Regulatory Guide 5.23

in Situ Assay of Plutonium Residual Holdup

ML13064A072
Person / Time
Issue date:	05/31/1974
From:	US Atomic Energy Commission (AEC)
To:
References
RG-5.023, Rev 1
Download: ML13064A072 (16)

May 1974 U.S. ATOMIC ENERGY COMMISSION

REGULAT(OR Y G U I D[E

'DIRECTORATE OF REGULATORY STANDARDS

REGULATORY GUIDE 5.23 IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP

A. INTRODUCTION

2. When the limit of error of Pu holdup is not compatible with constraints on the overall LEMUF, the Part 70, "Special Nuclear Material," of Title 10 of information obtained in the holdup survey can be used the Code of Federal Regulations requires licensees to locate principal Pu accumulations and to assure that authorized to possess more than one kilogram of other areas of the process contain less than the detectable plutornium to calculate a material balance based on a amount of plutonium. Once located, substantial accu- measured physical inventory at intervals not to exceed mulations can be recovered, transforming the plutonium two months. Further, these licensees are required to to a more accurately measurable inventory component.

conduct their nuclear material physical inventories in Having reduced the amount of plutonium holdup, the compliance with specific requirements set forth in Part limit of error on the remeasurement of the remaining

70. Inventory procedures acceptable to the Regulatory holdup may be sufficiently reduced to be compatible staff are detailed in Regulatory Guide 5.13, "Conduct of with overall LEMUF requirements.

Nuclear Material Physical Inventories."

B. DISCUSSION

Plutonium residual holdup is defined as the

,* plutonium inventory component remaining in and about Plutonium accumulates in cracks, pores, and zones process equipment and handling areas after those of poor circulation within process equipment. The walls collection areas have been prepared for inventory. of process vessels and associated plumbing often become Whenever possible, process equipment should be coated with plutonium during solution processing.

designed* and operated so as to minimize the amount of Surfaces internal and adjacent to process equipment, holdup. In this guide, procedures are detailed for the in especially glove box walls and floors, accumulate situ assay of the residual plutonium holdup. deposits of plutonium which can become appreciable.

Plutonium also accumulates in air filters and associated Assay information can be used in one of two ways: ductwork. The absolute amounts of plutonium holdup must be small for efficient processing and proper hazards I. When the limit of error of plutonium holdup is control. However, the total amount of plutonium compatible with constraints on the overall limit of error holdup may be significant in the context of the tolerable on the facility MUF (LEMUF), the material balance can facility MUF.

be computed using the measured contents of Pu holdup.

Additional cleanout and recovery for accountability will The measurement procedures detailed in this guide then not be necessary. are based on the controlled observation of gamma rays and neutrons which are spontaneously emitted by the

"Design features to minimize holdup in process equipment are plutonium isotopes. Because the gamma rays of interest the subject of a seriý of rgulatory guides. are emitted by Pu-239, garnma ray assay is the preferred Copies of published guides may be obtained by rsquast indicating the divisions USAEý REGULATORY GUIDES dosircd to the US. Atomic Enrgty Commission, Washington, D.C. 2054'.

Attention: Director of Regulatory Standards. Comments and suggestions for Regulatory Guides we issued to describe and make avaiille to the public inmprovements in thes guides ere encouraged and should be sent to the Secretary methods acceptable to the AEC Regulatory staff of implementing specific parts of of the Commission, U.S. Atomic Energy Commission. Washington. D.C. 20645.

the Commission's regulations, to delineate techniques .-.ed by the staff in Attention: Chief. Public Promedinga Staff.

evaluating specific problems or postulated accidents: or to provlde guidance to epplicents. Regulatory Guides we not substitutes for regulations arnd comoliancs The guidas ea issued in the following ton broad divisions:

with them is not required. Methods and solutions different from those sit out in the guides will be acceptable if they provide a basls for the findings requisot to S. Produects

1. Power eactors the issuanc or continuance of a pearmil or licemniby the Comnission. 2. Resmrch and Test Reactors

7. Transportation

3. Fuels and Materials Facilities S. Occupational l'slooh

• Published guidet will be revised periodically, as appropriate, to accommodatei 4. Envwonnmental and Siting 9. Antitrust Revow S. Materials and Plant Protection 10. General comments end to reflict new information or experience.

assay method whenever its acceptance criteria are system with sufficient resolution to measure the activity satisfied. To accomplish either gamma ray, or neutron from one or'two~isolopes o-Thinterest.

assay, it is essential to consider the facility in terms of a Gamma ray assay has an ' advantage , over series of zones which can be independently assayed.

neutron assay in that the emissions are primarily from Such zones are designated as "collection zones."

the principal isotopes qf linterest. -Because of the high emission rate of gammna rays, a detection sensitivity of

1. Delineation of Collection Zones less than one gram is generally attainable..

Typical plutonium process facilities comprise a The most useful portion of the spec trum for number of interconnected glove boxes which contain holdup assay is the Pu-239 gamma ray complex in the work areas and most process equipment, in-process 375-440 keV range. The-yields of these lines are given in storage areas, and self-contained process equipment. Table B.l.

Also, solution processing requires tanks, plumbing, and Table B.1 pumping equipment, which are often located in close proximity to.the glove box lines. Finally, storage areas PROMINENT GAMMA',RAYS FROM Pu-239 in for feed, scrap and waste, and final product are also ENERGY RANGE 375-440 keV

often located in close proximity to the plutonium process area. Energy Intensity (- /sec-g Pu-239)

Each facility can be divided into a series of 375.0 ........................ 3.59 x J04.l collection zones on the basis of a logical understanding 380.2 ...................... 0.70 x 10

of process activities. Individual glove boxes can be 382.7 ....................... 0.59 x 104 subzoned to improve assay performance, but for most 392 ..5 ................ ...... 0.26 x,104 applications, individual glove boxes are -examples of 393.1 .......... .... ..... 1.01 x104 .

suitable size areas for discrete collection zones. 413.7 ...................... 3.43 x I04

422.6 ............... ..0.27 x 104 Gamma ray assay for plutonium holdup measurement is practical when a collection zone consists Total 9.85 x 104 of a single structure of relatively uniform cross section.

