular and constant shape, their effect can generally be factored into the assay calibration. To be useful for the assay of plutonium holdup, the neutron production rate per gram of plutonium must be known. The spontaneous fission contribution to the total neutron production can be computed from basic nuclear data after the isotopic composition of the contained plutonium has been determined. Computing the (Alpha,n) contribution requires a knowledge of the chemical form of the plutonium and the amount and distribution of certain high-(Alpha,n)-yield target materials. The background count rate from neutron detectors may be a substantial part of the observed activity, often corresponding to as much as 20 grams of plutonium in typical holdup assays. Thus, neutron assay is primarily applicable to the measurement of significant accumulations of plutonium. The measured neutron yield from prepared calibration standards is used to calibrate each neutron assay collection zone. In the appendix to this guide, a method is given to calculate the anticipated neutron yield. This method provides the ability to calculate the neutron yield when the isotopic or impurity composition of the plutonium holdup is different from that of the calibration standards. The method can be used to calculate a ratio of the neutron production rate of the unknown material to the neutron production rate of the standard material. The yield from the holdup material is then determined by multiplying the measured "known" material yield by the computed ratio. 2.2.1Neutron Detection Instruments To effectively employ the spontaneous neutron yield as a measure of plutonium holdup, it is necessary to detect the neutrons in the presence of a more intense gamma ray background and to collimate the detector so that neutrons emanating from the collection zone under assay are preferentially detected. Holdup assay is performed under in-plant conditions where ruggedness, portability, high neutron efficiency, and low gamma ray sensitivity in the detectors are important. Gas-filled proportional counters containing (3)He or BF(3) are suitable for this purpose. Typical fill pressures are 1 to 4 atmospheres. One advantage of (3)He for in-plant applications is that the operating voltage of (3)He counters is about 75 percent of that required for BF(3) counters. The efficiency of (3)He and BF(3) counters increases as the energy of the neutrons decreases. Embedding gas-filled counters in polyethylene to moderate the incoming neutrons to thermal or epithermal energies will improve their efficiency. A nearly optimum design can be obtained by centering the counters in 10 cm of polyethylene with 2 to 3 cm of polyethylene between adjacent counters. To shield the detector from low-energy neutrons that may produce a complicated response pattern, the moderator material is covered with a thermal neutron absorber. Cadmium sheeting approximately 0.075 cm thick may be used for this application. 2.2.2Collimators for Neutron Detectors To assay a specific collection zone in the presence of other distributed sources of plutonium, it is necessary to collimate the detector. This is accomplished by stopping neutrons coming to the detector from all directions except the desired one. The cadmium surrounding the detector will stop essentially all neutrons striking the detector with energies below 0.4 eV. By adding moderator material around the outside of the detector in all directions except for the collimator channel, neutrons coming from unwanted directions will lose energy in this shield and will be absorbed in the cadmium cover. For each 6 inches (15 cm) of polyethylene added, the collimator assembly provides a factor of approximately 10 in the directionality of the response. An example of a collimated neutron detector assembly for plutonium holdup assay is shown in Figure 1. This assembly has a polyethylene shield thickness of 6 inches (15 cm) and a directionality of 10 to 1. The combined weight of the detector and collimator exceeds the requirements for a hand-held probe. For this reason and to provide for reproducible positioning at each assay, a sturdy cart housing such a detector and its associated electronics is recommended. In order to assay items at different heights, the capability to raise and lower the assembly to reproducible settings is needed. An example of a small commercially available hand-held detector is given in References 5 and 6. This Shielded Neutron Assay Probe (SNAP) is 12 inches (30.5 cm) high and 10 inches (25.4 cm) in diameter and contains two (3)He detectors. It includes a 2-inch-thick (5 cm) polyethylene shield that provides a directionality of 3 to 1. The SNAP has been used to measure plutonium holdup, UO(2)F(2) holdup, and UF(6) enrichment. It is recommended for the assay of well-defined concentrations of plutonium in pumps, grinders, pipe elbows, or other items of equipment where portability and accessibility are more important than directionality. (Due to database constraints, Figures 1 and 2 are not included. Please contact LIS to obtain a copy.) A third example of holdup measurement by neutron detection is given in Reference 7. In this case, a completely uncollimated polyethylene slab containing a row of (3)He detectors was suspended in midair in some of the processing rooms of an industrial plutonium facility. The response of the detector was found to be proportional to the total room holdup if the plutonium was reasonably uniformly distributed and if the room was isolated from external sources. The calibration procedure for the use of this detector will not be described here. However, it is recommended as a means for quickly verifying total room holdup when measurements of the holdup in individual items or equipment are not needed. 2.2.3Check Source for Neutron Assay To ensure the proper operation of the neutron assay system prior to making an assay, it is necessary to test the response of the instrument. An appropriate neutron assay check source can be measured, or the detector response for one or more collection zones can be recalibrated and compared to the results of previous calibrations. An appropriate neutron assay check source can be prepared by implanting a small encapsulated plutonium source (containing about 5 grams of plutonium) into the face of a plug of neutron moderating material (see Figure 2). The plug is fabricated to fit and close the collimator channel. When the response from the check source remains within the expected value, the previous calibration data are assumed to be valid. 2.3 Thermoluminescent Dosimeter (TLD) Assay Crystals of LiF, CaF(2), CaSO(4), or other compounds can store energy at manganese or dysprosium impurity centers when they are struck by gamma or neutron radiation. At some later time, the crystals can be heated rapidly to several hundred degrees centigrade to induce thermoluminescence. The light output at this time is proportional to the amount of radiation received. Thermoluminescent detectors that are primarily gamma sensitive, graded X-ray shields, read-out instrumentation, and other accessories are commercially available. TLDs have been used to measure the holdup in gloveboxes by placing them at regular intervals on the outside surfaces. The TLDs are left in place overnight in order to accumulate a measurable dose. Accuracies of plus or minus 20 percent relative to cleanout values are reported for plutonium of known isotopic composition. TLDs have also been used to measure the holdup in the interior of large furnaces that are not accessible by other means. For both of these examples, calibration requires either careful dose and geometry calculations or mockups of the actual collection zone. Because their use is relatively new and only a few published references exist (Refs. 8 and 9), TLDs will not be discussed further in this guide. However, they could be useful for special applications. 