Publication Date: 2/1/80
    Pages: 9
    Date Entered: 2/23/84
    Title: CONSIDERATIONS FOR ESTABLISHING TRACEABILITY OF SPECIAL NUCLEAR MATERIAL ACCOUNTING MEASUREMENTS (11/78)
    Revision 1
    February 1980
    U.S. NUCLEAR REGULATORY COMMISSION
    REGULATORY GUIDE
    OFFICE OF STANDARDS DEVELOPMENT
    REGULATORY GUIDE 5.58
    CONSIDERATIONS FOR ESTABLISHING TRACEABILITY OF SPECIAL
    NUCLEAR MATERIAL ACCOUNTING MEASUREMENTS
A. INTRODUCTION
    Part 70, "Domestic Licensing of Special Nuclear Material," of
    Title 10 of the Code of Federal Regulations requires that for approval
    to possess and use more than one effective kilogram of special nuclear
    material (SNM)(1) the licensee must provide an adequate material control
    and accounting system. Section 70.51, "Material Balance, Inventory, and
    Records Requirements," requires licensees to calculate material
    unaccounted for(2) (MUF) and the limit of error of the MUF(3) value
    (LEMUF) following each physical inventory and to compare the LEMUF with
    prescribed standards. Section 70.58, "Fundamental Nuclear Material
    Controls," requires licensees to maintain a program for the continuing
    determination of systematic and random measurement errors and for
    maintaining control of such errors within prescribed limits. Section
    70.57, "Measurement Control Program for Special Nuclear Materials
    Control and Accounting," provides criteria for establishing and
    maintaining an acceptable measurement and control system.(4) Reference
    1 describes the technical and administrative elements that are
    considered to be important in a measurement control program.
    Implicit in the criteria stated in Section 70.57 is the
    requirement of traceability of all SNM control and accounting systems to
    the national standards of measurement as maintained by the National
    Bureau of Standards (NBS) by means of reference standards.
    Reference standard is defined in Section 70.57(a)(3).
    Traceability is defined in Section 70.57(a)(4). These definitions are
    clarified as follows: Traceability means the ability to relate
    individual measurement results to the national standards of measurement
    through an unbroken chain of comparisons. Reference standard means a
    material, device, or instrument whose assigned value(5) is known
    relative to the national standards of measurement.
    ----------
    (*) Lines indicate substantive changes from previous issue.
    (1) For definitions, see paragraphs 70.4(m) and (t) of 10 CFR Part
    70.
    (2) Currently called inventory difference (ID).
    (3) Currently called the limit of error of the inventory
    difference (LEID).
    (4) The listed regulations do not apply to special nuclear
    materials involved in the operation of a nuclear reactor or in waste
    disposal operations or used in sealed sources. See paragraphs 70.51(e),
    70.57(b), and 70.58(a) of 10 CFR Part 70.
    ----------
    This guide presents conditions and procedural approaches
    acceptable to the NRC staff for establishing and maintaining
    traceability of SNM control and accounting measurements. No specific
    methods will be presented herein since the methodology to be used for
    any given measurement must be tailored to the needs and peculiarities of
    the relevant process material, reference standards, instrumentation, and
    circumstances. Rationales and pertinent analytical factors will be
    presented for consideration as to their applicability to the measurement
    at hand.
B. DISCUSSION
1. BACKGROUND
    SNM measurements for control and accounting are performed on a
    great variety of material types and concentrations, with a diversity of
    measurement procedures, by a large number of licensees at all the
    various industrial, research and development, and academic facilities
    involved. Accurate, reliable measurements are necessary to achieve valid
    overall accountability. To this end, all measurement systems must be
    compatible with the national standards of measurement through the
    national measurement system (NMS). To obtain this necessary
    compatibility for any SNM measurement task, reference materials
    appropriate for each SNM type and measurement system may be required.
    Table 1 defines the various types of reference materials.
    Traceability is a property of the overall measurement, including
    all Certified Reference Materials (CRMs), instruments, procedures,
    measurement conditions, techniques, and calculations employed. Each
    component of a measurement contributes to the uncertainty of the
    measurement result relative to national standards of measurement through
    the NMS. The NMS is composed of a number of components, including the
    NBS (which has the responsibility for maintaining the national standards
    of measurement), CRMs, national laboratories, calibration facilities,
    standards-writing groups, national standards, and the person making the
    ultimate measurement.
