Regulatory Guide 5.21

Nondestructive Uranium-235 Enrichment Assay by Gamma-Ray Spectrometry.

ML13064A082
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Issue date:	04/30/1974
From:	US Atomic Energy Commission (AEC)
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RG-5.021
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April 1974 U.S. ATOMIC ENERGY COMMISSION

REGULATORY

DIRECTORATE OF REGULATORY STANDARDS

GU I D E

REGULATORY GUIDE 5.21 NONDESTRUCTIVE URANIUM-235 ENRICHMENT ASSAY

BY GAMMA-RAY SPECTROMETRY

A. INTRODUCTION

energy and consequent low penetrating power of these gamma rays implies that most of those emitted within Section 70.51, "Material Balance, Inventory, and the interior of the material are absorbed within the Records Requirements," of 10 CFR Part 70, "Special material itself. These thick ' materials therefore exhibit Nuclear Material," requires, in part, that licensees a 185.7-keV gamma ray activity which approximates the authorized to possess at any one time more than one activity characteristic of an infinite medium: i.e., the effective kilogram of special nuclear material (SNM) activity does not depend on the size or dimensions of determine the material unaccounted for (MUF) and its the .material. Under these conditions, the 185.7-keV

associated limit of error (LEMUF) for each element and activity is directly proportional to the U-235 the fissile isotope for uranium contained in material in enrichment. A measurement of this 185.7-keV activity process. Such a determination is to be based on with a suitable detector forms the basis for an measurements of the quantity of the element and of the enrichment measurement technique.

fissile isotope folr uranium.

The thickness of the material with respect to the mean free path of the 185.7-keV gamma ray is the The majority of measurement techniques used in SNM accountability are specific to either the element or primary characteristic which determines the applicability of passive gamma-ray spectrometry for the measurement the isotope but not to both. A combination of of isotope enrichment. The enrichment technique is techniques is therefore required to determine the MUF

and LEMUF by element and by fissile isotope for applicable only if the material is thick. However, in addition to the thickness of the material, other

'uranium. Passive gamma-ray spectrometry is a conditions must be satisfied before the gamma-ray nondestructive ýmethod for measuring the enricdment, or enrichment technique can be accurately applied. An relative concentration, of the fihuile isotope U-235- in approximate analytical expression for the detected uranium. As such, this technique is used in conjunction

185.7-keV activity is given below. This expression has with an assay for the element uranium in order to been separated into several individual terms in order to determine the amount of U-235.

aid in identifying those parameters which may interfere with the measurement. Although approximate, 'this This guide details conditions for an acceptable relationship can be used to estimate the magnitude of U-235 enrichment measurement using gamma-ray interfering effects in order to establish limits on the spectrometry, and prescribes procedures for operation, range of applicability and to determine the associated calibration, error analysis, and measurement control. uncertainties introduced into the measurement. This relationship is:

. DISCUSSION

'Thick" and -thin" am used throughout this guide to refer to distances in relation to the mean free path of the 185.7 The alpha decay of U-235 to Th-231 is accompanied keV gammn ray in the material under consideration. The mean by the emission of a prominent gamma ray at 185.7 keV free path is the I/e-folding distance of the gamma-ray flux or, in

(4.3 x 104 of these 185.7-keV gamma rays are emitted other terms,'.the average distance a gamma ray traverses before per second per gram of U.235). The relatively low interacting.

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0. General

Effective source of 185.7.-ceV

pmrm rays men by the detector C E (a/tu) A [I + e (fa/4ir) e-PcIcd

(1)

enrichmftnt detecor container efecien/c absorption Physical are material geometrical constants defined by composition efficiency collimator where C = detected 185.7-keV activity E = enrichment of the uranium (. -1)

Pu,pi,pc = density of the uranium (u), matrix material (i), and container wall

3 (c), respectively, in (g/cm )

AuAi, Ac = mass attenuation coefficient for 185.7-keV gamma rays in uranium (u), matrix material (i), and container wall (c) in units of (cm 2 /g)

a = specific 185.7-keV gamma ray activity of U-235

= 4.3 x 104 gamma rays/sec-g e = net absolute detector full energy peak efficiency for detecting

185.7-keV gamma rays (< 1)

E2 = solid angle subtended by the detector (11 < 2w)