3. ISOLATION OF COLLECTION ZONES To ensure that each collection zone is independently assayed, it is necessary to shield the detector from all radiations except those radiations emanating from the collection zone being assayed. This is principally accomplished through the use of the collimators described in Sections B.2.1.2 and B.2.2.2. Two additional means exist to isolate a collection zone, detector positioning and shadow shielding. 3.1 Detector Positioning An unobstructed side view of a collection zone is preferred. When plutonium is located behind the zone under assay in another collection zone or a storage facility, an additional background assay may be performed with the detector above or below the collection zone and pointing at the material behind the zone under assay. It is important to prevent, or account for, moving objects within the field of view. If this is not done, variations in shielding and scattering can affect the measurement. 3.2 Shadow Shielding It may not be possible to avoid interfering radiations by collimator design or by choosing the detector position for assay. In such cases, it may be possible to move a shield panel between the source of interfering radiations and the collimator zone under assay. If the shield panel is very thick and its dimensions match or exceed the back side of the collection zone under assay, no interfering radiations will penetrate through the shadow shield to the detector. While such characteristics are desirable, the size of such a shield would limit its transportability. A rectangular panel mounted on wheels as an upright panel and containing ~5 cm of neutron moderator (e.g., benelex, WEP, or polyethylene) and ~0.5 cm of lead sheet is recommended. To use such a panel, it is necessary to measure the response of the collection zone with and without the shield in place. Also, the gamma and neutron transmission factors of the shield itself must be measured beforehand with a representative plutonium sample. From these measurements, the assay of the collection zone can be corrected for background radiation transmitted through the shield. 4. CALIBRATION FOR HOLDUP MEASUREMENTS 4.1 Basic Counting Geometries There are three fundamental counting geometries that can be used to represent most collection zones. These geometries are distinguished by the spatial distribution of the source material and the resulting dependence of the detector counting rate on the source-to-detector distance, r. 4.1.1Point Source If the material being assayed is distributed over an area with dimensions that are small compared with the source-to-detector distance and if the material resides entirely within the detector field of view, the zone can be treated as a point source. The detector count rate for a point source varies inversely as the square of the source-to-detector distance (count rate is proportional to 1/r(2)). Any equipment measured at great distances or any small pieces of equipment or equipment parts fall in this category.(4) 4.1.2Line Source If the material being assayed is distributed along a linear path so that only a segment of that distribution length is contained in the detector field of view, the zone can be treated as a line source. The detector count rate for a line source varies inversely as the source-to-detector distance (count rate is proportional to 1/r). Examples of this type of holdup geometry include isolated sections of piping and long, narrow ducts or columns. 4.1.3Area Source If the material being assayed is spread over an area so large that it covers the full field of view of the detector for a range of source-to-detector distances, the zone can be assayed as an area source. As long as the material being viewed is uniformly distributed, the detector count rate will be independent of the source-to-detector distance. However, for holdup applications, uniform material distribution is rare; so the source-to-detector distance can affect the instrument response and needs to be specified. Furthermore, when there are several measurement locations covering a large area (such as a floor), it is important to maintain the same source-to-detector distance (even if material distribution is uniform within a given measurement area) so that the number of measurement areas needed to cover the entire area remains constant. Examples of this type of assay geometry include floors, walls, glovebox floors, and large rectangular ducting. 4.2 Calibration of Detector Response 4.2.1Mockup of Known Material Distributions When a gamma ray assay is used and a collimator setting has been selected, the detector responses for the three basic source distribution geometries listed above need to be determined. For the point source, the response is expressed as (counts per minute)/gram of (239)Pu at a specified source-to-detector distance. For the line source, the response is expressed as (counts per minute)/(gram of (239)Pu per unit length) at a specified source-to-detector distance. For the area source, the response is expressed as (counts per minute)/ (gram of (239)Pu per unit area) at a specified source-to-detector distance. When neutron assay is used, the response for a point source is expressed as (counts per minute)/gram of (240)Pu effective at a specified source-to-detector distance. Calculation of (240)Pu effective from the plutonium isotopic composition is described in the appendix to this guide. Analogous expressions can be given for line and area sources although neutron assay is usually restricted to dense, isolated items of equipment that can be represented as point sources. For both neutron and gamma measurements, corrections to the point and line source calibrations for different detector distances are made using the 1/r(2) or 1/r count-rate dependence, respectively. For further detailed discussion of the measurement of detector responses for these basic geometries, see Reference 10. ---------- (4) Caution: small deposits of plutonium could exhibit very large gamma ray self-attenuation and could therefore require great care in analysis or could require neutron assay. ---------- For gamma ray assay, the calibration of the point source response can be accomplished with a well-characterized encapsulated standard plutonium foil smaller in size than the detector collimator opening. This foil can also serve as the check source for verification of the continued stability of the instrument settings in the field. It is important that care be taken in the preparation of this calibration standard to ensure that the amount of encapsulated (239)Pu is well known. It is also important to measure the gamma ray attenuation through the encapsulating material and the self-attenuation of the plutonium foil and to correct the calibration standard response to compensate for these effects. Enough (239)Pu needs to be encapsulated in this standard to provide count rates that will ensure good statistical precision of the calibration in a reasonable period of time. For neutron assay, it is probably necessary to