Regulatory Guide 5.37
| ML003739973 | |
| Person / Time | |
|---|---|
| Issue date: | 10/31/1983 |
| From: | Office of Nuclear Regulatory Research |
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| References | |
| RG-5.37, Rev 1 | |
| Download: ML003739973 (12) | |
Revision 1 October 1983 U.S. NUCLEAR REGULATORY COMMISSION
REGULATORY GUIDE
OFFICE OF NUCLEAR REGULATORY RESEARCH
REGULATORY GUIDE 5.37 (Task SG 047-4)
114 SITU ASSAY OF ENRICHED URANIUM RESIDUAL HOLDUP
A. INTRODUCTION
walls of process vessels and associated plumbing often become coated with uranium during processing of Part 70, "Domestic Licensing of Special Nuclear Mate solutions. Uranium also accumulates in air filters and rial," of Title 10 of the Code of Federal Regulations associated ductwork. Whenever possible, process equip
2 requires licensees authorized to possess more than 350 grams ment should be designed and operated to minimize the
235 U to conduct a physical inventory of all amount of holdup. The absolute amounts of uranium of contained holdup must be small for efficient processing and proper special nuclear material in their possession at intervals not to exceed 12 months. Certain licensees authorized to hazards control. However, the total holdup can be large (relative to the plant inventory difference (ID)) but have no possess more than one effective kilogram of special nuclear significance on the ID if it remains reasonably constant. It material are required to conduct more frequent measured physical inventories of their special nuclear materials. is the change in the holdup between beginning inventory and ending inventory that may impact the inventory Further, these licensees are required to conduct their difference. The amounts of holdup must therefore be nuclear material physical inventories in compliance with Inventory determined by an in situ assay.
specific requirements set forth in Part 70.
procedures acceptable to the NRC staff are described in Regulatory Guide 5.13, "Conduct of Nuclear Material Assay information can be used in one of two ways:
Physical Inventories."
1. When the standard error of uranium holdup is com Residual holdup is defined as the inventory component patible 3 with the plant standard error (estimator) of inven tory difference (SEID), the material balance can be com remaining in and about process equipment and handling puted using the measured contents of uranium holdup.
areas after those collection areas have been prepared for inventory. In situ assay is used to ensure that a measured Additional cleanout and recovery for accountability will then not be necessary.
value of residual holdup is included in each material balance. This guide describes procedures acceptable to the NRC staff for the in situ assay of the residual enriched 2. When the standard error of uranium holdup is not uranium holdup.
1 Because of the difficulty in measuring compatible with the plant SEID, the information obtained in-process holdup, the procedures described in this guide in the holdup survey can be used to locate principal ura for calibration and error evaluation differ from general nium accumulations. Once located, substantial accumula guidance on measuring residual holdup in more accessible tions can be recovered, transforming the uranium to a controlled situations. more accurately measurable inventory component. Having reduced the amount of uranium holdup, the standard error Any guidance in this document related to information collection activities has been cleared under OMB Clearance No. 3150-0009.
2
B. DISCUSSION
Design features to minimize holdup in drying and fluidized bed operations, equipment for wet process operations, and equipment for dry process operations are the respective subjects of Regulatory Guides 5.8, 5.25, and 5.42.
Uranium accumulates in cracks, pores, and zones of poor
3 circulation within and around process equipment. The Compatibility exists when the contribution of the standard error of the holdup to the total plant SEID is not large enough to
1Assay of residual plutonium holdup is the subject of Regulatory cause the overall SEID to exceed allowed limit
s. If the plant SEID
exceeds allowed limits because of an excessive contribution from Guide 5.23, "In Situ Assay of Plutonium Residual Holdup." A the holdup standard error, compatibility does not exist, and the proposed revision to this guide has been issued for comment as remedial steps of paragraph 2 need to be taken.
Task SG 045-4.
Comments should be sent to the Secretary of the Commission, USNRC REGULATORY GUIDES U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Attention: Docketing and Service Branch.
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on the remeasurement of the remaining holdup may be that the gamma rays experience a minimum or easily sufficiently reduced to be compatible with overall plant predicted attenuation in the apparatus being measured;
SEID requirements. and (3) the distribution of material in the zone is repre sented, as much as possible, by one of the distribution The measurement procedures described in this guide geometries used in the calibration procedure (see Sec involve the observation of the 185.7-keV gamma ray tion B.4.1 ).
emitted in association with the decay of 2 3 5 U. The amount of 235U holdup in a piece of equipment is proportional to 2. GAMMA RAY HOLDUP ASSAY
the measured intensity of this gamma ray after suitable corrections are made (1) for attenuation by intervening Two considerations are critical to the holdup assay.