A = cross-sectional area of material defined by the detector collimator d = container wall thickness A derivation of this expression, as well as other Calculated values of xc, the critical distance, for .

necessary background information relevant to this guide, several common materials are givn in Table 1.

may be found in the literature. 2 As evident in Eq. 1, the activity (C) is proportional to the enrichment (E)

'but is affected by several other characteristics as well. TABLE 13 Material Thicknm Effects Material Density Critical Material (g/cm 3 ) Distance Composition In order for Eq. 1 to be applicable, it is necessary xO lcm) 'Term that the material be sufficiently thick to produce strong Pi tai attenuation of 185.7-keV gamma rays. To determine .1 + 2:-

i Pu Mu whether this criterion is met, it is useful to compare the actual thickness of the material with a characteristic length xo, where xo is defined as that thickness of material which produces 99.5% of the measured U (metal) 18.7 0.20 1.000

185.7-keV activity, i.e., UF 6 4.7 1.08 1.040

U0 2 10.9 0.37 1,012 X0 I n(.005) = 5.29 X U3 08 7.3 0.56 1.015

(2)

Uranyl Nitrate 2.8 2.30 1.095 where IA = u.u + 7- plip (3)

Values of the mass attenuation coefficient, A, may be

2 L. A. Kull, "Guldejiws for Gamm&-gray Spectroscopy found in J. H. Hubbell, "Photon Cross Sections, Atteniation Coefficients, and Energy Absorption Coefficents From 10 keV

Measuremente of U-235 Enrichment," BNL-50414, July 1973. to 100 GeV," NSRDS-NBS 29, 1969.

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Note: Other nondestructive, techniques are capable of detector. The fractional change in the measured activity detecting SNM distributed within. a container. The AC/C due to a small change Ad in the container wall enrichment technique, however, is inherently a surface thickness can be expressed as follows:

measurement. Therefore, the "sample" observed-i.e., the surface, must be representative of all the material in the AC-- -ZcPcAd

.= (5$)

container. In this respect the enrichment mesurement is more analogous to chemical analysis than other NDA

techniques.

Calculated values of AC/C, corresponding to a Material Composition Effeb change in container thickness Ad of 0.0025 cm, for If the gamma-ray measurement is to be dependent common container materials, are given in Table 2.

only on the enrichment, the term related to -the composition of the matrix should be approximately equal to one, i.e.,

TABLE 2

+ pi'L C. I; Material Density

(4)

li P A- (g/cm 3 l C

Steel 7.8 - .003 Calculted values. of this quantity for common Aluminum 2.7 - .0009 materials are given in Table 1. The deviation of the Polyethylene 0.95 - .0004 numbers in Table I from unity indicate that a bias can'

be introduced by ignoring the difference in material composition.

Therefore, the container wall thickness should be Inhomogeneities in matrix material composition, known, e.g., by measuring an adequate number of the uranium density, and uranium enridunent within the containers before loading. In some cases an unknown measured volume of the maierial (as chariterized by container wall thickness can be measured using an the depth xo and the collimated area A) can produce ultrasonic technique and a simple correction applied to changes in the measured 185.7-keV activity and-affect the data to account for attenuation of the 185.7-keV

v-the accuracy of an enrichment calculated on the bais of gamma rays (see eq. 5). Commercial equipment is that activity. There is a small to negligble effect on the available to measure wall thicknesses ranging from about measurement accuracy due to variations in the content 0.025 to 5.0 cm to relative accuracies of approximately of low-atomic-number (Z<30) matrix materials. Care 1.0% to 0.1%, respectively.

should be exercised, however, in applyin this technique to materials having.high-atomro-number matria" (Z>50) Area and Geometrical Efficiency or materials having uranium concentuations 1. than approximately 75%. Inhomogeneities in uraium density The area of the material viewed by the detector and will also produce small to negligible effects on the the geometrical efficiency are variables which may be accuracy if the matrix isu of low-atomic-number adjusted, within limits, to optimize a system. It is elements. Sifjkuw inacraeieas cn. a.Ni, howem, important to be aware that once these variables are when the urnium enrichment itself ce. be expected to fixed, changes in these parameters will affect the results vary throughout the sample. of the measurement.