encapsulate a larger amount of material in the calibration standard because the spontaneous neutron production rate is significantly less than the 375- to 450-keV gamma ray production rate. A quantity of 50 to 100 grams of plutonium is adequate for most applications. Again, it is important to know the exact quantity and isotopic composition of the plutonium. Also, the neutron calibration standard may generate more neutrons than directly attributable to the spontaneous fission and (alpha,n) reactions. Because a relatively large quantity of PuO(2) is encapsulated in the neutron assay calibration standard, some spontaneous fission or (alpha,n) neutrons may be absorbed in (239)Pu or (241)Pu nuclei, producing additional neutrons through the induced fission reaction. The amount of multiplication depends in a complex manner on the amount and distribution of PuO(2) and on the surrounding medium (Ref. 11). For 50 grams distributed in the bottom of a 4-inch-diameter (10 cm) can, a self-multiplication of 0.5 percent of the total neutron output would be typical. At 100 grams, 1 to 2 percent may be expected. Thus, this effect is typically smaller than other errors associated with holdup measurements and can be neglected if the standard contains 100 grams or less of well-distributed material. The chemical and isotopic composition of the plutonium will have a larger effect, as described in the appendix to this guide. The measurement of the line source response is best accomplished by constructing a cylindrical surface distribution of plutonium with the aid of large foils. It is also possible to establish the line source response using a point source, as described in Reference 4. The line source geometry is closest to that of the pipes and ducts likely to be encountered in actual measurements. The area source response can be measured with the same plutonium foils laid flat to simulate the expected distribution on surfaces such as walls and floors. The area response can also be established using a point source. The point source is measured at different radial distances from the center of the field of view of the collimated detector. The response at each radial distance is weighted by the area of a concentric ring at that radius. From these weighted responses, it is then possible to calculate the area of a circular region of uniform plutonium deposition that would yield the same total response as the point source. From this equivalent area, the expected response/(gram of (239)Pu per unit area) can be derived. Further useful details on this procedure may be found in Reference 12. For both line and area calibrations, the self-attenuation of the foils or point sources also needs to be taken into account. There may be special material distribution geometries in the facility that are not readily represented by one of the three basic configurations described above. These special geometries may be mocked up as carefully as possible with large plutonium foils and point sources to produce a usable detector response calibration for these special cases. Examples of special cases might be concave or convex equipment surfaces or the internal volume of a rectangular cavity (see Ref. 10). Because material particle sizes (or material deposit thicknesses) have a significant effect on the self-attenuation of the gamma ray signals, it is important to use (whenever practical) well-characterized process material for preparing calibration standards and to duplicate to the extent possible process holdup distribution relative to particle size or thickness. Furthermore, holdup in floors is often deposited at various depths into the floor, rather than on the surface. Thus, calibration standards for such measurements need to incorporate the appropriate geometry and matrix effects. Core samples of a floor may be needed to establish typical concentrations at various floor depths. Calibration of the holdup measurement system using this procedure is recommended until a history of comparisons between predicted and recovered holdup quantities is developed. If it is possible to take holdup measurements before and after the cleanout of a piece of shut-down process equipment, they can be used to establish this comparison history and improve the accuracy of the calibration for each collection zone. 4.2.2Measurement of Calibration Sources in Actual Process Equipment One method for calibrating detector response to holdup radiation in process equipment is to place a known calibration source in various positions in that equipment and record the detector responses. In this way, the overall detector response (including all corrections for attenuation and geometry) is determined empirically. Unfortunately, this procedure is impractical, if not impossible, in process equipment already in operation. However, if those responsible for holdup assays are made aware of occasions when new equipment is brought into the plant for installation in the process, calibration sources can be conveniently placed in the equipment before its installation and the empirical measurements of the detector responses can be made. This procedure would be a valuable supplement to calibration data obtained from mockups of standard counting geometries and comparisons with cleanout recovery data. 5. HOLDUP MEASUREMENTS AND STANDARD ERROR The measurement of holdup in a complex plant environment can involve a very large number of measurements. In a stable plant environment where the process behavior is well known and well characterized, it may be possible to arrange the holdup measurement program so that: a. Careful and extensive holdup measurements are made infrequently (e.g., annually) and b. At more frequent intervals (e.g., at inventory times), careful measurements are made in known problem areas, and "spot check" measurements are made in the other, less used, zones where accumulations are known to be low. Such management of measurement resources can result in a very effective holdup measurement program at minimum cost. 5.1 Holdup Measurements In performing the holdup measurements, one must be aware of the large variability in holdup assays arising primarily from variability in the measurement conditions (e.g., background, geometry, gamma ray or neutron attenuation, material distribution). Accordingly, it is important to perform the assays from as many vantage points as possible for each collection zone. If this is impractical on a routine basis because of time or space constraints, one might consider multiple measurements initially on a collection zone, followed by fewer routine measurements at representative assay sites. Careful thought in the selection of measurement points and measurement strategy will minimize ambiguities in the interpretation of the data. 5.1.1Selection of Collection Zones and Detector Positions Location and configuration of collection zones are established on the basis of a detailed physical examination and a radiation survey of the physical layout of the facility. Preliminary measurements are needed to determine the optimum detector positions for the holdup assays. If nonuniform distribution of material in a collection zone is suspected or if the process apparatus is sufficiently complicated to require extensive attenuation corrections for certain counting geometries, multiple measurements are advisable for the collection zone. More than one detector position may be necessary. In the cases where radiation surveys have pointed out zones of high holdup collection, extra care will be necessary in the holdup measurements for those zones to minimize their contribution to the overall holdup variability. Where radiation surveys show little holdup, proportionately less time need be budgeted. Selecting optimum detector positions includes consideration of the need to conveniently measure the line-of-sight background by moving the detector to one side without changing its orientation. 