When a collection zone contains an item of equipment 2.1.1 -'Gamma Ray Detection Instruments.

having significant shielding properties and capable of contributing to the holdup, the uncertainty in the Gamma, ray detection-systems consist of a holdup prediction based on the observed response may scintillation or -semiconductor detector sensitive to become primarily due to attenuating the radiations in gamma rays and . appropriate -.electronics. 3 Required the internal structure. In such cases, neutron assay is electronics include lat least a single-ýchannel analyzer and applicable. a timer-scaler unit.- A second :single- channel analyzer used to determine the background radiation correction is

2. Applicable Methods and Instruments a time-saving feature. Battery powered systems are commercially. available and can provide operational Two ,considerations are critical to the selection of convenience, particularly in this application.

methods and instruments. First, to perform an assay, the plutonium radiations must reach the detector, and be The detection efficiency and res6lution of detected. Second, the observed response must be good Nal(Tl) detectors is'generally adequate for this attributable to the collection zone being assayed. application. CdTe, Ge(L), and-intrinsic 'Ge:detectors Therefore, the assay scheme is developed around have better resolution than Nal(TI) but: cost more, are penetrating radiations and the detector is collimated to generally less available, and are more difficult to operate.

provide for sufficient directionality in the response to resolve a collection zone from its neighbor zones and -' - The 332.3 keV- gamma-ray from U-237, a from the background. short-lived (6.75 d) daughter -of Pu-241, is usually the principal interference for. Pu-239 assay by Nal detection

2.1 Gamma Ray Assay of the 375-440 keV complex. If the U-237 is in equilibrium with Pu-241, the intensity of this gamma ray Under closely controlled conditions, the is 1.15 x 106 7t/sec-g Pu 124l.

measured plutonium gamma ray spectrum can be interpreted in terms of the abundance of each gamma Since this gamma ray is also emitted inthe ray emitter present in the sample. Because of the large decay of Am-241., the. interference from this decay number of gamma rays', 2 present, many regions of the branch may also be important in case -of preferential observed spectrum are characterized by overlapping americium holdups. To avoid this interference when lines. To accomplish the assay, it is necessary to select an using Nal detectors, the assay-energy window is adjusted appropriate spectral region and provide a detection to span the range from 390 to 440 keV.

5.23-2

Detector dimensions are selected to 2.1.4 Calibration Source for Gamma Ray provide a high probability for detecting the appropriate Assay gamma rays. The geometric detection efficiency increases as the square of the detector radius; however, To calibrate a collection zone, the observed the weight of the gamma ray shielding material required assay -response is compared to the response obtained to collimate the detector also increases ;when larger when the zone contains a known amount of plutonium.

detectors are used. The crystal depth is chosen such that most of the gamma rays of interest will lose all their Because of the complexity of the assay, the energy within the crystal; response is assumed to be linear. To be representative of typical holdup situations, the calibration standard is To reduce the pile-up of low energy prepared as an encapsulated disk with a bed thickness of radiations, the crystal face can be covered with an less than 0.2 cm. Care must be exercised in the appropriate shield (e.g., 0.075 cm cadmium). This preparation of the calibration standard to ensure that procedure will reduce counter dead time effects without the amount encapsulated of total plutonium, Pu-239, significantly affecting assay results. and the amount of Amn-241, is known. It is important to measure the gamma ray attenuation 'through the

2.1.2 Collimators for Gamma Rays encapsulating material and correct the calibration standard response to compensate for that attenuation.

A shaped shield constructed of any dense The amount of plutonium encapsulated in 'the gamma material is appropriate for gamma ray collimation. For ray calibration standard is selected to be representative cost, availability, and ease of fabrication, lead is of typical accumulations.

recommended. Less ,than 2% of all 400 keV gamma rays striking a 1.5-cm-thick sheet of lead will pass through 2.2 Neutron Assay without having suffered an energy loss.

Neutrons are emitted in the spontaneous fission The collimator will be most effective when of Pu-238, Pu-240, and Pu-242 and through the it is concentric about the crystal and photomultiplier interaction of emitted a particles with certain light and completely covers the photomultiplier base. nuclei. These neutrons suffer little attenuation in passing Extending the collimator forward of the crystal at least a through uranium or plutonium or through most distance equal to half the diameter of the crystal, and structural and containment materials. Glove box preferably the full diameter, is recommended. 4 Making windows may reduce the energy of emerging neutrons, this distance variable to reproducible settings will permit but because of their regular and constant shape, their adjustment over a range of collection zone sizes. effect can generally be factored into the assay calibration.

2.1.3 Check Source for Gamma Ray Assay To be useful for the assay of plutonium holdup, It is important to check the operation of the neutron production rate per gram of plutonium must the detection system prior to each inventory sequence.

be known. The spontaneous fission contribution to the Either recalibrating one or more collection zones and total neutron production can be computed from basic comparing the results'to previous analyses or testing the nuclear data, once the isotopic composition of the instrument with an appropriate check source is contained plutonium has been determined. Computing appropriate. When the performance remains within the the (an) contribution requires a knowledge of the expected value,, the previous calibration data are chemical form of the plutonium and the amount and, assumed to be valid. If not, theenergy window may have distribution of certain high (an) yield target materials.

shifted, or the unit may be in need of repair and recalibration.

The background count rate from neutron detectors may be a substantial part of the observed An appropriate check source enables the activity, often corresponding to as much as 20 g of stability of the assay instrument to be tested at any plutonium in typical holdup assays. Thus, neutron assay location. Such a source can be prepared by implanting a is primarily applicable to the measurement of significant small encapsulated plutonium source (containing -0.5 g accumulations of plutonium.

Pu) in the face of a plug of shielding material. The plug is shaped to fit and close the collimator channel, and the source is positioned to be adjacent to the crystal when The measured neutron yield from prepared the plug is in place. calibration standards is used to calibrate each neutron assay collection zone. In the Appendix, a method is The check source is fabricated in a manner given to calculate the anticipated neutron yield. This to ensure its internal stability. Other than .radiations method provides the ability to calculate the neutron increasing from the ingrowth of Am-241, the emission yield when the isotopic or impurity composition of the rate of the check source should remain constant. plutonium holdup is different from that of the

5.23-3

calibration standards. The method can be used to by stopping neutrons coming to the detector from all calculate a ratio of the neutron production rate of the directions --except the . -.desired one. The cadmium unknown material to the standard material neutron surrounding the detector will...stop essentially all production rate. The yield from the holdup material is neutrons striking, the, detector with energies below 0.4 then determined by multiplying the measured "known" eV. By adding moderator material around the.outside of material yield by the computed ratio. the,.<detector in -all -directions except .for the collimator channel, neutrons, coming from unwanted directions will

2.2.1 Neutron Detection Instruments lose energy~in 'this shield and will be absorbed in the Cd cover. For each six inches of polyethylene added, the To effectively employ the spontaneous collimator assembly provides a factor of approximately neutron yield as a measure of plutonium holdup, it is ten: in -the directionality of the response. An example of necessary to detect the neutrons in the vresence of a a collimated ,neutron detector assembly for plutonium more intense gamma ray background and to collimate holdup assay is shownin Figure B-I.

the detector so that the only neutrons being counted are emanating from the collection zone under assay. The weight of the .combined detector and collimator assembly. can easily exceed requirements for a

_,Holdup assay -is performed under in-plant hand-held detector probe. 4 For this reason, and to conditions where ruggedness, high detection efficiency, provide for reproducible positioning at each assay, a and high (-y,n) rejection, performance in the detectors is sturdy cart housing both the detector/collimator and the important. He-3 has one advantage over BF 3 detector* associated-'electronics is recommended; Further, as the tubes in that the operating voltage for He-3' tubes does items to be assayed will be at different, heights, the not increase as rapidly with increased gas pressure. ability to raise .and lower. the assembly to reproducible settings is ,recommended to expedite the assay and To increase the efficiency of the system, reduce the possibility of errors.

detector gas pressure in the tubes may be increased or multiple detectors can be connected in parallel to feed a 2.2.3 Check Source for Neutron Assay common preamplifier.