materials, (2) for gamma ray self-attenuation by the uranium, First, to perform an assay, the 2 3 SU gamma rays must
(3) for geometrical factors that relate the measurement and reach the detector and be detected. Second, the observed calibration configurations, and (4) for background radiation response must be attributable to the collection zone being from nearby sources of 2 3 5 U not being measured. The assayed. Therefore, the assay scheme is calibrated to proportionality factors are best determined prior to the compensate for the poor penetration of the 235U gamma holdup
235 measurement by assays of known quantities of ray. Also, the detector is collimated to separate a collec U distributed in well-defined and representative geom tion zone from its neighboring zones and from the radiation etries as discussed below. background. Finally, some effort may be necessary to employ external "shadow shielding" to block the field of Note that there may be some cases in which gamma ray view of the collimated detector from radiation being measurements with electronic detectors may not be prac produced in collection zones other than the one being tical or even feasible. Another gamma ray detection assayed.
technique that might be attempted is thermoluminescence dosimetry (see References 1 and 2). This technique may be For each gram of 2 3 5 U, the 185.7-keV gamma ray is useful either as an independent check of a conventional emitted at a rate of 4.3 x 1o4 per second. This gamma ray gamma ray assay or as a substitute for it in environments or is the only radiation emitted in the decay of 2 3 5 U that is geometries where conventional detectors cannot be used. useful for this assay application. Unless mixed with This technique will not, however, be discussed further in plutonium or thorium, all other gamma rays are usually this guide. attributable to the Compton scattering of high-energy gamma rays emitted by the 2 3 4 Pa daughter of 2 3 8 U. The The measurement of holdup in a complex plant background at 185.7 keV due to this source of radiation environment can involve a very large number of measure varies depending on the length of time between the separa- ments, as is implied in the text that follows. In a stable tion of the 2 3 8 U daughters from the uranium (as fre plant environment where the process behavior is well quently occurs during conversion processes) and the assay.
known and well characterized, it may be possible to arrange This interference is very important for low-enrichment the holdup measurement program so that careful and uranium but much less important at very high-enrichment extensive holdup measurements are made at infrequent values.
intervals (for example, annually); at more frequent intervals (for example, at inventory times), careful measurements are On uranium recycled after exposure in a nuclear reactor again made in the (presumably fewer) known problem (where plutonium will be produced from the fertile 2 3 8 U),
areas, and "spot check" measurements are made in the less a sufficient quantity of 2 3 2 U or 2 3 7 U may be present to problematic or less used zones where accumulations are emit a measurable amount of interfering gamma radiation.
known to be low. Such management of measurement resources can result in a very effective holdup measurement When uranium is mixed with thorium, the background program at minimum cost. gamma ray spectrum becomes much more comple
x. The background
232 spectrum may vary because the daughters of
1. DELINEATION OF COLLECTION ZONES Th can be volatilized to different extents during typical fuel processing. Further information on gamma ray inter To accomplish the gamma ray assay, it is essential to ferences at energies near 185.7 keV may be found in consider the facility in terms of a series of zones that can be Reference 3.
independently assayed. Such zones are designated "collec tion zones." Each uranium processing facility can be To ascertain total uranium holdup from the gamma ray conceptually divided into a series of contiguous collection assay of 235U holdup, it is necessary to determine from zones. Individual process machines, air filters, and separate separate measurements the enrichment of the uranium item areas that can be isolated from one another may be holdup and to measure the emitted 185.7-keV gamma ray suitable discrete collection zones. Great care needs to be with sufficient resolution to enable the intensity of that taken to define all collection zones in such a way that gamma ray to be determined in the presence of the interfer
(1) the assay of the zone can be performed with a minimum ing radiations encountered. However, there are situations of interference and background from nearby zones (i.e., so (for example, in multi-enrichment processing) in which that effective shadow shielding and reliable background holdup enrichment cannot be accurately measured because subtraction can be accomplished); (2) the gamma ray (1) the holdup enrichment may not be uniform and (2) the detector can be positioned reproducibly and in such a way samples of holdup material may not be representative.
5.37-2
In such cases, the uncertainty in the total uranium holdup and counting geometries so that one collimator setting will is increased drastically because of the uncertainty in the suffice for all measurements. This will simplify the calibra uranium enrichment used to convert the measured fissile tion procedures since the calibration constants depend holdup to total uranium holdup. strongly on the collimator settings.
2.1 Gamma Ray Detection Instruments The detector collimator serves the purpose not only of defining the effective field of view but also of shielding the Data processing electronics include a single-channel detector from unwanted radiation. To accomplish this analyzer for the 185.7-keV photopeak, a timer-scaler unit, latter purpose effectively, the collimator (heavy-element)
and a second single-channel analyzer used to determine the material must also cover the rear of the detector as much as possible. This is usually easy to achieve with portable background radiation correction. Both detection channels get their signals from the same detector, and the signals are Nal detectors but requires more effort when Ge(Li) or HPGe detectors are used.
processed through the same electronics system. Battery powered gamma ray analysis systems suitable for this application are commercially available and can enhance Often there is intensive X-ray radiation (in the energy range of 50 to 100 keV) coming from process equipment, operational convenience. Methods for determining energy window settings are provided in References 4 and 5. and this radiation can tie up the detector electronics unnecessarily. To alleviate this problem, a 1/32-in.-thick The detection efficiency and resolution of thallium (0.8-mm) layer of cadmium metal can be placed on the front face of the detector. This will absorb more than activated sodium iodide, NaI(T1), detectors are generally
90 percent of the (lower energy) X-rays incident upon the adequate for this application when the uranium is not detector with a much smaller effect on the 185.7-keV
mixed with plutonium or thorium. Cadmium telluride gamma ray and will therefore render the detection system (Refs. 6, 7, and 8) has better resolution than Nal and may more sensitive in the energy region of interest.