    ----------
    (5) The term "value" includes instrument response and other
    pertinent factors.
    ----------
    (Due to database constraints, Tables 1 and 2 are not included. Please
    contact LIS to obtain a copy.) If the NBS, as the legal caretaker of the national standards of
    measurement for the United States, is viewed as an entity capable of
    making measurements without error, traceability can be defined as the
    ability to relate any measurement made by a local station (e.g.,
    licensee) to the "correct" value as measured by the NBS. If it were
    possible for the NBS to make measurements on the same item or material
    as the local station, this relationship, and hence traceability, could
    be directly obtained. Since such direct comparisons are not ordinarily
    possible, an alternative means for achieving traceability must be
    employed. This necessary linkage of measurement results and their
    uncertainties to the NBS through the NMS may be achieved by:
    a. Periodic measurements by the licensee of CRMs or Standard
    Reference Materials (SRMs). The measurement, per se, of an SRM or CRM
    without rigorous internal control of measurements does not provide the
    necessary linkage. Adequate and suitable reference materials, along with
    reliable measurement methods and good internal measurement assurance
    programs, are necessary to ensure accuracy (Ref. 1).
    b. Periodic measurements of well-characterized process
    materials or synthesized artifacts that have been shown to be
    substantially stable and either being homogeneous or having small
    variability of known limits. The uncertainties associated with the
    values assigned to such process materials or artifacts are obtained by
    direct or indirect comparisons with Primary Certified Reference
    Materials (PCRMs).
    c. Periodic submission of samples for comparative measurement
    by a facility having established traceability in the measurement
    involved, employing one or both of the above procedures, and involving
    only samples not subject to change in their measured values during
    storage or transit. ("Round-robin" sample exchanges between facilities
    can be useful in confirming or denying compatibility of results, but
    such exchanges do not of themselves constitute the establishment or
    maintenance of traceability.) Valid assignment of an uncertainty value to any measurement result
    demands a thorough knowledge of all the observed or assigned
    uncertainties in the measurement system, including an understanding of
    the nature of the sources of these uncertainties, not just a statistical
    measure of their existence. It is not sufficient, for example, to
    derive a root-mean-square value for a succession of observed or assigned
    uncertainties (CRM, instrumental, and procedural) for which standard
    deviation values have been calculated by statistical methods for random
    events. To do so involves assumptions as to the randomness of these
    variances that may not be at all valid. The variances may, in fact, be
    due to a combination of systematic errors that appear to be randomly
    distributed over the long run but that are not at all random in their
    occurrence for a given analyst employing a given combination of
    standards, tools, and instruments. Thus, it is necessary to derive the
    uncertainty value of a measurement from methods that also involve a
    summation of the nonrandom (systematic) uncertainties, not from the
    mathematics of random events alone. The valid determination of the
    uncertainty of a measurement relative to the NBS, and thus of the degree
    of traceability, is not a rigorous procedure but is the result of sound
    judgment based on thorough knowledge and understanding of all factors
    involved.
    Obviously, the effects of systematic error can be reduced if
    Reference Materials (RMs) are included at least once in every series of
    related measurements by a given analyst and combination of tools,
    instruments, and conditions. The calibration and correlation factors so
    obtained cannot be applied uncritically to successive measurements. It
    also follows that the applicability of any given RM to a series of
    measurements of process material should be examined critically both
    periodically and with every change or hint of change in the measurement
    characteristics of the process material.
    It is doubtful that RMs can ever be exact representations of the
    material under measurement in any given instance, even for highly
    controlled process materials such as formed fuel pieces or uniform
    powdered oxide shown to be substantially uniform in both composition and
    measurement-affecting physical characteristics (e.g., density or shape
    for nondestructive assay (NDA) measurements). However, in most cases
    RMs that yield measurement uncertainties within the selected limits for
    the material in question can be achieved. Obviously, the errors
    resulting from mismatch of the RM with the measured material will be
    largest in heterogeneous matter such as waste materials, but in these
    cases the SNM concentrations normally will be low and the allowable
    limits of uncertainty correspondingly less stringent.