To ensure the proper operation of the He-3 and BF3 detectors have efficiencies neutron assay system prior to making an assay, it is which increase as 'the energy of the neutrons decrease. necessary to test the response of the instrument. An To take advantage of this characteristic, the detectors appropriate , neutron . assay check source can be can be surrounded by a 'neutron moderating material measured, Por one or more :collection zones can be (see Figure B1I). Polyethylene is recommended. The recalibrated and compared to the results of previous thickness of the moderator is important. When the calibrations.

moderating distance is short, a fraction of the higher energy neutrons pass through the gas chamber without An appropriate neutron assay check source being detected. Conversely, when the moderating can be prepared by implanting a small encapsulated distance is too long, a substantial number of low-energy plutonium source (containing about 5 g Pu).into the face neutrons are absorbed by the hydrogen contained in the of a plug. of neutron moderating material .(see Figure moderator. A balance -between these, two effects is B-2). The plug is .fabricated to fit and close the reached when -the spacing between adjacent tubes is collimator channel.

approximately one-inch of polyethylene, and the -spacing between the front of the unit and the detectors and the - 2.2.4 Calibration Source for, Neutron Assay back of the unit and the detectors is approximately I1%

inch when one-inch-diameter tubes are used, and To calibrate a neutron assay collection approximately one inch when two-inch-diameter tubes zone, the observed response is compared to the response are used. obtained when the zone contains an additional known amount of plutonium. Neutron assay is less sensitive to

'To -shield the detector, from low-energy attenuation than.is.,gamma ray assay.. It is important to neutrons which may produce a complicated response know how:..much plutonium is dencapsulated in the pattern, the moderator material is covered .with a neutron assay calibration standard, and the isotopic thermal neutron absorber. Cadmium sheeting approxi- composition of that plutonium.

mately .0.075 cm thick can be used for this application.

The spontaneous neutron production rate

2.2.2 Collimators for Neutron Detectors from typical reactor plutonium is significantly less than the production rate of 375440 keV gamma rays. To To assay a specific collection zone in the provide. ,.an adequate response for calibration, it is presence of other distributed sources of plutonium, it is therefore necessary, to encapsulate a larger amount of necessary to collimate the detector. This is accomplished plutonium. in the neutron assay calibration standard.

5.23-4

COLLIMATED NEUTRON DETECTOR ASSEMBLY FOR PLUTONIUM HOLDUP ASSAY

DETECTOR CABLE ACCESS CHANNEL

(TOP SECTION ONLY) --.-

-T-

17.5cm

- 5-1cm 1cmI " I

I

TOP VIEW

FRONT VIEW

4.5cm

.. .. . \NEUTRO N DETECTOR

TUBE C HANNELS

I I 2.6cm DIA METERý ITYP)

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I .1 FRONT VIEW

¢r 39cm I I

68 I I II

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I II -I

I I I " FIGURE B-1 I I lIl, I I 11 I II 1 14.l POLYETHYLENE BLOCK, COVERED ALL SIDES

'WITH 0.0756m CADMIUM SHEET

DETECTOR TUBE SUBASSEMBLY

NEUTRON DETECTOR/COLLIMATOR ASSEMBLY. ASSEMBLY INCLUDES THREE BF3 OR He-3 TUBES

(2.54cm DIAMETER) UNIT CAN BE MODIFIED TO INCREASE ORDECREASE THE NUMBER OF TUBES.

MODERATOR THICKNESS OF 15cm PROVIDES,10:? DIRECTIONALITY. ADDITIONAL POLYETHYLENE

CAN BE ADDED TO IMPROVE DIRECTIONALITY Io.p., 30*m POLYETHYLENE PROVIDES~100:1 DIRECTIONALITY). COMPONENTS ARE BOLTED OR STRAPPED TO REMAIN IN A FIXED CONFIGURATION.

5.23-5

MODERATOR\ NEUTRON COLLIMATOR to the same geometry as found in the neutron assay

/CHANNEL PLUG calibration standard. Each test sample is transferred to an empty glove box and positioned next to the window for measurement. The neutron assay probe is positioned as close as possible to the sample but outside the glove box. After the measurement is made, that sample is transferred from the glove box and the next sample is

1 CHECK SOURCE transferred in and positioned in the identical location for measurement. A plot of counts minus background as a TOP' VIEW

function of PuO2 mass is made and the points visually fitted using a French curve. If there is no multiplication, a straight line can be drawn through the. origin connecting all points. Multiplication is indicated when the curve turns upward, indicating an increase in counts per gram as the mass of PuO 2 increases. A correction term is obtained by determining the increase in counts CHECK SOURCE per gram at the mass value corresponding to the neutron COVER assay calibration standard mass. This increase is readily determined by plotting the straight line through the origin and the lowest mass sample response and reading the difference in counts between the two lines at the abscissa coordinate corresponding to the neutron assay CHECK SOURCE

calibration standard mass. All measurements relating to this standaid are thereafter reduced by the ratio of the difference in counts to the observed counts.

FRONT VIEW

3. Isolation of Collection Zones FIGURE B-2 NEUTRON COLLIMATOR CHANNEL To ensure that each collection zone is PLUG AND CHECK SOURCE independently assayed, it is necessary to screen all radiations from the detector except those radiations emanating from the collection zone being assayed. This While the amount needed is best determined through an is principally accomplished through the use of the evaluation of typical accumulations, 100 g Pu is collimators described in Sections B.2.1.2 and B.2.2.2.

adequate for most applications. Two additional means exist to further isolate a collection zone.