prove adequate to resolve the 185.7-keV gamma ray from thorium or plutonium gamma rays. Lithium-drifted
2.3 Check Source for Gamma Ray Assay germanium, Ge(Li), and high-purity germanium, HPGe (also referred to as intrinsic germanium, IG), semiconductor gamma ray detectors have very high resolution but are less It is important to check the operation of the detection efficient than the other detector types and are more diffi system each time the instrumentation is moved or other
4 cult to operate and maintain. wise disturbed (e.g., power outage) during the course of each inventory sequence. An appropriate check source enables the stability of the assay instrument to be tested at Detector crystal dimensions are selected to provide a any location. Such a source can be prepared by implanting
- high probability of detecting the 185.7-keV gamma ray and a small encapsulated uranium source (containing about a low probability of detecting higher energy radiation. For Nal, a crystal diameter of 1-1/2 in. (3.8 cm) and a thickness 0.5 gram of uranium) in the face of a plug of shielding material. The plug is shaped to fit and close the collimator of 1/2 to 1 in. (1.3 to 2.5 cm) is recommended. For Ge(Li)
channel, and the source is positioned adjacent to the crystal and HPGe detectors, a planar crystal approximately 10 mm in depth is recommended. when the plug is in place. When the response from the check source remains within the expected value, the previous calibration data are assumed to be valid. If not,
2.2 Collimators and Absorbers for Gamma Rays the energy window may have shifted, or the unit may be in need of repair and recalibration.
A shaped shield constructed of any heavy-element nonradioactive material is appropriate for gamma ray collimation. More than 98 percent of all 185.7-keV gamma 3. ISOLATION OF COLLECTION ZONES
rays striking a 0.35-cm-thick sheet of lead are absorbed or scattered. To ensure that each collection zone is independently assayed, it is necessary to screen all radiations from the detector except those radiations emanating from the The collimator will be most effective when it is con collection zone being assayed. This is principally accom centric about the crystal and, for Nal detectors, the photomultiplier and its base. Extending the collimator plished through the use of the collimators described in Section B.2.2. Two additional means exist to further forward of the crystal a distance equal to at least half the isolate a collection zone.
diameter of the crystal, and preferably the full diameter, is recommended (Refs. 4 and 5). Making this distance adjust able to reproducible settings will facilitate use of the
3.1 Detector Positioning collimated detectors for a range of collection zone sites.
However, it is highly desirable to select collection zones When uranium is located in back of the zone under assay
4For further information on germanium detectors, see Regula in another collection zone or in a storage facility, the detec tory Guide 5.9, "Specifications for Ge(Li) Spectroscopy Systems tor can be positioned between the radiation sources or for Material Protection Measurements," and references cited there in. A proposed revision to this guide has been issued for comment above or below the collection zone to isolate the zone as Task SG 042-2 with the title "Guidelines for Germanium Spec for assay.
troscopy Systems for Measurement of Special Nuclear Material."
5.37-3
3.2 Shadow Shielding needs to be specified. It should be further noted that, when there are several measurement locations covering a When it is not possible to avoid interfering radiations large area (such as a floor), it is important to maintain the through the collimator design or through choosing the same source-to-detector distance (even if material distribu detector position for assay, it may be possible to move a tion is uniform within a given measurement area) so that shield panel between the source of interfering radiations the number of measurement areas needed to cover the and the collimator zone under assay. If the shield panel is entire area remains constant. Examples of this type of sufficiently thick and its dimensions match or exceed the assay geometry include floors, walls, glovebox floors, and near portion of the collection zone under assay, no interfer any large-area pieces of equipment.
ing radiations will penetrate through the shadow shield to the detector. A lead sheet 0.4 to 0.5 cm thick, which might 4.2 Calibration of Detector Response be mounted on wheels as an upright panel, is generally adequate for this application. 4.2.1 Mockup of Known MaterialDistributions
4. CALIBRATION FOR HOLDUP MEASUREMENTS For a given collimator setting, the detector response for the three basic source distribution geometries listed above
4.1 Basic Counting Geometries needs to be determined. For the point source, the response is expressed as (counts per minute)/gram of 23SU at a There are three fundamental counting geometries that specified source-to-detector distance. For the line source, can be used to represent most of the distributions of the response is expressed as (counts per minute)/(gram of holdup in process equipment. These geometries are dis 235U per unit length) at a specified source-to-detector tinguished by the spatial distribution of the source material distance. For the area source, the response is expressed as and the resulting dependence of the detector counting rate (counts per minute)/(gram of 2 3 5 U per unit area) at a on the source-to-detector distance, r. specified source-to-detector distance. Corrections to the point and line source calibrations for different detector
4.1.1 Point Source distances in the actual holdup measurements are made using their 1/r2 and l/r count-rate dependences, respectively. For If the material being assayed is distributed over an area further detailed discussion of the measurement of detector with dimensions that are small compared with the source responses for these basic geometries, see Reference 9.