    The important truth being stressed here is that every measurement
    must be considered, in all aspects, as an individual determination
    subject to error from a variety of sources, none of which may be safely
    ignored. The all-too-natural tendency to treat successive measurements
    as routine must be rigorously avoided. Test object and device RMs, in
    particular, tend to be mistakenly accepted as true and unvarying, but
    they may well be subject to changes in effective value (measured
    response) as well as unrepresentative of the samples unless wisely
    selected and carefully handled.
    The characteristics required of CRMs include:
    a. Sufficiently small and known uncertainties in the assigned
    values. (Normally, the uncertainties of the CRMs will contribute only a
    small fraction of the total uncertainty of the measurement.) b. Predictability in the response produced in the measurement
    process. (Ideally, the measurement process will respond to the RMs in
    the same way as to the item or material to be measured. If there is a
    difference in measurement response to the measured parameter arising
    from other measurement-affecting factors, these effects must be known
    and quantifiable.) c. Adequate stability with respect to all measurement-affecting
    characteristics of the standard. (This is necessary to avoid systematic
    errors due to changes in such properties as density, concentration,
    shape, and distribution.) d. Availability in quantities adequate for the intended
    applications.
    It cannot be assumed that RMs will always remain wholly stable as
    seen by the measurement system employed, that working RMs will forever
    remain representative of the measured material for which they were
    prepared or selected, or that the measured material itself will remain
    unchanged in its measurement characteristics. Therefore, it is
    essential that these RMs, as well as the measurement instrumentation and
    procedures, be subject to a program of continuing confirmation of
    traceability. Many of the factors involved in such a program are
    discussed in Reference 2.(6)2. MASS AND VOLUME MEASUREMENTS
    The national systems of mass and volume measurements are so well
    established that RMs meeting the above criteria are readily available.
    Where necessary, the licensee can use the RMs to calibrate Working
    Reference Materials (WRMs) that more closely match the characteristics
    of the measured material in terms of mass, shape, and density in the
    case of mass measurements or are more easily adapted to the calibration
    of volume-measurement equipment.
    Specific procedures for the use of mass and volume RMs for the
    calibration of measurement processes and equipment are given in the
    corresponding national standards (Refs. 3 and 4). Factors likely to
    affect uncertainty levels in inventory measurements of mass and volume
    are discussed in regulatory guides (Refs. 5, 6, and 7).
3. CHEMICAL ASSAY AND ISOTOPIC MEASUREMENTS
    Methods for chemical analysis and isotopic measurement often are
    subject to systematic errors caused by the presence of interfering
    impurities, gross differences in the concentrations of the measured
    component(s) or of measurement-affecting matrix materials, and other
    compositional factors. Traceability in these measurements can be
    obtained only if such effects are recognized and either are eliminated
    by adjustment of the RM (or sample) composition or, in some cases, are
    compensated for by secondary measurements of the measurement-affecting
    variable component(s) and corresponding correction of the measured SNM
    value. The latter procedure involves additional sources of uncertainty
    and therefore should be employed only if it has a substantial economic
    or time advantage, if the interferences or biasing effects are small and
    limited in range, if the corrected method is reliable, and if the
    correction itself is verifiable and is regularly verified.
    ----------
    (6) Regulatory guides under development on measurement control
    programs for SNM accounting and on considerations for determining the
    systematic error and the random error of SNM accounting measurements
    will also discuss the factors involved in a program of continuing
    confirmation of traceability.
    ----------
    Systematic measurement calibration errors frequently arise and can
    be ascribed to improper use, handling, or treatment of reference
    materials. These errors are independent of the effect of impurities,
    concentration differences, etc., and are almost impossible to detect via
    an internal measurement control program. Interlaboratory measurement
    comparison programs where carefully characterized materials are used are
    means by which these systematic errors may be identified and corrective
    action initiated.
    3.1 National Standards - Uses and Limitations
    PCRMs generally are not recommended for use directly as WRMs, not
    only because of cost and required quantities but also because of
    differences in composition (or isotopic ratios) compared to the process
    materials to be measured. PCRMs are more often used to prepare RMs of
    composition and form matching the process material or to evaluate (and
    give traceability to) non-NBS but substantially identical material from
    which matching WRMs are then prepared. This is necessary because of both
    the wide diversity of process materials encountered and the very small
    number and variety of SNM PCRMs available. These RMs may be used
    directly as WRMs, if appropriate, or may be reserved for less frequent
    use in the calibration of suitable synthetic or process-material WRMs of
    like characteristics, as well as for verifying instrument response
    factors and other aspects of the measurement system. However, each
    level of subsidiary RMs adds another level of uncertainty to the overall
    uncertainty of the SNM measurement.