The neutron assay calibration standard may generate more neutrons than directly attributable to the spontaneous fission and (an) reactions. Because a 3.1 Detector Positioning relatively large quantity of PuO2 is encapsulated in the neutron assay calibration standard, some of the An unobstructed side view of a collection zone spontaneous fission or (an) neutrons may be absorbed in is preferred. When plutonium is located behind the zone Pu-239 or Pu-241 nuclei, producing additional neutrons under assay in another collection *zone or a storage through the induced fission reaction. The amount of facility, either consider positioning the detector above or multiplication depends in a complex manner on the below the collection zone, or consider the use of shadow amount and distribution of PuO 2 and on the surrounding shielding.

medium. The potentially significant calibration error arising by having too large a neutron yield per gram of 3.2 Shadow Shielding plutonium will be corrected in the long term through assay verification tests. In the initial phase of assaying It may not be possible to avoid interfering holdup, a rough correction for this yield can be radiations through the collimator design or through measured by preparing two additional PuO2 sources choosing the detector position for assay. In such cases, it containing 1/3 and 2/3 of the neutron assay calibration may be possible to move a shield panel between the standard mass. These samples need not be encapsulated, source of interfering radiations and the collimator zone as they will be measured only once and can then be under assay. If the shield panel is very thick and its returned to the process stream. dimensions match or exceed the back side of the The PUO2 used in this test is taken from collection zone under assay, no interfering radiations the same batch used to prepare the neutron assay will penetrate through the shadow shield to the detector.

calibration standard. After weighing out the proper While such characteristics are desirable, the size of such a quantities, the PuO 2 is put into containers having close shield would limit its transportabilit

y. A rectangular

5.23-6

panel containing -5 cm of neutron moderator (e.g., The calibration obtained through this procedure is benelex, WEP, or polyethylene) and -0.5 cm lead sheet recommended until a history of comparisons between is recommended, mounted on wheels as an upright predicted and recovered holdup quantities is developed, panel. To use such a panel, two measurements are as described in Section B.5 of this~guide.

required.* --

4.1 Detector Positioning R1 - Rcz + Rlnterference (1) To calibrate each collection zone, the best position or series of positions is selected to observe the R2 = Rcz + TRinterference (2) collection zone with .the least amount of interference from principal structural components. It is important to where view the collection zone with the detector located between the collection zone and all areas used for Pu R1 is the assay response obtained before the shadow storage during inventory. A three-dimensional approach shield is moved into position, can be investigated, positioning the detector on top of or below the collection zone if it is not possible to have an R2 is the assay response obtained with the shadow unobstructed, interference-free side view of the shield in position, collection zone. The use of shadow 'shielding can be explored if it is not possible to get a clear view of each Rez is the response component attributable to the collection zone for assay.

collection zone under assay, On the basis of a detailed examination of the Rinterference is the response component physical layout of the facility, some preliminary attributable to the interfering radiations, and measurements are made to determine optimum detector positions for holdup assay. Once the assay positions for T is the transmission through the shadow shield. the detector and shadow shields -are established, Note that T represents a measured transmission-T.r permanently marking the assay positions will. facilitate for gamma rays or Tn for neutrons. Ty. and Tn are subsequent measurements.

measured by counting radiations from any arbitrary source of plutonium with the shield between the 4.2 Calibration Sources source and detector and again with the shadow shield removed: Since this assay is to measure the amount of plutonium holdup, it is appropriate to use plutonium as T = (R.?,) shield in/(R,,) shield out (3) the calibration standard material. Further, as the plutonium holdup will generally be distributed over a T, = (Rn) shield in/(Rn) shield out (4) large surface area, it is recommended that the gamma ray calibration standard be fabricated to resemble this To correct for the interference, subtract R2 from R 1 , characteristic, as described in Section B.2 of this guide.

and solve for 'Rlnterference:

(R2 - R) (" 4.3 Calibration Procedures Rlnterference (1 -T) " (5) Once the principal items containing plutonium have been removed and the detector located in its assay To ensure that this correction is sufficiently accurate, it position, the response from a calibration standard may be necessary to extend the length of the normal combined with the plutonium already held up is counting period .to accumulate sufficient counting obtained. When the collection zone is appropriately statistics (1% statistics are generally adequate for this isolated, two factors influence the observed response application). from the calibration standard:

I. the location of the calibration standard within the

4. Calibration of Collection Zones collection zone, and Euch collection zone is independently calibrated, as 2. the shielding of radiations from the calibration hackground-lfaclors and the compos*tion of each zone standard caused by the items comprising the vary widely from zone to zone. A collection zone is best collection zone.

calihlaled throngh the in situ measurementi of' known The gcomelric response variation is measured t'iilih)lU in lanltdads. When such a program is not by observing lie response from-one calibration standard piositlve,. Ihli callibration can it , based on the calculalion with the other standard removed from the collection of Ithe anticipated response or through measuring a zone under investigation. The calibration standard mockup ot the collection zone of interest. 5 response is measured with the standard positioned in various parts of the collection zon

e. avoiding internal

• Response tierms refer to neutron or gamma response, as items which may attenuate the radiation emanating .from appropriate. the standard.

5.23-7

When neutron assay is employed or when the To use this relationship, the detector is first collection zone consists of a hollow box, pipe, or duct, positioned at point d and a reading is taken. Point d is attenuation is either relatively uniform or negligibly the center of the first subzone, selected to coincide with small. The calibration of each collection zone then the physical edge of the calibration' zone. The detector is becomes a matter of appropriately averaging the then moved a distance 2D along the traverse to the geometric response variations. The average response of center point of the second subzone, and the second the entire collection zone is assumed to properly measurement taken. The cycle*is repeated to include all represent that zone. -'If, however, it is known that of the larger collection zone. The value interpreted for plutonium accumulates in one particular location within calibration for each subzone *.corresponds to the a collection 'zone, the response of the standard is maximum of the traverse across each subzone because emphasized when located near the principal collection the response has been flattened. The content of the site. entire collection zone is the sum of the contributions from the subzones.

If the item to be assayed consists of a large 5. Estimation of the Holdup Error unit, assay performance may be enhanced by subdividing the unit into smaller contiguous measurement zones. The overall uncertainty associated with the The repeat dimensions of the subzones are determined measured plutonium holdup is due to (1) the by measuring the rcsponse while moving the standard uncertainty in. the observed response and (2) the along an axis perpendicular to the detector centerline. uncertainty in the interpretation of that response. The By studying the response curve, the distance D is random uncertainty components in this application are selected as the point beyond which sufficient activity is .frequently negligible in comparison with the geometric detected to flatten the response within the subzone. uncertainty and the uncertainty in the isotopic Each subzone will measure 2D across its face. An composition. In this assay application, it is appropriate example is illustrated 'in Figure B-3. As the response to estimate the assay error components by assuming the about the centerline is assumed to be symmetrical, only measured range -(Ri) of the ith fluctuation constitutes an half of the traverse is indicated. In Figure B-3, D is interval four standard deviations wide. The midpoint of selected such that the area under the curve to the right the range estimates the mean effect, and the' distance of D is approximately equal to the area above the curve from the "midpoint to each extreme comprises an to the leftof D (Area A 1 = Area A 2 ). Note: the distance estimated 95% confidence interval. 'The error from the collection zone to the detector or the distance attributable to this effect is then approximately from the crystal face to the end of the collimator, or 2 both, can be varied to divide the collection zone into an 2=(R)

(6)

integral number of subzones.