5.1.2Holdup Measurement Procedure The measurement and analysis of gamma or neutron radiation from a collection zone may be carried out by treating the material distribution as a point, line, or area source, as described in Section B.4.1, or as one of the special cases that may have been measured, as mentioned in Section B.4.2. If the nature of the material distribution is uncertain for a particular detector position, a measurement of the detector counting-rate dependence on the source-to-detector distance, r, may reveal the most appropriate counting-rate geometry with which to interpret the data. After the assay positions for the detector and shadow shields are established for each collection zone, permanent markings that indicate detector location (including height) and orientation will ensure reproducibility of subsequent measurements for these positions. Uniquely labeling each assay site will facilitate unambiguous reference to each measurement and its location in the assay log. Furthermore, assay site labels and markings can indicate whether neutron or gamma ray measurements are to be made. Alphabetic labels (for example, "G" for gamma and "N" for neutron) and color-coded tape markings of the sites would be useful. Protecting the markings (for example, with clear epoxy) will ensure their long-term durability. After measuring the gamma or neutron radiation intensity at each detector position in a given collection zone, the line-of-sight background is measured by moving the detector and collimator to one side (still pointing in the same direction as during the assay) and measuring the radiation intensity from the surrounding materials. During the background measurement, the vessel in which the holdup is being measured must not be in the field of view of the detector. Because uncertainties in geometry, attenuation, or sample matrix will usually dominate the total response variability, the counting period need not be long. Having 1000 to 10,000 net counts is generally sufficient for most holdup applications. This procedure is repeated at all measurement positions and in all counting geometries designated for each collection zone. The final holdup value for the zone is obtained from the average of the individual measurements (each one being corrected for the effects of attenuation and any variation in geometry relative to the calibration measurement). Whenever possible, the collection zone is assayed in a variety of ways. For example, one could measure an apparatus up close and treat it as an area source; the measurement could then be repeated at a large distance, treating the zone as a point source. It may be better to measure some zones from several different directions-especially if complicated attenuation corrections are called for in some of the counting geometries. Several independent measurements of one zone can provide an average holdup value that is better than the individual measurements. Further, the variability between these measurements can provide an indication of the measurement uncertainty. 5.1.3Gamma Ray Attenuation Corrections To obtain useful assay results by detecting 375- to 450-keV gamma rays, it is necessary to correct each assay for attenuation of the signal, either within the plutonium holdup material or by structural materials. Without this critical correction, the assay is no more than a lower limit on the true holdup value. The attenuation correction may be based on calculations of known attenuation in uniform materials, on earlier measurements of materials similar to those found in the plant equipment, or on direct measurements of gamma ray transmission through the actual equipment. Details on establishing an appropriate attenuation correction are given in Laboratory Exercise No. 4 of Reference 4. Additional treatment of gamma ray attenuation corrections is given in Reference 13. 5.1.4Gamma Ray Interferences Variability in the observed gamma ray response may arise as a result of the presence of extraneous gamma ray emitters or as a result of fluctuations in the background from the Compton scattering of higher energy gamma rays. The magnitude of this effect is generally small. It can be monitored by observing the spectrum with a multichannel analyzer, but, unless data on periodically recovered holdup accumulations are in error, this contribution can be ignored. 5.1.5Matrix Effects on Neutron Assay A change in the neutron yield for a plutonium sample of fixed isotopic content can be caused by a change in the concentration of high-(alpha,n)-yield impurities in the matrix. If it is possible to estimate the range of permissible impurity concentrations, the variation in a typical neutron yield can be calculated using the method given in the appendix to this guide. 5.1.6Effect of Isotopic Uncertainty Gamma ray measurements of plutonium holdup provide a direct determination of the fissile plutonium (i.e., (239)Pu and (241)Pu) holdup in the zone under consideration. On the other hand, neutron techniques measure only the (240)Pu effective content, and chemical techniques provide elemental analysis without consideration of the isotopic makeup. Thus, knowledge of the isotopic composition of the plutonium is necessary to correlate holdup measurements with chemistry and accountability values. Gamma ray assays must be divided by the (239)Pu isotopic fraction, and neutron assays must be converted from (240)Pu effective to total plutonium in order to express holdup in terms of total plutonium. If the process equipment is thoroughly cleaned each time the isotopic composition is changed, the holdup may consist primarily of the current material. In that case, the declared isotopic composition can be used. When mixing occurs, use of the stream-averaged isotopic composition is appropriate. Bounds on the isotopic composition are estimated by considering the batches of highest and lowest composition and computing the corresponding range. This measure of variability must then be incorporated into the estimated holdup standard deviation before making direct comparisons with the chemical analyses. The variability in isotopic composition can be expressed as an estimated standard deviation defined as one-half the observed range and then combined in quadrature with the standard deviation given by Equation 1 in Section B.5.2. In general, gamma ray measurements of (239)Pu will be less sensitive to isotopic variations than neutron measurement of (240)Pu. 5.2 Assignment of Standard Error The assignment of a standard error to a holdup measurement is extremely difficult on a rigid statistical basis. This is because the only statistically predictable fluctuations (e.g., counting statistics) in this application are frequently negligible in comparison with variability due to counting geometry (including material distribution), gamma ray attenuation, gamma ray background and interferences, neutron matrix effects, and instrument instabilities. It is important to recognize that the variability can be large and