The above ,gonclusions about the effects of It is also important to note that the placement of inhomogeneities are based on the assumption that the the material within the container will affect the detected thickness of the material exceeds the critical distance, activity. The 'material should fill the volume of the xo, and that the inhomogeneities exist within this depth. container to a certain depth, leaving no void spaces In the case of extremely inhomogeneous materiah much between the material and the container wall.

as scrap, the condition of sufficient depth may not always be fulfllled,-or inhomogeneitiesmay exist beyond Net Deteetw Bffidncy the depth xo; i.e., the "sample" is not representative.

Therefore, this technique is not applicable to such Thallium-activated sodium iodide, NaI(T1),

inhomogeneous materials. scintillationw detectors and lithium-drifted germanium, Ge(LI), solid-state detectors have been used to perform Container Wafl Effects these measurements. The detection systems are generally conventional gamma-ray spectrometry systems presently Variations in the thickness of the container walls commercially available in modular or single-unit

-can significantly affect the activity measured by the construction.

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The following factors influence detector selection live-time s intervals. The pile-up or overlap of electronic and the control required for accurate results. pulses is a problem which also results in a loss of counts in the full-energy peak for Ge(Li) systems. A pulser may be used to monitor and correct for these losses.

1. Background Radiation which provides, no useful -information can be selectively attenuated by filters; e.g., a one-millimeter- a. Compton Background. This background is thick cadmium filter will reduce x-ray interference, predominately produced by'the 765-keV and ICOl-keV eliminating this source of count-rate losses.

gamma rays of Pa-234m, a daughter of U-238. Since, in most cases, the Compton background behaves smoothly 3. Instability in Detector Electronics. The gain of a in the vicinity of the 185.7-keV peak, it can be readily photomultiplier tube is sensitive to changes in subtracted, leaving only the net counts in the 185.7-keV temperature, count rate, and magnetic field. Provision full-energy peak. can be made for gain checks and/or gain stabilization for enrichment measurement applications. Various gain stabilizers that automatically adjust the system gain to b. Overlapping Peaks. The observable peak from keep a reference peak centered between two preset certain gamma rays may overlap that of the 185.7-keV energy limits are available.

peak due to the finite energy resolution of the detector;

i.e., the difference in energies may be less than twice the

C. REGULATORY POSITION

FWHM. This problem is common in enrichment measurements of recently separated uranium from a Passive gamma-ray spectrometry constitutes an reprocessing plant. The peak from a strong 208-keV acceptable means for nondestructively determining gamma ray from U-237 (half-life of 6.75 days)- can U-235 enrichment, if the following conditions are overlap the 185.7-keV peak when an Nal detector is satisfied:

used. Analytical separation of the two unresolved peaks, i.e., peak stripping, may be applied. An alternative Range of Application solution is to use a Ge(Li) detector so that both peaks are clearly resolved. 1. All material to be assayed under a certain calibration should be of similar chemical form, physical The U-237 activity ;present in reprocessed form, homogeneity, and impurity level.

uranium will depend on the amount of Pu-241 present before reprocessing and also on the time elapsed since 2. The critical distance of the material should be separation. determined.. Only those items of the material having dimensions greater than -this critical distance should- be assayed by this technique.

c. Ambient Background. The third source of background originates from natural sources and from 3. The material should be homogeneous in all respects other uranium-bearing materials located in the vicinity on a mnacroscopic 6 scale.- The material should be of the measuring apparatus. This last source can be homogeneous'with respect to uranium enrichment' on a particularly bothersome since it can vary with time microscopic -wscale.

within wide limits depending on plot operating conditions. 4. The containers should all be of similar size, geometry, and physical and chemical composition.

2. Count-Rate LoAmes. Calculation of the detector System Requirements count rates for purposes of making dead time estimates requires that one calculate the total count rate, not only I. Nal('I) scintillation detectors having a resolution of that due to U-235. Total count rate estimates for FWHM < 16% at the 185.7-keV peak of' U-235 are low-enrichment material must therefore take into account the relatively important background from U-238 gamma rays. If other radioactive materials are s"Live time" means that portion of the measurement present within the sample, their contributions to the period during which the instrument can record detected events.

total count rate must also be considered. Dead time refers to that portion of the measurement period during which the instrument is busy processing data already recehed anldcannot accept new data. in order to compare Count-rate corrections can be made by determining 6fferent data for which dead times are appreciable, one must the dead time or by making measurements for known compare counts measured for equal live-time periods.