to-detector distance and if the material resides entirely within the detector field of view, the zone can be treated as The measurement of the point source response can be a point source. The detector count rate for a point source accomplished with an encapsulated uranium foil smaller in varies inversely as the square of the source-to-detector size than the detector collimator opening. This foil can also distance (count rate is proportional to I /r 2 ). Any equip serve as the check source for verification of the continued ment measured at great distances or any small pieces of stability of the instrument settings in the field. 5 Care must equipment or equipment parts fall in this category. (Caution: be taken in the preparation of this calibration standard to small-sized sources of uranium could have very large self ensure that the amount of 235U is well known. It is also attenuation of the 185.7-keV gamma ray and could therefore important to measure the gamma ray attenuation through require great care in analysis.) the encapsulating material and the self-attenuation of the uranium foil and to subsequently correct the calibration
4.1.2 Line Source standard response to compensate for these effect
s. Enough
23 SU needs to be present in this standard to provide count If the material being assayed is evenly distributed along a rates that will ensure good statistical precision of the cali linear path so that only a segment of that distribution bration in a reasonable period of time.
length is contained in the detector field of view, this material can be treated as a line source. The detector The measurement of the line source response is best count rate for a line source varies inversely as the source accomplished by constructing a cylindrical surface distribu to-detector distance (count rate is proportional to l/r). tion of 2 3 5 U with the aid of large uranium foils rolled into Examples of this type of holdup geometry include isolated a cylindrical shape. (It is also possible to establish the sections of piping and long, narrow trays or columns. line source response using a point source, as described in Reference 5). The line source geometry is closest to that of
4.1.3 Area Source the pipes and ducts likely to be encountered in actual measurements. The amount of 23SU in the foils must be If the material being assayed is spread over an area large well known to ensure an accurate calibration. The area enough for the material to cover the full field of view of the source response can also be measured with the same detector for a range of source-to-detector distances, the uranium foils laid flat to simulate the expected uranium material can be assayed as an area source. As long as distribution on surfaces such as walls and floors. Self the material being viewed is uniformly distributed, the attenuation by the foils must also be taken into account in detector count rate will be independent of the source-to order for the calibration to be as accurate as possible.
detector distance. However, for holdup applications, uniform material distribution is rare; therefore, the source 5 Note that a calibration source can be used as a check to-detector distance can affect the instrument response and source, but a check source cannot be used as a calibration source.
5.37-4
There may be special material distribution geometries in routine basis because of time or space constraints, one might the facility that are not readily represented by one of the consider multiple measurements on a collection zone initially three basic configurations described above. These special followed by fewer routine measurements at representative geometries may be mocked up as carefully as possible with assay sites. Careful thought in selecting measurement points uranium foils and point sources to produce a usable and measurement strategy will minimize ambiguities in the detector response calibration for these special cases. interpretation of the data.
Examples of special cases might be concave or convex equipment surfaces or the internal volume of a rectangular 5.1.1 Detector Positioning cavity (see Reference 9). Because material particle sizes (or material deposit thicknesses) have a significant effect on the Location and configuration of collection zones are estab self-attenuation of the gamma ray signals, it is important to lished on the basis of a detailed physical examination and a use (whenever practical) well-characterized process material radiation survey of the physical layout of the facility. Pre for preparing calibration standards and to duplicate to the liminary measurements are needed to determine the optimum extent possible process holdup distribution relative to detector positions for the holdup assays. If nonuniform particle size or thickness. Furthermore, holdup in floors is distribution of material in a collection zone is suspected or often deposited at various depths into the floor, rather than if the process apparatus is sufficiently complicated to on the surface. Thus, calibration standards for such meas require extensive attenuation corrections for certain count urements need to incorporate the appropriate geometry ing geometries, multiple measurements are advisable for and matrix effects. Core samples of a floor may be needed that collection zone and more than one detector position to establish typical concentrations at various floor depths. may be necessary. If radiation surveys have indicated zones of high holdup, extra care will be necessary in the holdup Calibration of the holdup measurement system using this measurements for these zones to minimize the contribution procedure is recommended until a history of comparisons of their holdup uncertainties to the SEID. Selecting opti between predicted and recovered holdup quantities is mum detector positions includes consideration of the need developed. If it is possible to take holdup measurements to conveniently measure the line-of-sight background by before and after the cleanout of a piece of shut-down moving the detector to one side without changing its process equipment, these can be used to establish this orientation.
history of comparisons and improve the accuracy of the calibration for each collection zone. 5.1.2 Holdup Measurements
4.2.2 Measurement of Check Sources in Actual Process The measurement and analysis of the 185.7-keV gamma Equipment ray intensity from a collection zone may be carried out by treating the material distribution as one of 'the three One method for calibrating detector response to holdup basic geometries described in Section B.4.1 or as one of the radiation in process equipment is to place a known calibra special cases that may have been measured, as mentioned in tion source in various positions in that equipment and Section B.4.2. If the nature of the material distribution is record the detector responses. In this way, the overall uncertain for a particular detector position, a measurement detector response (including all corrections for attenuation of the detector counting-rate dependence on the source and geometry) is determined empirically. Unfortunately, to-detector distance, r, may indicate the most appropriate this procedure is impractical, if not impossible, in process counting-rate geometry with which to interpret the data.