guard against underestimating the standard deviation of the overall holdup value in a collection zone. Careful measurements must be carried out during the calibration procedure to determine the range of detector responses resulting from variations in measurement parameters. A useful discussion of these ideas is presented in Reference 10. A reasonable estimate of the standard deviation of the measured holdup for a given collection zone may be obtained by consideration of the range of holdup values obtained from the variety of measurements performed on that collection zone, as suggested in the previous section. The mean value for the holdup is defined as the average of the various (corrected) measurement results on the collection zone. The standard deviation, sigma, for that mean value is estimated as one-half the range of holdup values obtained in the measurements. This estimate is conservative if a large number of measurements have been made. For a small number of measurements, the actual standard deviation can be larger than one-half the range. In such cases the actual standard deviation of the holdup values must be calculated. In some cases, it may be unavoidable that the counting statistics are so poor that they contribute significantly to the measurement variability. In such an instance, the overall holdup standard deviation, sigma(h-u), is defined as the square root of the sum of the squares of the standard deviation due to counting, sigma(stat), and the standard deviation due to measurement fluctuations, sigma(meas); that is, (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)5.3 Estimation of Bias When a single collection zone is cleared out, it is desirable to perform a holdup assay before, H(before), and after, H(after), the cleanout if possible. By comparing the amount of plutonium removed, Pu(r), to the recovery amount predicted through the in situ holdup assays, Pu(a), the collection zone calibration can be updated, and the calibration and assay standard deviations can be based on relevant data. The amount of plutonium recovered, Pu(r), during the cleanout of a specific collection zone can be assayed through sampling and chemical analysis, through calorimetry, or through other applicable nondestructive assay methods (e.g., spontaneous fission coincidence detection or gamma ray assay). The assay value for the recovered amount is computed as the difference in the holdup assays before and after the cleanout: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.) The percent difference, Delta, between the assay and recovery values for the plutonium holdup is then computed: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.) A running tabulation of the quantities Pu(a), Pu(r), and Delta (as well as their standard deviations, sigma(a), sigma(r), and sigma(Delta) is kept in the assay log for each collection zone. The average value, Overline Delta, of the percent differences between Pu(a) and Pu(r) will serve as an estimate of the bias in the holdup assay for that collection zone and will also provide quantitative justification for revision of the assay calibration for that zone to remove the bias. The root-mean-square deviations, sigma(Delta), of the percent differences, sigma(i), from their mean value, Delta, serve as a check on the appropriateness of the size of the estimated standard deviation of the holdup measurements. To the extent that the standard deviation of Pu(r( is small compared with the uncertainty in Pu(a) (usually an adequate assumption), the quantity s (Delta) should be comparable in size to the standard deviation of Pu(a). For K measurements of the percent differences, Delta(i), for a given collection zone, the quantity s(Delta) is given by: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)Equation 4 assumes that all the sigma(Delta)'s are equal. For a calculation of s(Delta) using weighted sums, see Reference 14. Note that, if the holdup measurements (i.e., H(before) or H(after)) contain a constant bias, their difference can still provide useful information in the comparison with Pu(r). However, a small difference between Pu(a) and Pu(r) does not necessarily mean that the bias associated with H is small. This ambiguity is reduced in importance if the cleanout is such that H(after) is much smaller than H(before). In addition, the use of several holdup measurements from varying vantage points, as suggested earlier, will help to minimize the bias associated with incorrect geometrical or attenuation corrections in one measurement configuration. C. REGULATORY POSITION To develop a program for the periodic in situ assay of plutonium residual holdup as a method acceptable to the NRC staff for measuring this inventory component, it is necessary to consider the factors in the following sections. Care must be exercised during the fabrication and use of check sources and calibration standards to ensure their continued integrity and to prevent contamination. In addition, the usual precautions for safeguarding plutonium should be taken. 1. DELINEATION OF COLLECTION ZONES AND ASSAY SITES Preliminary radiation survey measurements of the plutonium processing facility should be used to budget the measurement time to emphasize high-holdup areas, to establish independent collection zones, and to determine detector positions within the zones. 1. At each collection zone, detector positions (assay sites) should be chosen so that the material holdup can be measured from several vantage points around the zone. At each assay site, the detector should have an exclusive view of the collection zone being assayed. If necessary, shadow shielding should be used to isolate the region being assayed from other collection zones. Detector positions should be chosen to minimize the measurement ambiguities, as described in Section B.5.1.1. 2. Each assay site should be permanently marked with paint or colored tape on the floor to ensure reproducible assay positions. The markings should be protected (for example, with clear epoxy) to ensure their long-term durability. Detector height and orientation should be clearly indicated in the assay log for each measurement site and, if possible, included in the site markings. 3. Each assay site should be uniquely labeled to facilitate unambiguous reference to that site in the assay log. A labeling and color-coding convention should be established to distinguish neutron assay sites from gamma ray assay sites. 4. Gamma ray assay should be used for collection zones containing less plutonium than the neutron detection limit. Also, gamma ray assay should be used for all structures that do not contain irregularly shaped components capable of significantly attenuating the emerging gamma rays. Neutron assay should be used for all structures not suitable for gamma ray assay. There may be some large structures such as furnaces that can be measured only with small interior probes or with thermoluminescent dosimeters. 5. Areas may be denoted as problem areas so that careful holdup measurements will be made in these areas each time plant holdup is to be determined; or the area may be labeled as a spot-check zone, where accumulations are known to be low and careful holdup assays are needed less frequently. 2. ASSAY INSTRUMENTS Neutron and gamma ray assay capability can be provided, if desired, using separate or compatible electronics with interchangeable detector probes. Compatible electronics can provide for both (3)He or BF(3) neutron detection and NaI(Tl) gamma ray detection. The electronics unit should have a temperature coefficient of less than 0.1 percent per degree centigrade. Battery-powered electronics can expedite assays. 