AREA A1 If a.severe effect is~noted, the response can often be corrected for the variation in the corresponding

100,

parameter by measuring the. value of that particular parameter at the time of the assay. Using a measured relationship between the response and the value of that parameter, the observed response is corrected.

5.1 Response Uncertainties S A MEASUREMENT DATA POINTS

'5.1.1 Counting Statistics VISUAL FIT TO MEASUREMENT DATA

The magnitude of the uncertainties attributable to variations in the geometric distribution A and in the attenuation of the radiations are expected to dominate the total 'response uncertainty. 'The relative S RA .

standard deviation due to counting statistics can usually be made as small as desired through '(I) using more AREA A2 efficient detectors or (2) extending the counting period.

Having 1000 to't0,000 net counts is generally sufficient for most holdup assay applications.

5.1.2 Instrument Instabilities

0 25 5D 75 100

DISTANCE FROM DETECTOR CENTERLINE TO POINT SOURCE, CENTIMETERS

- Fluctuations in ambient temperature, FIGURE B-3 EQUIVALENT DIAMETER SUSZONE TO ACHtEVE A FLAT

PLANAR RESPONSE. SELECT D SUCH THAT AREA A 1 = A2. humidity, electronic noise, and line voltage (for

5.23-8

non-battery-powered electronic units) generally affect 5.2.1 Interfering Radiations the stability of electronic systems. The magnitude of this uncertainty can be estimated by monitoring the check 5.2.1.1 Gamma Ray Assay standard response and determining the range of variability as described in Section B.5 of this guide, An uncertainty in the observed gamma ray response may arise due to the presence of extraneous

5.1.3 Geometric Uncertainty gamma ray emitters or due to fluctuations in the background from the' Compmon scattering of The geometrical variation in the observed higher-energy gamma rays. The shape of the background response is measured by moving the calibration source gamma ray spectrum may change in such cases to such within the bounds of each collection .zone. Two -cases are an extent that even with the energy windows stabilized.

described below.

the background correction is irregular and uncertain.

The magnitude of this effect is generally smal

l. It can be

5.1.3.1 Isolated Collection Zones .monitored by observing the spectrum with a multichannel analyzer, but unless the data on When a single unit comprises a periodically recovered 'holdup accumulations are in collection zone, the standard is moved .to all .sites within error, this contribution can be ignored.

the zone at which an accumulation of plutonium might occur. With sufficient collimation, the response for. the collection zone under investigation is independent of its 5.2.1.2 Neutron Assay neighbor zones. The average of the response, weighted to reflect 'prejudgments on the likelihood of accumulation A change in the neutron yield for a sites, is then used as the calibration point. As shown in plutonium sample of fixed isotopic content is primarily Section B.5, the range of values can be assumed to attributable to the fluctuation in the concentration of comprise an expectation interval four standard high (an) yield impurities.* Judgment can be used to deviations wide. The geometric error is then estimated determine the range *of permissible impurity using Equation 6. concentrations. The variation in a typical neutron yield can then be predicted using the methods discussed in the

5.1.3.2 Overlapping Collection Zones Appendix of this guide. Again, the range of permissible variations is assumed to constitute an acceptance interval When a collection zone is subdivided from which the component error is computed using into overlapping subzones, the geometric uncertainty Equation 6.

due to the dimension perpendicular to the detector collection zone centerline is eliminated through the

5.2.2 Isotopic Uncertainties area-averaging calibration method described in Section

4.3.

If the process equipment is cleaned each The uncertainty in the depth time the isotopic composition of the plutonium feed is dimension in each subzone can be determined through varied, the holdup will consist primarily of the current the procedure outlined 'for isolated collection zones. material. New calibration standards can be prepared or Judgment can be used to weight the calibrationdata to the previous yield data can be normalized using the emphasize principal accumulation sites. methods presented in tht Appendix to correct tor t*his effect. When mixing occurs, use of the stream-averaged

5.1.4 Attenuation Uncertainty isotopic composition is appropriate. The uncertainty bounds are estimated by considering the highest .,id If the attenuation is not extreme, it can be lowest fissile isotopic batches and computing the measured in situ, mocked up, or computed for the corresponding range.

different conditions encountered. The worst and best cases can be assumed 'to determine the range of 5.3 Holdup and Its Associated Error permissible effects. Using Equation 6, the magnitude of Ihis uncertainty component can then be estimated. The amount of Pu holdup can be ,measured Again, judgmaent is appropriate to weight the correction through the systematic application of the program factor. developed in conjunction with the principles and pitfalls discussed herein. For each collection zone, measured

5.2 Interpretation Uncertainties holdup and its error can be determined.

Two factors are central to the issue here, assuming that the calibration standard material is similar *Over a long period of time the a-particle production ratc to the held-up material. increases due to the ingrowth of Am-24 1.

5.23-9

5.3.1 Initial Operations measurement method for this inventory component, it is necessary to consider -the -factors' in -the -following During the initial phase of operations, the sections.

error associated with the in situ assay of plutonium holdup is estimated by combining the component errors Note: Care must be exercised during the fabrication and determined in the preceding sections of this guide (B3.5.1 use of check sources and-calibration. standards to ensure and B.5.2). their continued integrity and to prevent contamination.

5.3.2 Routine Operations 4. Delineation of Assay Collection Zones To ensure the validity of assay predictions A plan of each plutonium processing facility should and to more realistically estimate the uncertainty in be examined.,to establish, independent collection zones.

those predictions, it is necessary to establish a program Individual glove boxes and similar containment to measure, the amount of plutonium recovered when a structures should be so-identified.. Using the layout and collection zone is cleaned out. By comparing the aniount touring the facility, -an. assay. site(s) for. each collection of plutonium recovered to the recovery amount zone should be selected:

predicted, the collection zone calibration can be updated and the assay error can be based on relevant verification 1. Assay site(s) - should afford a clear, unobstructed tests. view ,9f-the collection zone with no other collection or

....storage. areas in_ the line- of sight of the .collimator The update data is computed as the assembly. Location of the. detector probe above or difference in the assays before and after cleanout: below, the -collection zone- should be considered if an unobstructed side , view is not possible. If an (PU)assay = Rbefore - Rafter , (7) - unobstructed view is not, possible, shadow, shielding should be used to isolate the collection zone, for assay.