equipment already in operation. However, those respon sible for holdup assays need to be aware of occasions when After the assay positions for the detector and shadow new process equipment is brought into the plant for installa shields are established for each collection zone, permanent tion. At that time, before installation, calibration sources markings that indicate detector location (including height)
can conveniently be placed in the equipment and the and orientation will ensure reproducibility of subsequent empirical measurements of the detector responses can be measurements for these positions. Uniquely labeling each made. This procedure would be a valuable supplement to assay site will facilitate unambiguous reference to each calibration data obtained from mockups of standard count measurement and its location in the assay log.
ing geometries and comparisons with cleanout recovery data.
After measuring the 185.7-keV gamma ray intensity at
5. HOLDUP MEASUREMENTS AND STANDARD ERROR each detector position in a given collection zone, the line-of sight background is measured by moving the detector and
5.1 Assay Measurements collimator to one side of the zone (still pointing in the same direction as during the assay) and counting the 185.7-keV
In performing the holdup measurements, one must be gamma ray intensity from the surrounding materials.
aware of the large uncertainties in holdup assays arising During the background measurement, the vessel in which primarily from variability in the measurement conditions the holdup is being measured must not be in the field of (e.g., background, geometry, gamma ray attenuation, view of the detector. This procedure is repeated at all material distribution). Accordingly, every effort should be measurement positions and in all counting geometries mnade to perform the assays from as many vantage points as designated for each collection zone. The final holdup value Spossible for each collection zone. If this is impractical on a for the zone is obtained from the average of the individual
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measurements (each one being corrected for the effects of A reasonable estimate of the uncertainty in the meas attenuation and any variation in geometry relative to the ured holdup for a given collection zone may be obtained calibration measurement). by considering the range of holdup values obtained from the variety of measurements performed on that collection K-
Whenever possible, the collection zone is assayed in a zone as suggested in the previous section. The mean value variety of ways. For example, one could measure an for the holdup is defined as the average of the various apparatus at close range, treating it as an area source, and (corrected) measurement results on the collection zone. In then repeat the measurement at long range, treating the view of the well-known uncertainty of holdup measure zone as a point source. It may be better to measure some ment, the standard deviation for that mean value is con zones from several different directions-especially if compli servatively estimated as one-half of the range of holdup cated attenuation corrections are called for in some of the values obtained in the measurements.
counting geometries. The averaging of several independent measurements of one zone helps both to smooth out In some cases, counting statistics may be so poor that imprecisions due to incomplete knowledge of the measure they contribute significantly to the measurement vari ment conditions and to provide a measure of the magni ability. In such an instance, the standard deviation of the tudes of the fluctuations in the measurement results as an overall holdup c (h-u) is defined as the square root of the estimate of the measurement variability. sum of the squares of the standard deviation due to count ing 0 (stat) standard deviation due to measurement fluctua
5.1.3 Gamma Ray Attenuation Corrections tions CF(meas); that is, To obtain useful assay results by detecting the 185.7-keV
/_ 2 2 gamma ray, it is necessary to correct each assay for attenua C(h-u) = I/ cr(stat) + (meas) (1)
tion of the signal, either within the uranium holdup mate rial or by structural materials. Without this critical correc tion, the assay is no more than a lower limit on the true 5.3 Estimation of Bias holdup value. Details for establishing an appropriate attenuation correction are given in Laboratory Exercise When a single collection zone is cleaned out, it is desirable No. 4 of Reference 5. Additional treatment of gamma ray to perform a holdup assay before (Hbefore) and after (Hafter)
attenuation corrections is given in Reference 10. the cleanout if possible. By comparing the amount of uranium removed, Ur, to the amount predicted through the
5.1.4 Uranium Enrichment in situ holdup assays, Ua, the collection zone calibration can be updated, and the calibration and assay standard The 185.7-keV gamma ray measurement provides the deviations can be based on relevant data. The amount of amount of 2 3 5 U holdup. Total uranium holdup is obtained uranium recovered, Ur, during the cleanout of a specific by dividing the assay result by the declared material enrich collection zone can be assayed through sampling and ment in 2 35 U. For accountability of fissile uranium, this chemical analysis or 6through other applicable nonde step is not necessary. However, as is discussed below, con structive assay methods.
version of the measurement results to total uranium is necessary when making comparisons with other assay tech The update data is computed as the difference in the niques that use elemental analysis. Also note that, at low holdup assays before and after the cleanout:
enrichment (and to a lesser extent at high enrichment),
some knowledge of the total amount of uranium holdup is Ua = Hbefore - Hafter (2)
necessary to facilitate proper gamma ray self-attenuation corrections to the measurements. The percentage difference between the assay and recovery values for the uranium holdup,
5.2 Assignment of Standard Error A = 100(Ua - Ur)/Ur (3)
The assignment of a standard error (i.e., twice the stan dard deviation, (j) to a holdup measurement is extremely is then computed. A running tabulation of the quantities difficult on a rigid statistical basis. This situation exists Ua, Ur, and A (as well as their standard deviations, 0 a' a 0 r'
because the only statistically predictable fluctuations and GiA) is kept in the assay log for each collection zone.