2.1 Gamma Ray Assay Gamma ray assay should be based on the activity observed in the energy range from 375 to 450 keV, excluding the composite gamma ray complex centered at 333 keV. Yield data for appropriate gamma rays are presented in Section B.2.1 of this guide. 2.1.1Detector Selection Gamma ray detectors for holdup measurements should have FWHM (full width at half maximum) resolution better than 10 percent at 662 keV ((137)Cs gamma ray). NaI(Tl) detectors can exhibit resolutions as good as 7 percent and are suitable for this application. The crystal depth should be sufficient to detect a significant percentage of 400-keV gamma rays. For NaI(Tl), the minimum depth should be 1 inch (2.5 cm); a 2-inch (5-cm) depth is recommended. The crystal should be stabilized with a suitable radioactive source. An internal seed containing (241)Am is recommended for this application. The electronics should be capable of stabilizing on the reference radiation emitted by the seed. The crystal face (external to the cover) should be covered with 0.75 mm of cadmium and 1.5 mm of lead to filter low-energy radiations. Two single-channel analyzers should be provided with lock-set energy windows. One channel should be set to admit gamma rays from 375 to 450 keV. Unless equilibrium of the (237)U and (241)Pu can be ensured, the 333-keV region of the gamma ray spectrum should be completely excluded. The second channel should be set above the first window to provide a background subtraction for the assay window. This second window should be set from approximately 475 to 575 keV. The width and position of this window is a matter of personal preference in how the background subtraction should be done. These analyzers should be packaged as one integral unit. 2.1.2Gamma Ray Collimator A cylinder of shielding material such as lead should be made concentric with the gamma ray detector. The end of the cylinder opposite the crystal should be blocked with the shielding material. The thickness of the collimator should be chosen to provide sufficient directionality for the specific facility (1.5 cm of lead thickness should be sufficient for most applications). The collimator sleeve should be fixed over the end of the detector crystal at a reproducible setting identical to that used in the calibration measurements. 2.1.3Gamma Ray Calibration and Check Sources Standard sources of (239)Pu should be provided for calibration of the measurement system for the basic measurement geometries described in Section B.4. A small encapsulated plutonium sample can be used both as a calibration standard for the point source counting geometry and as a check source for verification of instrument stability. For the line and area calibrations, large plutonium foils can be used, or the calibrations can be derived from a series of measurements made with the point source. The gamma ray self-attenuation correction should be clearly specified for all foils and samples. 2.2 Neutron Assay 2.2.1Neutron Detector Selection Neutron detectors should have high detection efficiency and be capable of operating in the presence of gamma radiation. BF(3) and (3)He neutron detectors are recommended for this application. Neutron detectors should be surrounded by a layer of neutron moderator material to enhance their detection efficiency. The neutron moderator layer should be covered with a low-energy neutron absorber to filter out extraneous neutrons from the desired signal. 2.2.2Neutron Collimator A slab collimator or concentric cylinder collimator of polyethylene should completely surround the detector, leaving open only a detection channel in one direction. The moderator thickness should be selected to provide the directionality required for each facility. A directionality profile providing a 10:1 response ratio is desirable. However, for portable detectors a 3:1 ratio may be used. 2.2.3Neutron Calibration and Check Source A 50- to 100-gram sample of plutonium should be adequate both as a point source calibration standard and as a check source. The isotopic composition, (241)Am content, and high-(alpha, n)-yield impurity composition should be representative of the plutonium being processed. The neutron yield of the standard should be independently measured, if possible, and also computed using the method described in the appendix of this guide. If the measured and calculated yields differ by more than 20 percent, any future yield calculations should be normalized to be consistent with this measurement. 2.3 Service Cart A cart carrying electronics and both detector probes should be provided. The capability to raise or lower the probes to reproducible settings should be included. 3. CALIBRATION 3.1 Instrument Check The stability of the neutron and gamma ray detection systems should be tested prior to each inventory by comparing the observed counts obtained from the check source, minus the counts with the shaped shield in place but without the check source, to the readings obtained prior to previous inventories. If the measurement is consistent with previous data (i.e., is within plus or minus 2 single-measurement standard deviations of the mean value of previous data), all previously established calibrations using this detection system should be considered valid. If the measurement is not consistent, the operation of the unit should be checked against the manufacturer's recommendations and repaired or recalibrated, as required. These check source measurements should be supplemented with regular remeasurements of instrument calibrations to ensure continued proper instrument performance over the entire operating range. 3.2 System Response Calibration The response of the detection system should be determined with well-known quantities of plutonium in the basic measurement geometries described in Section B.4. If there are special counting geometries in the facility that are not readily represented by one of the basic configurations, these geometries should also be mocked up and measured during the calibration procedure. 4. ASSAY PROCEDURES 4.1 Assay Log An assay log should be maintained. Each collection zone should have a separate section in the assay log, with the corresponding calibration derived on the page facing the assay data sheet. Recording space should be provided for the date of measurement, gross counts, corrected counts, and the corresponding grams of plutonium from the calibration in addition to position and instrument electronic setting verification. There should also be provision for recording data from recovery operations and holdup assay comparisons, as described in Section B.5.3. 4.2 Preassay Procedures Prior to inventory, the isotopic composition of the plutonium processed during the current operational period should be determined. Variations in the neutron and gamma ray yield data from the calibration standard should be calculated. Either the calibration data or the predicted holdup should then be corrected to reflect this difference. Prior to each inventory, the operation of the neutron and gamma ray assay detection systems should be checked, as described in Regulatory Position 3.1. Prior to any assay measurements, feed into the process line should be stopped. All in-process material should be processed through to forms amenable to accurate accountability. All process, scrap, and waste items containing plutonium should be removed to approved storage areas to minimize background radiations. 