The difference.(A) in assay and recovery,

2. The assay site should be set back as far as possible A = (PII)assay - (Pu)recovery (8) from each collection zone to reach a compromise between interference from neighbor zones and efficient is then computed. counting..

The standard deviation in the A values (s.) 3.' Gamma ray assay should ,be applied to measure the is computed separately for" each collection zone, plutonium- held up in all collection. zones containing less including no more than the twelve preceding than the neutron- detection limit- and for' single measurement tests: containment structures which do not contain irregularly shaped structural components capable of significantly

-,*attenuating. the emerging gamma rays. Neutron assay should: be applied to measure the accumulation of sA (K- 1 (9) plutonium holdup in all structures not suitable, for gamma ray assay. - .

When a value of A is determined, it is used 4. Each collection zone should be uniquely numbered.

to update the estimate sb. The -standard, deviation (Neutron collection zones could be preceded by an "N",

estimate s. can be used to estimate the~error in. the assay gamma ray collection zones by a "G". Subzones should prediction for the collection zone for which it has been be identified by an alphabetic suffix to the collection established. - zone identification.) .

The amount of plutonium collected during .5. Each assay site should be' marked with paint or the cleanout of a specific collection zone can be assayed colored tape on the floor. (To be consistent, blue tape through sampling and chemical analysis, through should be used for neutron assay sites, orange for gamma calorimetry, or through other applicable nondestructive ray sites.) The height setting for midpoint assay should assay methods (eg.,. spontaneous fission coincidence be recorded in the measurement log corresponding to detection or gamma ray assay). Each of these topics is each assay site....

the subject of a Regulatory Guide.

2.., Assay Instruments

C. REGULATORY POSITION

Neutron and gamma ray assay capability should be To develop a program for the periodic in situ assay provided using separate or compatible' electronics with of plutonium residual holdup as an acceptable interchangeable detector probes. Compatible electronics

5.23-10

should provide for both He-3 or BF3 neutron detection 2.1.3 Gamma Ray Check Source and Nal(TI) gamma ray detection. The electronics unit should have a temperature coefficient of less than 0.1% To ensure the continued normal operation per 'C. Battery-powered electronics should be provided of each system, an encapsulated plutonium check source to expedite assays. should be provided. The source should be small enough to be implanted in a section of shielding material so

2.1 Gamma Ray Assay shaped as to close off the collimator opening. The check source should be positioned adjacent to the detector.

Gamma ray assay should be 'based on the The source should contain an amount of plutonium activity observed in the energy range from 375 keV to sufficient to provide a gross count rate of 1000 to

440 keV, excluding the composite gamma ray complex 10,000 counts per second.

centered at 333 keV. Yield data for appropriate gamma rays are presented in Section B.2.1 of this guide. 2.1.4 Gamma Ray Calibration Source To permit the calibration of gamma .ray

2.1.1 Detector Selection assay collection zones, a calibration standard should be fabricated by encapsulating plutonium oxide in a disk.

Gamma ray detectors should have FWHM The isotopic composition of the plutonium and the resolution equal to or better than 7.5% at 662 keV abundance of Am-241 should be measured and be (Cs- 137 gamma ray). NaI(TI) can meet such chosen to be nominally representative of the plutonium specifications and is suitable for this application. The being processed. The total amount of plutonium crystal depth should be sufficient to detect a significant encapsulated should be closely monitored. Attenuation percentage of 400-keV gamma rays. For NaI(TI), the losses within the bed of PuO 2 and through the minimum depth should be one inch. A two-inch depth is encapsulating material should be measured and the recommended. calibration standard response normalized to counts per gram incorporating these corrections.

The crystal should be stabilized with a suitable radioactive source. An'ý internal Cs] seed 2.2 Neutron Assay containing Am-241 is recommended for this application.

The electronics should be capable of stabilizing on the

2.2.1 Neutron Detector Selection reference radiation emitted by the seed. The crystal face (external to the cover) should be covered with 0.075 to Neutron detectors should have high

0.150 cm cadmium sheet to filter low-energy radiations.

detection efficiency and be capable of operating in the presence of gamma radiation. He-3 and BF 3 neutron Two single-channel analyzers should be

.provided with lock-set energy windows. One channel detectors are recommended for this application. Multiple detector tubes with matdhed operating performance should be set to admit gamma rays from 390 keV to 440

should be connected in parallel to a single preamplifier keV unless equilibrium of the U-237 and Pu-241 can be to increase the overall detection efficiency obtainable assured. The 333-keV region of the gamma ray spectrum from a single detector tube. Neutron detectors should be should be excluded. With Nal detectors, it is necessary surrounded by a layer of neutron moderator material to to exclude the 375 keV gamma ray to ensure that the enhance their detection efficiency. The neutron tail from the 333 keV complex is not added. The second moderator layer should be covered with a low-energy channel should be set above the first window to provide neutron absorber to filter out extraneous neutrons from a background correction for the assay window. This the desired signal. A recommended configuration is second window should be set from approximately 450

diagrammed in Figure B-I.

keV to 600 keV.

2.2.2 Neutron Collimator

2.1.2 Gamma Ray Collimator A cylinder of shielding material such as A slab collimator or concentric cylinder lead should be made c(ncentric with the gamma ray collimator of a suitable neutron moderator material detector. The end of the cylinder opposite the crystal (e.g., polyethylene) should be constructed to completely should be blocked with the shielding material. The surround the detector with its associated moderator and thickness of the collimator should -be chosen to provide filter assembly, 'leaving open orly the collimator sufficient directionality for the specific facility (1.5 cm channel. A recommended 'configuration is shown in of lead thickness should be sufficient for most Figure B-1.

applications). The collimator sleeve should be extendible over the end of the crystal to reproducible settings to The moderator thickness should be vary the degree of collimation for different collection selected to provide. the directionality required for each zones. facility. A directionality profile providing a 10:1

5.23-11

response ratio (six inches of polyethylene) should be so that the response from the calibration standards will adequate for most applications; however, each situation not be influenced by the in-process material.

should be evaluated as discussed in Part B of this guide.

3.1 Instrument Check

2.2.3 !NeutronCheck Source The stability of the neutron and gamma ray Any neutron source which emits detection systems should be tested prior to each approximately 100-10,000 neutrons/second is inventory by comparing the observed counts obtained acceptable for this application. The source should be from the check source, minus the counts with the small enough to be contained within a section of, shaped shield in place but without the check source, to neutron moderator material so shaped as to completely the readings obtained prior to previous inventories. If fill the collimator channel of the detector assembly. The the measurement is consistent with previous data (i.e., is source should be implanted ,directly adjacent to the within plus or minus two single-measurement standard neutron detectors, outside the cadmium thermal neutron deviations of the mean value of previous data), all filter. A recommended configuration for this assembly is previously established calibrations using this detection diagrammed in Figure B.2. system should be considered valid. If the measurement is not consistent, the operation of the ..unit should be

2.2.4 Neutron Assay Calibration Standard checked against the manufacturer's recommendations and repaired or recalibrated, as required.