    PCRMs can be used to "spike" process samples or WRMs to determine
    or verify the measurability of incremental changes at the working SNM
    level. However, because of possible "threshold" or "zero error" effects
    and nonlinearity or irregularity of measurement response with
    concentration, this process does not of itself establish traceability.
    3.2 Working Reference Materials
    WRMs that closely match the effective composition of process
    material, or a series of such WRMs that encompass the full range of
    variation therein, serve as the traceability link in most chemical
    analyses and isotopic measurements. The WRMs derive traceability through
    calibration relative to either PCRMs, Secondary Certified Reference
    Materials (SCRMs), or, more often, synthesized RMs containing either
    PCRMs or other material evaluated relative to the PCRM (see Section
    B.3.1 of this guide).
    The characteristics required of a WRM are that it be chemically
    similar to the material to be measured (including interfering
    substances), that it be sufficiently stable to have a useful lifetime,
    and that it have sufficiently low uncertainty in its assigned value to
    meet the requirements of the measurement methods and of the
    accountability limits of error.
    WRMs can be prepared (a) from process materials characteristic of
    the material to be measured or (b) by synthesis using known quantities
    of pure SNM. The former method offers the advantage that the WRM will
    include all the properties that can affect the measurement such as
    impurities, SNM concentration level, and chemical and physical form; it
    suffers from the disadvantage that the assigned value is determined by
    analyses subject to uncertainties that must be ascertained. The latter
    method involves preparations using PCRMs (not usually economical unless
    small amounts are used) or SCRMs with the appropriate combination of
    other materials to simulate the material to be measured. The advantages
    of the latter method include more accurate knowledge of the SNM content
    and better control of other variables such as the amount of impurities
    and the matrix composition. The chief disadvantage is that the
    synthesized WRM may not possess all the subtle measurement-affecting
    characteristics of the process material. Moreover, the preparation of
    synthesized WRMs may be substantially more costly than the analysis of
    WRMs prepared from process material. Detailed procedures for preparing
    plutonium and uranium WRMs are described in References 8, 9, 10, and 11.
    The primary concern in the use of a WRM to establish traceability
    in SNM measurements is the validity of the assigned value and its
    uncertainty. Considerable care is necessary to ensure that the WRMs are
    prepared with a minimal increase in the uncertainty of the assigned
    value above that of the PCRM upon which the WRM value is based. If the
    assigned value of a WRM is to be determined by analysis, the use of more
    than one method of analysis is necessary to enhance confidence in the
    validity of the assigned value. The methods should respond differently
    to impurities and to other compositional variations. If the WRM has
    been synthesized from a PCRM or other reference materials, the
    composition and SNM content can be verified by subsequent analyses.
    The composition of a WRM can change with time, e.g., changes in
    oxidation state, crystalline form, hydration, or adsorption. These
    changes and their effects on measurement are minimized by appropriate
    packaging and proper storage conditions. Additional assurance is
    attained by distributing premeasured amounts of the material into
    individual packets at the time of preparation, and these packets can be
    appropriately sized so that the entire packet is used for a single
    calibration or test. Even among such subsamples, there may be
    variability in SNM content, and this variability must be taken into
    account in determining the uncertainty of the assigned value.
    3.3 Standard Laboratories and Sample Interchange
    Traceability of chemical assay and isotopic analysis values also
    may be obtainable through comparative analyses of identical samples
    under parallel conditions. A comparative-measurement program may take
    either or both of two forms:
    a. Periodic submission of process samples for analysis by a
    facility having demonstrated traceability in the desired measurement.
    b. Interfacility interchange and measurement of
    well-characterized and representative materials with values assigned by
    a facility having demonstrated traceability in the measurement.
    Round-robin programs in which representative samples are analyzed
    by a number of laboratories do not establish traceability but can only
    indicate interlaboratory agreement or differences, unless traceability
    of one or more of the samples in a set has been established as above.