4.3 Measurements Before beginning the holdup measurements, it is advisable to conduct a preliminary gamma survey of the collection zones to point up the zones where holdup accumulations are the highest (and therefore where the most careful measurements should be made). In zones where accumulations are shown to be very low by the survey, spot-check measurements may be adequate, as pointed out earlier. Before assaying each collection zone, the operator should verify the floor location, probe height, and probe orientation. The electronic settings should be verified every 1 or 2 hours with the check source. During the actual assay of the collection zones, the check source should be removed or shielded so as not to interfere with the measurement. Prior to taking a measurement, a visual check of the zone and the line of sight of the detector probe should be made to ensure that no obvious changes have been made to the process area and that no unintended accumulations of plutonium remain within the collection zone. The operator should initial the measurement log to ensure compliance for each collection zone. When the preceding steps have been completed, the measurement at each collection zone should be taken, recorded, and converted to grams of plutonium. If each value is within an expected or permissible range, the assayist can proceed to the next collection zone. However, if the collection zone contains an unexpectedly large amount of plutonium, it should be cleaned to remove the accumulation for conversion to a more accurately accountable material category. After the cleanout has been completed, the zone should be reassayed. 5. ESTIMATION OF HOLDUP ERROR During the initial implementation of the holdup measurement program, the holdup uncertainty for each collection zone should be estimated from the range of values obtained in the various measurements on that zone, as described in Section B.5.2. As a history of comparisons between holdup measurements and cleanout recovery data becomes available, these data should be used to adjust for bias and to revise the magnitudes of the holdup uncertainties, as described in Section B.5.3. During each physical inventory, the calibration in at least 10 percent of the collection zones should be updated on the basis of the comparison between holdup and cleanout recovery measurements. In any case, all calibrations should be updated at least once per year. REFERENCES 1. R. Gunnink et al., "A Re-evaluation of the Gamma-Ray Energies and Absolute Branching Intensities of U-237, Pu-238, -239, -240, -241, and Am-241," Lawrence Livermore Laboratory, UCRL-52139, 1976. 2. J. E. Cline, R. J. Gehrke, and L. D. McIsaac, "Gamma Rays Emitted by the Fissionable Nuclides and Associated Isotopes," Aerojet Nuclear Co., Idaho Falls, Idaho, ANCR-1069, July 1972. 3. L. A. Kull, "Catalogue of Nuclear Material Safeguards Instruments," Battelle National Laboratories, BNL-17165, August 1972. 4. R. H. Augustson and T. D. Reilly, "Fundamentals of Passive Nondestructive Assay of Fissionable Material," Los Alamos Scientific Laboratory, LA-5651-M, 1974; also T. D. Reilly et al., "Fundamentals of Passive Nondestructive Assay of Fissionable Material: Laboratory Workbook," Los Alamos Scientific Laboratory, LA-5651-M, Suppl., 1975. 5. R. B. Walton et al., "Measurements of UF(6) Cylinders with Portable Instruments," Nuclear Technology, Vol. 21, p. 133, 1974. 6. C. H. Kindle, "In Situ Measurement of Residual Plutonium," Nuclear Materials Management, Vol. 5, No. 3, p. 540, 1976. 7. J. W. Tape, D. A. Close, and R. B. Walton, "Total Room Holdup of Plutonium Measured with a Large-Area Neutron Detector," Nuclear Materials Management, Vol. 5, No. 3, p. 533, 1976. 8. H. E. Preston and W. J. Symons, "The Determination of Residual Plutonium Masses in Gloveboxes by Remote Measurements Using Solid Thermoluminescent Dosimeters," United Kingdom Atomic Energy Authority, Winfrith, England, AEEW-R1359, 1980. 9. A. Ohno and S. Matsuura, "Measurement of the Gamma Dose Rate Distribution in a Spent Fuel Assembly with a Thermoluminescent Detector," Nuclear Technology, Vol. 47, p. 485, 1980. 10. W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A Method for Surveying for (235)U with Limit of Error Analysis," Nuclear Materials Management, Vol. 2, p. 395, 1973. 11. N. Ensslin, J. Stewart, and J. Sapir, "Self-Multiplication Correction Factors for Neutron Coincidence Counting," Nuclear Materials Management, Vol. VIII, No. 2, p. 60, 1979. 12. M. S. Zucker et al., "Holdup Measurements for Nuclear Fuel Manufacturing Plants," Nuclear Materials Management, Vol. X, p. 239, 1981. 13. J. L. Parker and T. D. Reilly, "Bulk Sample Self-Attenuation Correction by Transmission Measurement," Proceedings of the ERDA X- and Gamma-Ray Symposium, Ann Arbor, Michigan (Conf. 760639), p. 219, May 1976. 14. P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, 1969. APPENDIX A. NEUTRON YIELD COMPUTATIONS The following model for the calculation of the total spontaneous neutron yield from plutonium-bearing materials assumes that the plutonium is widely dispersed. With this condition, there will be no significant neutron production through induced fission of (239)Pu or (241)Pu. The total neutron yield of plutonium holdup will then be the sum of the spontaneous fission and (alpha,n) contributions: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)1. SPONTANEOUS FISSION NEUTRONS To determine the spontaneous neutron yield of plutonium, the isotopic composition must be known. (The contribution from (238)U spontaneous fission is usually negligible even if uranium is present in large quantities.) The yield from the plutonium isotopes is given by: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)where M(i) is the total mass of the ith plutonium isotope, and Q(i) is the spontaneous fission neutron yield per gram of the ith isotope. Using the yield data from Table A-1, Equation 2 can be rewritten as: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.) The coefficients 2.50 and 1.70 are the spontaneous fission yields of (238)Pu and (242)Pu relative to (240)Pu. The concept of effective (240)Pu mass reflects the fact that most of the spontaneous fission yield is due to that isotope. 