(i.e., counting statistics) in this application are frequently negligible compared with the variability due to counting The average value, A, of the percentage differences geometry (including material distribution), gamma ray at between Ua and Ur will serve as an estimate of the bias in tenuation, gamma ray background and interferences, and the holdup assay for that collection zone and will also instrument instabilities. Therefore, the variability can be large provide quantitative justification for revision of the assay and it is necessary to guard against underestimating the stan calibration for that zone to remove the bias. The root dard deviation of the overall holdup value in a collection mean-square deviations, sA, of the percentage differences, zone. Careful measurements are needed during the calibra
6 tion procedure to determine the range of detector responses See, for example, Regulatory Guide 5.11, "Nondestructive resulting from variations in measurement parameters. A Assay of Special Nuclear Material Contained in Scrap and Waste."
A proposed revision to this guide has been issued for comment as useful discussion of these ideas is presented in Reference 9. Task SG 043-4.
5.37-6
Ai, from their mean value, A, serves as a check on the holdup as a method for measuring this inventory compo appropriateness of the size of the estimated standard nent, the factors in the following sections should be deviations of the holdup measurements. To the extent that considered.
the standard deviation of Ur is small compared with the Sstandard deviation of Ua (usually an adequate assumption), 1. DELINEATION OF COLLECTION ZONES AND
the quantity sA should be comparable in size to the stan ASSAY SITES
dard deviation of Ua. For K measurements of the percent age differences Ai for a given collection zone, the quantity Preliminary radiation survey measurements of the SA is given by: uranium processing facility should be used to establish independent collection zones and detector positions within the zones.
[KA )2-K 1] 1i.
= (4)
At each collection zone, detector positions (assay sites)
should be chosen so that the material holdup can be meas Equation 4 assumes that all of the G*A'S are equal. For a ured from several vantage points around the zone. At each calculation of sA using weighted sums, see Reference 11. assay site, the detector should have exclusive view of the collection zone being assayed. If necessary, shadow shield Note that, if the holdup measurements (i.e., Hbefore or ing should be used to isolate the region being assayed from Hafter) contain a constant bias, their difference can still other collection zones. Detector positions should be provide useful information in the comparison with Ur. chosen to minimize the measurement ambiguities, as de However, a small difference between Ua and Ur does not scribed in Section B.5.1.1.
necessarily mean that the error associated with H is small.
This ambiguity is reduced in importance if the cleanout is Each assay site should be permanently marked with such that Hafter is much smaller than Hbefore. In addition, paint or colored tape on the floor to ensure reproducible the use of several holdup measurements from various vantage assay positions. Detector height and orientation should be points, as suggested earlier, will help to minimize the bias clearly indicated in the assay log for each measurement associated with incorrect geometrical or attenuation correc site and, if possible, included with the site markings. The tions in one measurement configuration. markings should be protected (for example, with clear epoxy) to ensure their long-term durability.
5.4 Effect of Enrichment Uncertainty Each assay site should be uniquely labeled to facilitate unambiguous reference to that site in the assay log.
- > The gamma ray measurements described here provide a direct determination of the fissile uranium (i.e., 2 3 SU) Areas may be denoted as "problem areas" so that careful holdup in the zone under consideration. However, the holdup measurements will be made there each time plant comparison and verification measurements made with holdup is to be determined, or an area may be labeled as a chemical techniques provide elemental analysis without "spot check" zone where accumulations are known to be consideration of the isotopic makeup of the samples. low and careful holdup assays are needed less frequently.
While 235U accountability does not require it, knowledge of the uranium enrichment of the material being measured
2. ASSAY SYSTEM
is necessary for meaningful comparisons with the chemical analyses. Thus, the holdup assays must be divided by the 2.1 Detector Selection material enrichment to make the comparisons outlined in Equations 2 through 4, which are expressed in terms of NaI(TI) detectors are generally suitable for this applica total uranium. tion when the uranium is not mixed with thorium or plutonium. The crystal depth should be sufficient to detect If the process equipment is thoroughly cleaned each a significant percentage of 185.7-keV gamma rays. For time the enrichment of the uranium feed is changed, the Nal(TI), a 1-inch (2.5-cm) crystal depth is recommended.
holdup will consist primarily of the current material. In Cadmium telluride, Ge(Li), or HPGe detectors should be that case, the declared enrichment can be used. When used
235 when Nal resolution is inadequate to separate the mixing occurs, use of the stream-averaged enrichment is U activity from interfering radiations.
appiopriate. The enrichment uncertainty bounds are estimated by considering the batches of highest and lowest The crystal should be stabilized with a suitable radioac enrichment and computing the corresponding range. The tive source. An internal seed containing 241Am is recom uncertainty in the material enrichment must then be mended for Nal applications. The electronics should be incorporated into the quoted holdup uncertainty before capable of stabilizing on the reference peak produced by making direct comparisons with the chemical analyses. the seed.