    The Safeguards Analytical Laboratory Evaluation (SALE) program as
    administered by the Department of Energy New Brunswick Laboratory (NBL)
    is an example of an acceptable comparative-measurement program.
4. NONDESTRUCTIVE ASSAY
    Nondestructive assay (NDA) measurement methods are those that
    leave the measured material unchanged (e.g., gamma emission methods) or
    with no significant change (e.g., neutron activation) relative to its
    corresponding unmeasured state (Ref. 2). NDA offers the advantages that
    the same RM or the same sample can be measured repeatedly and yields
    valuable data on system uncertainties not otherwise obtained, that the
    measurement made does not consume process material, and that
    measurements can be made more frequently or in greater number, usually
    at a lesser unit cost than with destructive chemical methods. These
    advantages often yield better process and inventory control and enhanced
    statistical significance in the measurement data. However, like chemical
    measurement methods, NDA methods have many sources of interferences that
    may affect their accuracy and reliability. The interferences and their
    sources must be identified before valid traceability can be assured.
    In nearly all NDA methods, the integrity and traceability of the
    measurements depend on the validity of the RMs by which the NDA system
    is calibrated. Calibrations generally are based on WRMs that are or are
    intended to be well characterized and representative of the process
    material or items to be measured. While the matching of RMs to process
    items, and consequent valid traceability, is not difficult to achieve
    for homogeneous materials of substantially constant composition (e.g.,
    alloys) having fixed size and shape (e.g., machined pieces), such ideal
    conditions are not obtained for most SNM measurements. Many of the
    materials and items encountered are nonhomogeneous, nonconforming in
    distribution, size, or shape, and highly variable in type of material
    and composition. In order to ensure traceability of the measurement
    results through the NMS, variations in the physical characteristics and
    composition of process items and in their effects upon the response of
    the NDA measurement system must be evaluated and carefully considered in
    the selection or design of WRMs and measurement procedures (Refs. 12 and
    13).
    WRMs usually (a) are prepared from process materials that have
    been characterized by measurement methods whose uncertainties have been
    ascertained through the NMS (i.e., are traceable) or (b) are artifacts
    synthesized from well-characterized materials to replicate the process
    material.(7) However, calibration of the NDA method by means of such RMs
    does not automatically establish continuing traceability of all process
    item measurement results obtained by that method. The effects of small
    variations in the materials being assayed may lead to biased results
    even when the WRM and the material under assay were obtained from
    nominally the same process material. It therefore may be necessary
    either (a) to establish traceability of process item measurement results
    by comparing the NDA measurement results with those obtained by means of
    a reliable alternative measurement system of known traceability, e.g.,
    by total dissolution and chemical analysis (see Section B.4.1) or (b) to
    establish adequate sample characterization to permit the selection of a
    similarly characterized WRM for method calibration (see Section B.4.2).
    4.1 Traceability Assay by a Second Method
    Any NDA method would be of little practical use if every
    measurement also required a confirmatory analysis. However, in cases in
    which there are a number of items or material samples of established
    similar characteristics, it is practical to establish traceability for a
    series of measurements by means of second-method evaluations of an
    appropriate proportion of randomly selected samples. If the correlation
    between the two methods is then found to be consistent, traceability is
    established for all NDA measurements on that lot of SNM and on other
    highly similar material.
    For nominally uniform process or production material of which
    multiple subsamples can be obtained from a gross sample, the uniformity
    can be deduced from the distribution of the NDA measurement data. For
    thus characterized material, traceability can be established for all
    subsamples that approximate the mean(8) from the separate traceable
    second-method analysis of a few of the subsamples. Other like
    subsamples can then be selected as traceable WRMs whose assigned values
    are related to the separately analyzed subsamples through their
    respective NDA measurement results.
    For subsample populations exhibiting a range of NDA values,
    especially where a destructive second-method analysis is used, the
    "twinning" method of sample selection may be employed. In this method,
    pairs of subsamples are matched by their NDA measurement values, and the
    matches are confirmed by NDA reruns. One member of each pair is
    evaluated by the traceable second-method analysis; the other member of
    that pair is then assigned the value determined for its twin and may
    serve thereafter as a traceable WRM for the measurement of that process
    material by that NDA method.
    ----------
    (7) The advantages stated for similarly derived WRMs (see Section
    B.3.2) also apply here.