(actual measurement period) - (dead time) = live ,time

4 FWHM- full width of the spectrum peak at half its 6 Macroscopic refers to distances greater than the critical maximum height. distance; miuoscopic to distances les than the critical distance.

5.21-4

generally adequate for measuring the enrichment of neighboring peaks, and to optimize the system stability uranium containing more than the natural (0.71%) and the signal-to-background ratio.

abundance of U-235. Crystals With a thickness of ~-1.25 cm are recommended for optimum efficiency. If other 3. The net response attributed to 185.7-keV gamma

-1- radionuclides Which emit significant quantities of gamma rays should be the accumulated counts in the peak radiation in an energy region E = 185.7 keV +/- 2 FWHM region minus a multiple of the counts accumulated in a at 185.7 keV are present: nearby background region(s). A single upper background region may be monitored or both a region above the a. A higher-resolution detector. e.g., Ge(Li), peak region and one below may be monitored.

should be used, or If only an upper background region is monitored, the b. A peak stripping procedure should be used to net response, R, should be given by subtract the interference. In this case, data should be provided to. show the range of concentration of -the R = G-bB

interfering radionuclide, and the accuracy and precision of the stripping technique over this range. where G and B are the gross counts in the peak region and the background region, respectively, and b is the

2. The detection system gain should be stabilized by multiple of the background to be subtracted. This net monitoring a known reference peak. response, R, should then be proportional to the enrichment, E, given by

3. The system should measure live time or provide a means of determining the count-rate losses based on the E = C, R = C, (G-bB)

total counting rate.

where C, is a calibration constant to be determined (see

4. Design of the system should allow reproducible Calibration, next section). The gross counts, G and B,

positioning of the detector or item being assayed.. should be measured for all the standards. The quantities G/E should then be plotted as a function of the

5. The system should be capable of determining the quantities B/E and the slope of a straight line through gamma-ray activity in at least two energy regions to the data determined. This slope is b, the multiple of the allow background subtraction. One region should upper background region to be subtracted, i.e..

encompass 185.7 keV, and the other region should be above this but not overlapping. The threshold and width G/E = b(B/E) + I/CI

of the regions should be adjustable.

The data from all the standards should be used in

6. The ýsystem should have provisions for filtering determining this slope.

low-energy radiation which could interfere with the

185.7-keV or background regions. If both an upper and a lower background are monitored, the counts in each of these regions should be Data Reduction used to determine a straight line fit to the background.

Using this straight line approximation, the area or I. if the total counting rate is determined primarily by number of counts under this line in the peak region the 185.7-keV gamma ray, the counting rate should be should be subtracted from the gross counts, G. to obtain restricted (absorbers, decreased geometrical efficiency) the net response. An adequate technique based on this below those rates requiring correction. The system principle is described in the literature.

sensitivity will be reduced by these measures and, if no longer adequate,' separate calibrations should be made in Calibration s two or more enrichment regions.

1. Calibration standards should be obtained by:

Ifrthe total counting rate is determined primarily by events other than those due to 185.7-keV gamma rays, a. Selecting items from the production material. A

counting rate corrections should be made. group of the items selected should, after determination G. Gunderson, 1. Cohen, M. Zucker, "Proceedings: 13th

2. To determine the location and width of the Annual Meeting, Institute of Nuclear Materials Management,"

185.7-keV peak region and the background region(s), Boston, Mass. (1972) p. 221.

the energy spectrum from each calibration standard (see Calibration, next section) should be determined and the " None of the calibration techniques or data reduction position of the 185.7-keV peak and neighboring peaks procedures exclude the use of automated direct-readout systems for operation. The procedures described in this guide should be noted. The threshold and width of each energy region used for adjustment and calibration of direct-readout should then be selected to avoid including any instruments.