To permit the, calibration of neutron assay

• collection zones, a calibration standard should be

3.2 Zone Calibration

-fabricated by encapsulating PuO 2 . The PuO 2 should be nominally representative of the plutonium being The geometric response profile for each processed in isotopic composition, in Am-241 content,

"and in the content of high (a,n) yield target materials. collection zone should be determined by measuring the variation in the response as a calibration standard is The amount of plutonium to be encapsulated should be moved within the defined limits of the collection zone.

chosen to be representative of the amounts of plutonium The. response variation should then be averaged to estimated to be held up in typical neutron assay determine the response per gram of plutonium for that collection zones.

collection zone. The averaging should be weighted to reflect known local accumulation sites within each

'The' neutron yield of the calibration collection-zone. The response per gram should be used standard should be measured and also computed using to directly translate the observed response to grams oi the. method described in the Appendix. The observed plutonium, after the response is corrected for neutron. count rate should be normalized. 6 If the background.

,predicted response differs by more than 10%76, the response should be normalized as discussed in Section

3.2.1 Subzone Calibration B.2.2.4.

2.3 Service Cart When a collection zone is too large to be accurately measured in a single assay, the collection zone A cart carrying electronics and both detector should be divided into overlapping subzones. The repeat probes should be provided. The capability to raise or dimensions of each subzone perpendicular to the lower the probes to reproducible settings should be detector-to-collection-zone line should be determined so included. that the response variation across that distance is nulled.

Using this procedure, the residual geometric uncertainty

2.4 Notation of Operating Parameters should be determined by measuring the response as a calibration standard is moved along the depth When compatible electronics are used to coordinate. The calibrated response should then reflect facilitate neutron and gamma ray assay, a notation of the average of the depth response, weighted to reflect athe respective settings should be affixed to the known accumulation sites.

electronics unit. To decrease the likelihood of incorrect settings, the neutron probe and the -appropriate 4. Asmy Procedures electronics settings should be color-coded blue; the gamma ray probe and :corresponding electronics settings 4.1 Ammy LoA

should be coded orange.

An assay log should be maintaine

d. Each

3. Calibration collection zone or subzone should have a separate page in the amy log, with the corresponding calibration Each collection zone should be independently derived on the page facing the assay data sheet.

calibrated when all in-process material has been located Recording space should be provided for the date of

5.23-112

measurement, gross counts, corrected counts, and the collection zone. The operator should initial the corresponding grams plutonium from the calibration in measurement log to assure conmpliance for each addition to position and instrument electronic setting collection zone.

verification.

Having met all preceding requirements, the

4.2 Preassay Procedures measurement at each site should be taken, recorded, and converted to grams plutonium. If each value is within an Prior to inventory, the isotopic composition of expected or permissible range, -the cart should be moved the plutonium processed during the current operational to the next site and the cycle repeated. If a high period should be determined. Variations in the neutron response is noted, the cause should be investigated. If and gamma ray yield data from the calibration standard the collection zone contains an unexpectedly large should be calculated. Either the calibration data or the content of plutonium, that collection zone should be predicted holdup should then be corrected to reflect this cleaned to remove the accumulation for conversion to a difference. more accurately accountable material category. After the cleanout has been completed, the zone should be Prior to each inventory, the operation of the reassayed and the recovered material quantity used to neutron and gamma ray assay detection systems should test the validity of the zone calibration.

be checked.

5. Estimation of the Holdup Error Prior to any assay measurements, feed into the process line should be stopped. All in-process material During the initial implementation of this program, should be processed through to forms amenable to the error quoted for the holdup. assay should be accurate accountability. All process, scrap, and waste computed on the basis of estimating the error items containing plutonium should be removed from the components, as described in SectionsB.5.1 and B.5.2.

process areas to approved storage areas to minimize Prior to the cleanout of any collection zone for background radiations. whatever purpose, that zone should be prepared for assay and measured as described in:Section C.4 of this

4.3 Measurements guide. Following this assay, the collection zone should be cleaned out and the collected plutonium should then The assay cart should:be moved in sequence to be assayed using an appropriately accurate assay the assay site(s) corresponding to each collection zone. method. When the collection zone has been cleaned and Assaying all gamma ray sites before assaying neutron the collected plutonium removed,, the collection zone sites (or Vice versa) is recommended. should be reassayed. The recovered plutonium should be used to update the calibration and,. from the. sixth test Before assaying each collection zone, the on, should serve as the assay error estimate. Separate operator should verify the floor location, probe records should be maintained for each collection zone to selection, probe height, and electronics settings. All estimate the error in assaying the plutonium holdup.

check and calibration sources should be sufficiently removed so as not to interfere with the measurement. To ensure that error predictions remain current, Prior to taking a measurement, a visual check of the only data of the twelve preceding independent tests zone and the line of sight of the detector probe should should be used to estimate the assay error. Collection be made to assure that no obvious changes have been zones not cleaned for other purposes should be cleaned made to the process area and that no unintended for assay verification at intervals not to exceed two accumulations of plutonium remain within the months.

REFERENCES

1. R. Gunnink and R. J. Morrow, "Gamma Ray 4. An example of a collimator for uranium gamma ray Energies and Absolute Branching Intensities for assay is found in R. B. Walton, et al, "Measurements

, 1Pu and 2 4 1 Am," UCRL,51087

238 2 39 240 24

, , of UF 6 Cylinders with Portable Instruments," Nucl.

(July 1971). Technol., 21, 133 (1974).

2. J. E. Cline, R. J. Gehrke, and L. D. Mclsaac,

5. W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A

"Gamma Rays Emitted by the Fissionable Nuclides Method for Surveying for Uranium-235 with Limit and Associated Isotopes," ANCR-1069 (July 1972).

of Error Analysis," Gulf-GA-A12641 (June 1973).

3. L. A. Kull, "Catalogue of Nuclear Material Safeguards Instruments," BNL-17165 (August

1972).

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APPENDIX

NEUTRON YIELD COMPUTATIONS

The following model for the calculation of the total 2. (a,n) Neutrons spontaneous neutron yield from plutonium-bearing materials assumes that the plutonium is widely The maior contribution to the total neutron dispersed. With this condition, there will be no production from (ax) reactions will typically be due to significant neutron production created through induced the 048 (an) Ne-21 reaction when the plutonium exists fission of Pu-239 or Pu-241. The total neutron yield per as the oxide. The yield from this reaction per gram of gram of plutonium holdup will then be the sum of the plutonium can be calculated using the isotopic spontaneous fission and (an) contributions: weight fractions (Wj) and the Yi yield data given in Table 1.