2. (alpha,n) NEUTRONS When the plutonium holdup is in the form of oxide, the major contribution from (alpha,n) reactions will be due to the 0-18(alpha,n) (21)Ne reaction. The additional neutron yield is typically 50 to 100 percent of the spontaneous fission yield. The (alpha,n) yield can be calculated from the yields per gram of each isotope of Pu(Y(i)) given in Table A-1: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)The summation over M(i) should also include (241)Am, which is a strong alpha emitter. In addition to (alpha,n) production in the oxide itself, certain low-Z impurities in the oxide can contribute substantially. Values for the yields of neutrons obtained in bombarding thick targets of these elements with 5.2-MeV alpha particles are given in Table A-2. Further research may change these values somewhat, but they are sufficient for computing the approximate effect of these elements if they exist as impurities in PuO(2). One method for doing this is to compute the impurity (alpha,n) yield relative to the oxide (alpha,n) yield: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)(Due to database constraints, Tables A-1-A-4 are not included. Please contact LIS to obtain a copy.) In Equation 6, P(j) is the (alpha,n) neutron yield in the impurity element, and P(o) is the yield in oxygen (0.050 neutrons/10(6) alphas); A(j) is the atomic weight of the impurity element, and A(o) that for oxygen (16); I(j) is the concentration of the impurity expressed in parts per million (by weight) of plutonium oxide, and I(o) is oxide (118,000 ppm). If the impurity concentration is expressed as ppm of plutonium, it can be converted to ppm of plutonium oxide by multiplying by the gravimetric dilution factor, 0.882. To summarize the calculation of (alpha,n) neutron yields in oxide that also contains impurities, Y(alpha,n) from all sources is given by: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)Elements other than those listed in Table A-2 yield no neutrons by (alpha,n) reactions for the alpha energies obtained from plutonium and americium decay. Also note that the summation over i must include (241)Am and that the summation over j includes only the oxygen that is not bound up as plutonium oxide. 3. SAMPLE CALCULATION FOR PuO(2)-UO(2) Consider the case of 1 gram of recycle plutonium blended to 3 percent by weight of PuO(2) in a UO(2) matrix where the isotopic composition is as given in Table A-3. For mixed oxides, the oxygen density is approximately the same as in PuO(2) alone. Also, plutonium and uranium have similar atomic numbers. For these reasons, it may be assumed that the oxygen (alpha,n) yield in mixed oxide is the yield in PuO(2), further reduced by the blending ratio, PuO(2)/(PuO(2) + UO(2)). Using the isotopic composition given in Table A-3 and using Equation 3, the spontaneous fission neutron yield can be found to be 218 n/sec for 1 gram of plutonium. Then the neutron production in the oxide can be calculated using the masses M(i) of each isotope and the yields Y(i) from the fifth column of Table A-1. The result of 112 n/sec is given in the last column of Table A-3. Note that the alpha particle yield of plutonium is nearly constant in time, but that, because (241)Am builds up in time, the total alpha production increases at a rate of roughly 0.3 percent per month in typical reactor fuel. The impurity (alpha,n) yields are calculated in Table A-4. The calculation is based on impurities in PuO(2) only. The mixed oxides are assumed to consist of blended PuO(2) and UO(2) particles approximately 40 @m in diameter where most alpha particles stop within the PuO(2) particles. If the particle size were smaller or the mixed oxide were created through coprecipitation, the uranium impurity content would also contribute to the plutonium (alpha,n) yield. In the present example, it is sufficient to use the neutron yields P(j) from Table A-2, the concentrations I(j) from Table A-4, and Equation 6 or 7. The total neutron yield from 1 gram of plutonium in PuO(2) is then 218 + 112 + 47 = 377 n/sec. Using the gravimetric dilution factor of 0.882, this is 333 n/sec for 1 gram of PuO(2). If the PuO(2) is blended so that PuO(2)/ (PuO(2) + UO(2)) = 0.03, the neutron yield from 1 gram of mixed oxide is 10 n/sec. The impurity (alpha,n) yields, P(j), used in this example are currently known to about 10 percent accuracy for most elements and 50 percent accuracy for the others. The oxide (alpha,n) yields, Y(j), are known to 10 percent or better. Both yield calculations must assume perfect mixing, however. For these reasons, neutron yield calculations are accurate to 10 percent at best, and the neutron holdup measurement calibration should be based on representative standards rather than calculation wherever possible. B. CONVERSION OF MEASURED M(240) (EFFECTIVE) TO TOTAL PLUTONIUM To convert a measured effective (240)Pu mass to actual total plutonium, one must use both the relationship between these two quantities, as shown in Equation 4, and the known isotopic composition of the samples being measured. Let f(238), f(239), f(240), f(241), and f(242) represent the weight fractions of the respective plutonium isotopes in the unknown sample. The (240)Pu effective weight fraction, f(240) (effective), can be defined as: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)Generally, as previously mentioned in this guide, the relative measurement uncertainty of M(240) (effective) in a holdup measurement will be much larger than that of f(240) (effective), so the relative error in M(Pu) (total) is essentially equal to that of M(240) (effective). As an example calculation, the sample of isotopic composition given in Table A-3 has an effective fraction given by: (Due to database constraints, this equation is not included. Please contact LIS to obtain a copy.)Thus, a holdup measurement of 35 plus or minus 10 grams (240)Pu effective corresponds to 166 plus or minus 47 grams total plutonium, where the relative error in the total plutonium result was taken to be equal to that of the M(240) (effective) result. VALUE/IMPACT STATEMENT 1. PROPOSED ACTION 1.1 Description Licensees authorized to possess at any time more than 1 kilogram of plutonium are required by Part 70, "Domestic Licensing of Special Nuclear Material," of Title 10 of the Code of Federal Regulations to calculate a material balance based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70. Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inventories." Plutonium residual holdup is defined as the plutonium inventory component remaining in and about process equipment and handling areas after those collection areas have been prepared for inventory. This regulatory guide describes procedures acceptable to the NRC staff for the in situ assay of the residual plutonium holdup. 1.2 Need for Proposed Action Regulatory Guide 5.23 was published in 1974. The proposed action, a revision to this guide, is needed to bring the guide up to date with respect to advances in measurement methods, as well as changes in terminology. 1.3 Value/Impact of Proposed Action 1.3.1NRC Operations The regulatory positions will be brought up to date. 1.3.2Other Government Agencies Not applicable. 1.3.3Industry Since industry is already applying the methods and procedures discussed in the guide, updating these methods and procedures should have no adverse impact. 1.3.4Public No adverse impact on the public can be foreseen. 1.4 Decision on Proposed Action The regulatory guide should be revised to reflect improvements in measurement techniques and to bring the language of the guide into conformity with current usage. 2. TECHNICAL APPROACH Not applicable. 3. PROCEDURAL APPROACH Of the procedural alternatives considered, revision of the existing regulatory guide was selected as the most advantageous and cost effective. 4. STATUTORY CONSIDERATIONS 4.1 NRC Authority The authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Energy Reorganization Act of 1974, as amended, and is implemented through the Commission's regulations, in particular 10 CFR Part 70. 4.2 Need for NEPA Assessment The proposed action is not a major action that may significantly affect the quality of the human environment and does not require an environmental impact statement. 5. RELATIONSHIP TO OTHER EXISTING OR PROPOSED REGULATIONS OR POLICIES The proposed action is one of a series of revisions of existing regulatory guides on nondestructive assay techniques. 6. SUMMARY AND CONCLUSIONS Regulatory Guide 5.23 should be revised. 16 |