    (8) Subsamples whose measured values markedly deviate from the
    mean (i.e., "flyers") are not used for second-method analysis or for
    WRMs.
    ----------
    4.2 Characterization by a Second Method
    If the process items or materials being measured are subject to
    non-SNM variations that affect the SNM measurement, it may be possible
    to employ one or more additional methods of analysis to measure these
    variations and thus to characterize process materials in terms of such
    analysis results. If the secondary analyses also are of an NDA method,
    they may often be performed routinely with the SNM measurements. In
    many cases, the results of secondary analyses may be used to derive
    simple corrections to the SNM measurement results. Correction also may
    be obtained and traceability preserved by the judicious modification of
    RMs so as to incorporate the same variable factors, i.e., so that they
    can produce the same relative effects in the SNM and non-SNM
    measurements as do the process variable(s).
    Alternatively, it may be advantageous to prepare WRMs that span
    the normal range of variability of the measurement-affecting non-SNM
    parameter(s) (and also the SNM-concept range, if appropriate). These
    WRMs can then be characterized on the basis of their non-SNM measurement
    results or of some function(s) of SNM and non-SNM measurement results
    and can be assigned a correspondingly "characteristic figure." If this
    procedure can be carried out with adequate sensitivity and specificity
    relative to the interfering factors and within acceptable limits of
    uncertainty, the process material can be routinely characterized in like
    manner and the appropriate WRM selected on the basis of such
    characterization.
5. CONTINUING TRACEABILITY ASSURANCE
    Initial or occasional demonstration that a laboratory has made
    measurements compatible with the NMS is not sufficient to support a
    claim of traceability. Measurement processes are by their nature
    dynamic. They are vulnerable to small changes in the skill and care
    with which they are performed. Deterioration in the reliability of
    their measurement results can be caused by (a) changes in personnel
    performance, (b) deterioration in or the development of defects in RMs,
    instrumentation, or other devices, or (c) variation in the environmental
    conditions under which the measurements are performed. The techniques
    discussed in preceding sections ensure traceability only if they are
    used within a continuing program of measurement control (Ref. 1).
C. REGULATORY POSITION
    The measurement control program (Ref. 1) used by the licensee
    should include provisions to ensure that individual measurement results
    are traceable to the national standards of measurement through the
    national measurement system (NMS). RMs used to establish traceability
    of measurement results through the NMS should have assigned values whose
    uncertainties are known relative to the national standards of
    measurement. To meet this condition, the licensee should maintain a
    continuing program for calibrating each measurement process, using RMs
    that meet the criteria in the following paragraphs.
1. REFERENCE MATERIALS
    1.1 The National Bureau of Standards
    Devices and instruments calibrated by, and CRMs certified by, NBS
    along with reference material data supplied are acceptable RMs(9) for
    calibrating either methods or WRMs. However, it is very important that
    the licensee be able to demonstrate that the RMs are stable under the
    conditions for which they are used, that their validity has not been
    compromised, and that they meet the accuracy requirements of the
    intended applications.
    1.2 Secondary Certified Reference and Working Reference Materials
    SCRMs or WRMs that have been produced by the licensee or by a
    commercial supplier are acceptable provided their uncertainties relative
    to PCRMs are known.
    A statement of uncertainty should be assigned to each RM based on
    an evaluation of the uncertainties of the calibration process. The
    statement should contain both the standard deviation and the estimated
    bounds of the systematic errors associated with the assigned value
    similar to the statistical information contained within the most recent
    NBS PCRM certificates.
    1.2.1RMs for Chemical and Isotopic Analyses
    WRMs used for calibrating chemical assay and isotopic measurements
    may be prepared from standard reference materials (SRMs) supplied by NBS
    or from other well-characterized materials available to the industry.
    Such WRMs should be prepared under conditions that ensure high
    reliability and should be packaged and stored in a way that eliminates
    any potential for degradation of the WRM.
    The assigned values of WRMs prepared from process materials should
    be determined by analysis, using two different methods whenever
    possible. A sufficient number of analyses should be done by both
    methods to allow a reliable estimate of the components of random
    variation that affect the measurement. If two methods are not
    available, as may be the case for isotopic analysis, it is recommended
    that a verification analysis be obtained from another laboratory.