5.21-5

of the gamma-ray response, be measured by an 5. All containers should be agitated, or the material independent, more accurate technique traceable to, or mixed in some manner, if possible, prior to counting.

calibrated with, NBS standard reference material, e.g., One container from every ten should be measured at two mass spectrometry. The other items should be retained different locations. Other items may be measured at as working standards. only one position. (If containers am scanned to obtain an average -enrichment, the degree of inhomogeneity b. Fabricating standards which represent the should still be measured by this method.)

material to be assyed in chemical form, physical form, homogeneity, and impurity level. TheU-235 enrichment The difference between the measurements at of the material used in the fabrication of the standards different locations should be used to indicate a lack of should be determined by a technique traceable to, or the expected homogeneity. If the two responses differ calibrated with, NBS standard reference material, e.g., by more than three times the expected standard mass spectrometry. deviation (which should include the effects of the usual or expected inhomogeneity), repeat measurements

2. The containers for the standards should have a should be made to verify that an abnormal geometry, dimensions, and composition which inhomogeneity exists. If the threshold is exceeded, the approximate the mean of these parameters in the container should be rejected and investigated to containers to be assayed. determine the cause of the abnormal inhomogeneity. 9

3. The values of enrichment for the calibration 6. In the event that all containers are not filled to a standards should span the range of values encountered in uniform height, the container should be viewed at a normal operation. No less than three separate standards position such that material fills the entire volume viewed should be used. by the detector. The procedure for determining the fill of the container should be recorded' e.g., by visual

4. Each standard should be measured at a number of inspection at the time of filling and recording on the different locations, e.g., for a cylinder, at different container tag.

heights and rotations about the axis. The mean of these values should be used as the response for that 7. The container wall thickness should be measured.

enrichment. The dispersion in these values should be The wall thickness and location of the measurement used as an initial estimate of the error due to material should be indicated, if individual wall thickness and container inhomogeneity. measurements are made, and the gamma-ray measurement made at this location. If the containers are

5. The data from the standards, i.e., the net response nominally identical, an adequate sampling of these Il attributed to 185.7-keV gamma rays and the known containers should be representative. The mean of the uranium enrichment, should be used to determine the measurements on these samples constitutes an constants in a calibration function by a weighted acceptable measured value of the wall thickness which least-squares technique. may be applied to all containers of this type or category.

Operations 8. The energy spectrum from a process item selected at random should be used to determine the existence of

1. The detection system and counting onometry unexpected interfering radiations and the approximate (collimator and container-to-detector distance) should magnitude of the interference. The frequency of this test be identical to those used in calibration. should be determined by the following guidelines:

2. The data reduction technique and count-rate loss a. At leat one item in any new batch of material.

corrections, if included, should be identical to those b. At ieast one item if any chanps in the material used in calibration. procesing occur.

c. At least one item per material balance period.

3. Data from all measurements should be recorded in an appropriate log book. If an interference appears, either a higher-resolution detector must be acquired or an adequate peak stripping

4. At least two working standards, should be measured routine applied. In both cases additional standards which during each eight-hour operating shift. The measured include the interfering radiations should be selected and response should beý compared to the expected response the system recalibrated.

(value used in calibration) to determine if the difference exceeds three times the expected standard deviation. If this threshold is exceeded, repeat measurements should be made to verify that the response is significantly The difference nmy also be due to a large variation in wall different and that the system should be recalibrated. thickness.

5.21-6

9. No item should be assayed if the mesured response 3. The item-to-item error due to the uncertainty in exceeds that of the highest enrichment standard by more wall thickness should be determined. The uncertainty in than tvice the standard deviation in the reponse from the wall thickness may be the standard deviation about this standard. the mean computed from measurements on randomly selected samples, or it may be the uncertainty in the Error Anysis thickness measurement of individual containers. This uncertainty in wall thickness should be multiplied by the I. A least4quares technique should be used to effect of a unit variation in wall thickness on the determine the uncertainty in the calibration constants. measured 185.7-keV response to determine this component uncertainty.

2. The measurement.to-measurement error should be determined by periodically observing the net response 4. Item-to-item errors other that those measured, e.g.,

from the standards and repeating measurements on wall thickness, should be determined by periodically (see selected process items. Each repeat measurement should guidelines in paragraph 8. of the Operation Section)

be made at a different location on the container surface, selecting an item and determining the enrichment by an at different times of the day, and under differing independent technique traceable to, or calibrated with, ambient conditions.' "The standard deviation should be NBS standard reference material. A recommended determined and any systematic trends corrected for. approach is to adequately sample and determine the U-235 enrichment by calibrated mass spectrometry. In addition to estimating the limit of error from these

' The statistical error due to counting (Including backipound) and the erron due to inhomopamsity, ambient comparative measurements, the data should be added to conditions, etc. will be include in this measurement- the data used in the original calibration and new to-measurement error. calibration constants determined.

5.21-7