C. REGULATORY POSITION
Two single-channel analyzers should be provided with lock-set energy windows. These analyzers should be
"-> To develop a program acceptable to the NRC staff for integrally packaged, should get their signals from the same the periodic in situ assay of enriched uranium residual detector, and should process the signals with the same
5.37-7
electronics. One channel should be set to admit the checked against the manufacturer's recommendations and
185.7-keV gamma rays from 2 3 5 U. The second channel repaired or recalibrated as required.
should be set above the first window -to provide a back ground correction for the assay window. The electronics 3.2 System-Response Calibration unit should have a temperature stability of less than 0.1 percent per degree Celsius. The response of the detection system should be deter mined with well-known quantities of 2 3 SU in the basic
2.2 Gamma Ray Collimator measurement geometries described in Section B.4. If there are special counting geometries in the facility that are not A cylinder of shielding material such as lead should be readily represented by one of the basic configurations, made coaxial with the gamma ray detector. The end of the these geometries should also be mocked up and measured cylinder opposite the crystal should be blocked with the during the calibration procedure.
shielding material. The thickness of the collimator should be chosen to provide sufficient directionality for the
4. ASSAY PROCEDURES
specific application (a lead thickness of 0.35 cm should be sufficient for most applications). The collimator shield 4.1 Assay Log should be fixed over the end of the detector crystal at a reproducible setting identical to that used in the calibration An assay log should be maintained. Each collection measurements. zone or subzone should have a separate section in the assay log, with the corresponding calibration derived on the page An absorber should be placed over the front face of the facing the assay data sheet. Recording space should be detector to filter out unwanted low-energy photons. A provided for the date of measurement, gross counts,
1/32-in.-thick (0.8-mm) layer of cadmium metal is corrected counts, and the corresponding grams of uranium recommended. from the calibration in addition to verifying the position and electronic setting of the instrument. Also, space
2.3 Gamma Ray Calibration and Check Sources in the log should be provided for recording data from recovery operations and holdup assay comparisons as Standard sources of 23SU should be provided for described in Section B.5.3.
calibrating the measurement system for the basic measure ment geometries described in Section B.4. A small encapsu 4.2 Preassay Procedures lated foil of enriched uranium can be used both as a cali bration standard for the point-source counting geometry Prior to inventory, the enrichment of the uranium and as a check source for verification of instrument stabil processed during the current operational period should be ity.7 Standard sources for the line and area material determined. Variations in the gamma ray yield data from distributions should be well-characterized uranium samples the calibration standard should be calculated. Either the such as large-area uranium foils of well-known thickness calibration data or the predicted holdup should then be and 2 3 5 U content, which can be arranged in reproducible corrected to reflect this change.
configurations for the calibration measurements. Other well-characterized samples taken from the process may also Before each inventory, the operation of the gamma ray be advisable for use as calibration standards to reflect more assay detection systems should be checked, as described in realistically actual process conditions. The gamma ray Regulatory Position 3.1.
self-attenuation correction (or equivalently, the effective mass of 23SU without application of the correction) should Prior to any assay measurements, feed into the process be clearly specified for all of the uranium foils and check line should be stopped. All in-process material should be sources. processed through to forms amenable to accurate account ability. All process, scrap, and waste items containing
3. CALIBRATION uranium should be removed to approved storage areas to minimize background radiations.
3.1 Instrument Check
4.3 Measurements The stability of the gamma ray detection system should be tested prior to each inventory. If the check-source Before beginning the holdup measurements, it is advis measurement is consistent with previous data (i.e., is within able to conduct a preliminary gamma survey of the collec plus or minus two single-measurement standard deviations tion zones to point up the zones where holdup accumula of the mean value of previous data), previously established tions are the highest (and therefore where the most careful calibration data should be considered valid. If the measure measurements should be made). In zones where accumula ment is not consistent, the operation of the unit should be tions are shown to be very low by the survey, spot check measurements may be adequate, as pointed out earlier.
7 Recall that a calibration source may be used as a check source, Before assaying each collection zone, the operator but a check source should never be used for calibration. should verify that floor location, probe height, probe
5.37-8
orientation, and electronics settings correspond to previous with the recovered material quantity to test the validity of measurements. All check and calibration sources should be the zone calibration as described in Section B.5.3.
sufficiently removed (or shielded) to prevent interference with the measurement. Prior to taking a measurement, a 5. ESTIMATION OF HOLDUP UNCERTAINTY
-- < visual check of the zone and the line of sight of the detec tor probe should be made to ensure that no obvious changes During the initial implementation of the holdup have been made to the process area and that no unintended measurement program, the holdup uncertainty for each accumulations of uranium remain within the collection collection zone should be estimated from the range of zone. The operator should initial the measurement log to values obtained in the various measurements on that zone ensure compliance for each collection zone. as described in Section B.5.2. As a history of comparisons between holdup measurements and cleanout recovery data When all the preceding steps have been completed, the becomes available, these data should be used to adjust measurements at each collection zone should be taken and for bias and to revise the magnitudes of the holdup uncer recorded. An attenuation correction measurement should tainties as described in Section B.5.3.