    The components of variance (random variation) of measurements used
    to assign a value to an RM should be known in advance. The statistical
    design of an RM characterization plan requires that measurement
    precision, etc., be known in order to calculate the number of
    measurements to be performed and the number of samples to be analyzed so
    that the desired uncertainty in the mean value assigned to the RM can be
    achieved. The maximum uncertainty permitted by the proposed end use of
    the RM must be an assumption that is factored into the characterization
    plan.
    ----------
    (9) International RMs and reference material such as IAEA RMs are
    included, if accepted by NBS.
    ----------
    If WRMs are prepared from NBS SRMs or other PCRMs, they should be
    analyzed to verify that the makeup value is correct, i.e., that no
    mistakes have been made in their preparation. For this verification, at
    least five samples should be analyzed using the most reliable method
    available. Should the analytical results differ significantly from the
    makeup value, the WRM should not be used. Typical statistical and
    analytical procedures acceptable to the NRC staff for preparing WRMs are
    found in References 8, 9, 10, and 11.
    Storage and packaging of WRMs should follow procedures designed to
    minimize any changes likely to affect the validity of the assigned
    values. Whenever practical, the WRM should be divided into small
    measured quantities at the time of preparation, and the quantities
    should be of appropriate size so that each entire unit is used for a
    single calibration or calibration test (Refs. 8, 9, 10, and 11).
    1.2.2Nondestructive Assay
    RMs for NDA should be prepared from well-characterized materials
    whose SNM contents have been measured by methods that have been
    calibrated with CRMs or from synthetic materials of known SNM content.
    The NDA RMs should closely resemble in all key characteristics the
    process items to be measured by the system. Since destructive
    measurements ordinarily cannot be made on NDA RMs in order to verify
    makeup, as required for WRMs for chemical assay and isotopic analyses,
    RMs should be prepared in sets of at least three using procedures that
    guard against errors common to all members of the set. If all three RMs
    respond consistently to the NDA system, one RM could be used as the
    intended NDA RM, the second could be kept in reserve, and the third
    characterized using destructive chemical measurement techniques whenever
    possible. If destructive analysis is not possible, the consistency of
    the NDA system response to all the RMs in the set would provide a basis
    for judging the validity of the set of RMs. If one or more of the RMs
    in the set differs significantly from the expected response, no RMs from
    that set should be used. Statistical tests for this comparison can be
    found in References 8, 9, 10, and 11.
    The design and fabrication of the RMs should take into account the
    measurement process parameters affecting the response of the system
    (Ref. 2), including:
    a. SNM content,
    b. Isotopic content,
    c. Matrix material,
    d. Density,
    e. Container material and dimensions,
    f. Self-absorption effects, and
    g. Absorption and moderation effects.
    Studies should be carried out in sufficient detail to identify the
    process item characteristics and the variations of the characteristics
    that can cause systematic error. The results of the studies should be
    used to establish reasonable bounds for the systematic errors.
    NDA systems whose uncertainties relative to the national standards
    of measurement cannot be satisfactorily established directly through the
    calibration process should be tested by comparative analysis. This test
    should be done by periodically analyzing randomly selected process items
    with the NDA system in question and by another method with known
    uncertainty. The verification analysis can be done on samples obtained
    after reduction of the entire item to a homogeneous form. In some
    cases, verification analysis by small-sample NDA or by other NDA methods
    may be acceptable if the uncertainties of the verification method are
    known relative to the national standards of measurement.
2. MEASUREMENT ASSURANCE
    The traceability of each measurement process through the NMS
    should be maintained by a continuing program of measurement assurance
    (Ref. 1). This program should include planned periodic verifications of
    the assigned values of all RMs used for calibrations.
    2.1 Verification of Calibrations
    A formal program fixing the frequency at which calibrations and
    calibration checks are performed should be established. The required
    frequencies are strongly dependent on system stability and should be
    determined for each case by using historical performance experience.
    Current performance of the measurement system based on measurement
    control program data may signal the need for more frequent
    verifications. Also, the effects of changes in process parameters such
    as composition of material or material flows should be evaluated when
    they occur to determine the need for new calibrations.
    WRMs that are subject to deterioration should be recertified or
    replaced on a predetermined schedule. The frequency of recertification
    or replacement should be based on performance history. If the integrity
    of an RM is in doubt, it must be discarded or recalibrated.