Yn = YSF + Y(,t,n) (1)

Y(an) Oxy WiYi (3)

1. Spontaneous Fission Neutrons To determine the spontaneous neutron yield per gram of plutonium held up within a collection zone, the The yield per gram of Put 2 is calculated by multiplying the yield per gram of plutonium by the isotopic composition of the plutonium and uranium (if gravimetric dilution factor (Pu/PuO 2 - 0.882).

present) must be known. The contribution from spontaneous fission can generally be calculated by neglecting the contribution from U-238: The presence of certain impurities can contribute substantially to the total (atn) production rate.

YSF = W 2 3 8 Q 2 3 8 + W240Q240 + W242Q242 Approximate values of (a,n) impurity yields for the highest yield (an) target materials are given in Table 2.

(2) To compute the impurity (an) contribution, the total a particle production is determined. Production rates of a where particles per gram of the principal nuclides of interest are shown in Table 1. This contribution to the total Wi = weight fraction of the ith plutonium neutron yield can be computed using the relationship:

isotope. For reactor fuel applications, W2 3 8 + W239

+ W240*'W241 +W242* I

Qi = spontaneous fission neutron yield per gram of Y(a,n) Impurity = Y 0 TPjlj i

(4)

the ith plutonium isotope (see Table 1).

TABLE 1 a Particle and Spontaneous Fission Neutron Yields

8 Half-life Alpha Activity PuO 2 (mn) Yield Spontaneous Fission Nuclide (yr) (r/sec-gram) (n/sac-ram) (n/sec-gram)

Pu-238 87.78 6.33 x 1011 1.71 x 104 2.57 x 103 Pu-239 24,150 2.30 x 109 54.5 2.22 x 10-2 Pu-240 6,529 8.43 x 109 202.1 1.03,x 103 Pu-241 14 . 3 5 b 9.39 x 10' 2.03 2.43 x 10-2 Pu-242 379,000 1.44 x 108 3.13 1.75 x 103 Am-241 433.8 1.27 x 10 11 3.46 x 103 6.05 x 10-1 U-234 2.47 x 105 2.29 x 108 4.65 5.67 x 10-3 U-235 7.1 x 108 7.93 x 104 1.37 x 10-3 5.96 x 10-4 U-238 4.51 x 109 1.23 x 104 1.93 x 10--4  !.12 x 10-2 a - Oxygen yield from PuO2 form only.

b - &-branching ratio - 2.46 x 10-5

5.23-14

where Pj = (an) yield per ppm of the impurity j (see Table 2)

Y,, = total a production Ii = impurity j content, expressed in ppm (weight) of plutonium.

= WWiai + WArnm'Am i 3. Sample Calculation (PuO 2 -UO 2 )

Wi = Pu isotopic weight fractions Consider the case of recycle plutonium blended t6

3 wt %Pu in a normal U0 2 matrix, Where the isotopic WAm = Am weight fraction = Am/Pu composition is Pu-238 (.25%). Pu-239 (75.65%), Pu-240

(18.48%), Pu-241 (4.5%), Pu-242 (1.13%), and Am-241 ai = a yield per gram of nuclide i (see Table 1) (.28% of Pu).

For mixed oxides, the oxygen density is approximately the same for the case ofPuO.. This fact, TABLE 2 together with the atomic similarity of uranium and (Q,n) Yield Rats of Low-Z Impurities in Pu02a plutonium, justifies the assumption that the oxygen (a,n) yield per gram of mixed oxide is the yield per gram of PuO 2 , further reduced by the blending ratio, P. Pu/(Pu + U).

I mpurity (n/a-ppm)

Using the values given in Table I, the spontaneous Li ......................... 6.29 x 10-12 fission yield and total a production per gram of Be ......................... 2.00 x 10 -' 0 plutonium can be computed. Results are shown in Table

3.

B .... ...................... 4.63 x 10-11 C .......................... 2.77 x 10-13 The a particle yield of plutonium is constant in time

0b ....... . ................. 1.56 x b0-"13 for all intents. However, the Am-241 a production in- F ........................... 2 .44 x 1O-Il creases at a rate which results in approximately a 0.3%;.

Na ......................... 3.00 x 10-12 increase per month in the total a production, for the g........................... 2.67 x 10-12 range of plutonium isotopic compositions intended for if ......................... 1.45 x 10-1 2 reactor fuel application.

Si ......................... 3.25 x 10-13 In the present example, the impurity levels of the aAssumnes zero yield from all other impurities. principal (a.n) target materials are shown in Table 4. The bOxygen not contained in oxide. neutron yields attributable to (an) interactions on those TABLE 3 Sample Calculation Spontaneous Fission Alpha Production PuO 2 1a,n)a Nuclide W (nsec-g Pu) (cx/sec-g Pu) (n/sec.- Pu)

Pu7238 .0025 6.4 1.58 x 199 42.6 Pu-239 .7565 <.05 1.74 x 109 41.3 Pu-240 .1848 189.4 1.56 x l09 37.3 Pu-241 .0450 <.05 4.23 x 106 0. i Pu-242 .0113 19.8 1.63 x 106 <0.05 Ain-241 .0028 <.05 3.56 x 108 9.7 rotal Yields 215.6 5.26 x 109 131.0

-- oxygen yield only.

impurities are also shown in Table 4, calculated using the TABLE 4 a particle production rate of 5.3 x 109 a/sec-g Pu, Impurity (.,n) Yield computed above. In this example, the mixed oxides are composed of blended PuO 2 and U0 2 particles Arbitrary approximately 40 microns in diameter. If the particle size were smaller or the mixed oxide was created Concentration (a,n) Yield Impurity (ppm) Wnisec-g. Pu)

through coprecipitation, the uranium impurity content would also contribute to~the plutonium(an) yield. This contribution can be ignored for large particles and Li 9 0.30

estimated by combining the impurities for small particles Be 8 8.42 and coprecipitatedoxides. B 10 2.44 C 200 .30

The total neutron yield in this example is 380 F 125 16.0

n/sec-g Pu. In this example, the percentage of plutonium OR ... 4600 3.77 tq the total Pu + 0 is 0.8835. Using this gravimetric Na 120 1.90

dilutign factor, the neutron yield is 336 n/sec-g PuO2 . If the PuO 2 is blended with U0 2 to 3%, i.e., PuO 2 /PuO 2 + Total 33.1 U0 2 = 0.03, the neutron yield. from the blend will be

10.1 n/sec-g MO. aOxygen present in moisture, not as oxide.

5.23-16