be made, and the corrected response should be converted to grams of 2 3 5 U and recorded in the assay log. To convert During each physical inventory, the calibration in at the result to grams of uranium, divide the previous result by least 10 percent of the collection zones should be updated the declared uranium enrichment. If a high response is on the basis of the comparison between holdup and clean noted, the cause should be investigated. If the collec -out recovery measurements. In small plants with less than tion zone contains an unexpectedly large content of ten collection zones, at least one zone should be updated uranium, that collection zone should be cleaned to remove during each physical inventory.
the accumulation for conversion to a more accurately accountable material category. After the cleanout has been To ensure that error predictions remain current, data completed, the zone should be reassayed and the assay from only the 12 preceding independent tests should be difference before and after cleanout should be compared used to estimate the assay uncertainty.
5.37-9
REFERENCES
.H. E. Preston and W. J. Symons, "The Determination 6. W. tliginbothaln, K. Zanio, and W. A. Kutagawa, of Residual Plutonium Masses in Gloveboxes by "CdTe Gamma Spectrometers for Nondestructive Remote Measurements Using Solid Therinolumi Analysis of Nuclear Fuels," IEEE Transactions on nescent Dosimeters," UKAEA-Winfrith Report No. Nuclear Science, Vol. 20, p. 510, 1972.
AAEW-R 1359, 1980.
7. H. B. Serreze et al., "Advances in CdTe Gamma Ray
2. A. Ohno and S. Matsuura, "Measurement of the Detectors," IEEE Transactions on Nuclear Science, Gamma Dose Rate Distribution in a Spent Fuel Vol. 21, p. 404, 1974.
Assembly with a Thermoluminescent Detector,"
Nuclear Technology, Vol. 47, p. 485, 1980. 8. M. de Carolis, T. Dragnev, and A. Waligura, "IAEA
Experience in the Development and Use of CdTe
3. T. D. Reilly, "Gamma Ray Measurements for Uranium Gamma Spectrometric Systems for Safeguards Enrichment Standards," in "Measurement Technology Application," IEEE Transactions on Nuclear Science, for Safeguards and Material Control," in Proceedings Vol. 23, p. 70, 1976.
of the ANS Topical Meeting held at Kiawah Island, S.C., November 1979, National Bureau of Standards 9. W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A
Special Publication 582, p. 103, June 1980. Method for Surveying for U-235 with Limit of Error Analysis," Nuclear Materials Management, Vol. 2,
4. R. B. Walton et al., "Measurements of UF Cylinders p. 395, 1973.
with Portable Instruments," Nuclear Technology, Vol. 21, p. 133, 1974. 10. J. L. Parker and T. D. Reilly, "Bulk Sample Self Attenuation Correction by Transmission Measure
5. R. H. Augustson and T. D. Reilly, "Fundamentals of ment," in Proceedingsof the ERDA X- and GammaRay Passive Nondestructive Assay of Fissionable Mate Symposium, Ann Arbor, Michigan (Conf 760639),
rial," Los Alamos Scientific Laboratory, LA-565 l-M, p. 219, May 1976.
1974; and its supplement, T. D. Reilly et al., "Funda mentals of Passive Nondestructive Assay of Fission 11. P. R. Bevington, Data Reduction and Error Analysis able Material: Laboratory Workbook," Los Alamos for the Physical Sciences, McGraw-Hill, New York, Scientific Laboratory, LA-565 1-M, 1975. 1969.
5.37-10
VALUE/IMPACT STATEMENT
1. PROPOSED ACTION
2. TECHNICAL APPROACH
1.1 Description and Need Not applicable.
Regulatory Guide 5.37 was published in August 1974.
3. PROCEDURAL APPROACH
The proposed action, a revision to this guide, is needed to bring the guide up to date with respect to advances in Of the procedural alternatives considered, revision of measurement methods and changes in terminology. the existing regulatory guide was selected as the most advantageous and cost effective.
1.2 Value/Impact Assessment 4. STATUTORY CONSIDERATIONS
1.2.1 NRC Operations 4.1 NRC Authority The regulatory positions will be brought up to date. Authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Energy
1.2.2 Other Government Agencies Reorganization Act of 1974, as amended, as implemented through the Commission's regulations.
Not applicable.
4.2 Need for NEPA Assessment
1.2.3 Industry The proposed action is not a major action that may Since industry is already applying the methods and significantly affect the quality of the human environment procedures discussed in the guide, updating these should and does not require an environmental impact statement.
have no adverse impact.
5. RELATIONSHIP TO OTHER EXISTING OR PROPOSED
1.2.4 Public REGULATIONS OR POLICIES
No adverse impact on the public can be foreseen. The proposed action is one of a series of revisions of existing regulatory guides on nondestructive assay techniques.
1.3 Decision on the Proposed Action
6. SUMMARY AND CONCLUSIONS
Revised guidance should be developed to reflect the improvement in measurement techniques and to bring the A revised guide should be prepared to bring Regulatory language into conformity with current usage. Guide 5.37 up to date.
5.37-11
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