    2.2 Recertification or Replacement of CRMs
    Objects, instruments, or materials calibrated by NBS or other
    authoritative laboratories and used as CRMs by the licensee should be
    monitored by intercomparisons with other CRMs to establish their
    continued validity. In any case, the values should be periodically
    recertified by the certifying agency or compared with other CRMs by the
    licensee in accordance with Table 2.
    2.3 Interlaboratory Exchange Programs
    The licensee should participate in interlaboratory exchange
    programs when such programs are relevant to the types of measurements
    performed and the materials analyzed in his laboratory. The values
    assigned to the materials that are to be analyzed in the interlaboratory
    exchange programs should be carefully and traceably certified so that
    any deviation that may occur can be readily identified and quantified.
    The data obtained through this participation and other comparative
    measurement data (such as shipper-receiver differences and inventory
    verification analyses) should be used to substantiate the uncertainty
    statements of his measurements.
    When statistically significant deviations indicating lack of
    consistency in measurements occur in the results of the comparative
    measurements, the licensee should conduct an investigation. The
    investigation should identify the cause of the inconsistency and, if the
    cause is within his organization, the licensee should initiate
    corrective actions to remove the inconsistency. The investigation may
    involve a reevaluation of the measurement process and the CRMs to locate
    sources of bias or systematic error or a reevaluation of the measurement
    errors to determine if the stated uncertainties are correct.
3. RECORDS
    The licensee should retain all records relevant to the uncertainty
    of each measurement process for 5 years [Section 70.51(e)(4)(iv) and
    (v); Section 70.57(b)(12)]. The records should include documents or
    certificates of CRMs, the measurement and statistical data used for
    assigning values to WRMs, and the calibration procedures used in
    preparing the WRMs.
    REFERENCES
1. R.J. Brouns, F.P. Roberts, J.A. Merrill, and W.B. Brown, "A
    Measurement Control Program for Nuclear Materials Accounting," NRC
    report NUREG/CR-0829 (1979).
2. Regulatory Guide 5.11, "Nondestructive Assay of Special Nuclear
    Material Contained in Scrap and Waste" (1973).
3. ANSI Standard N15.18, "Mass Calibration Techniques for Nuclear
    Material Control," American National Standards Institute, 1430
    Broadway, New York, New York (1975).
4. ANSI Standard N15.19, "Volume Calibration Techniques for Nuclear
    Material Control," American National Standards Institute, 1430
    Broadway, New York, New York (1975).
5. Regulatory Guide 5.25, "Design Considerations for Minimizing
    Residual Holdup of Special Nuclear Material in Equipment for Wet
    Process Operations" (1974).
6. Regulatory Guide 5.42, "Design Considerations for Minimizing
    Residual Holdup of Special Nuclear Material in Equipment for Dry
    Process Operations" (1975).
7. Regulatory Guide 5.48, "Design Considerations-Systems for
    Measuring the Mass of Liquids" (1975).
8. G.C. Swanson, S.F. Marsh, J.E. Rein, G.L. Tietjen, R. K. Zeigler,
    and G. R. Waterbury, "Preparation of Working Calibration and Test
    Materials-Plutonium Nitrate Solution," NRC report NUREG-0118
    (1977).
9. S. S. Yamamura, F. W. Spraktes, J. M. Baldwin, R. L. Hand, R. P.
    Lash, and J. P. Clark, "Preparation of Working Calibration and
    Test Materials: Uranyl Nitrate Solution," NRC report NUREG-0253
    (1977).
10. J. E. Rein, G. L. Tietjen, R. K. Zeigler, G. R. Waterbury, G. C.
    Swanson, "Preparation of Working Calibration and Test Materials:
    Plutonium Oxide," NRC report NUREG/CR-0061 (1978).
    11. J. E. Rein, G. L. Tietjen, R. K. Zeigler, G. R. Waterbury,
    "Preparation of Working Calibration and Test Materials: Mixed
    Oxide," NRC report NUREG/CR-0139 (1978).
    12. ANSI Standard N15.20, "Guide to Calibrating Nondestructive Assay
    Systems," American National Standards Institute, 1430 Broadway,
    New York, New York (1975).
    13. Regulatory Guide 5.53, "Qualification, Calibration, and Error
    Estimation Methods for Nondestructive Assay" (1975).
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