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)
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RG-5.023, Rev 1
Download: ML13064A072 (16)

May 1974U.S. ATOMIC ENERGY COMMISSIONREGULAT(OR Y G U I D[E'DIRECTORATE OF REGULATORY STANDARDSREGULATORY GUIDE 5.23IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP

A. INTRODUCTION

Part 70, "Special Nuclear Material," of Title 10 ofthe Code of Federal Regulations requires licenseesauthorized to possess more than one kilogram ofplutornium to calculate a material balance based on ameasured physical inventory at intervals not to exceedtwo months. Further, these licensees are required toconduct their nuclear material physical inventories incompliance with specific requirements set forth in Part70. Inventory procedures acceptable to the Regulatorystaff are detailed in Regulatory Guide 5.13, "Conduct ofNuclear Material Physical Inventories."Plutonium residual holdup is defined as theplutonium inventory component remaining in and aboutprocess equipment and handling areas after thosecollection areas have been prepared for inventory.Whenever possible, process equipment should bedesigned* and operated so as to minimize the amount ofholdup. In this guide, procedures are detailed for the insitu assay of the residual plutonium holdup.Assay information can be used in one of two ways:I. When the limit of error of plutonium holdup iscompatible with constraints on the overall limit of erroron the facility MUF (LEMUF), the material balance canbe computed using the measured contents of Pu holdup.Additional cleanout and recovery for accountability willthen not be necessary."Design features to minimize holdup in process equipment arethe subject of a seriý of rgulatory guides.2. When the limit of error of Pu holdup is notcompatible with constraints on the overall LEMUF, theinformation obtained in the holdup survey can be usedto locate principal Pu accumulations and to assure thatother areas of the process contain less than the detectableamount of plutonium. Once located, substantial accu-mulations can be recovered, transforming the plutoniumto a more accurately measurable inventory component.Having reduced the amount of plutonium holdup, thelimit of error on the remeasurement of the remainingholdup may be sufficiently reduced to be compatiblewith overall LEMUF requirements.

B. DISCUSSION

Plutonium accumulates in cracks, pores, and zonesof poor circulation within process equipment. The wallsof process vessels and associated plumbing often becomecoated with plutonium during solution processing.Surfaces internal and adjacent to process equipment,especially glove box walls and floors, accumulatedeposits of plutonium which can become appreciable.Plutonium also accumulates in air filters and associatedductwork. The absolute amounts of plutonium holdupmust be small for efficient processing and proper hazardscontrol. However, the total amount of plutoniumholdup may be significant in the context of the tolerablefacility MUF.The measurement procedures detailed in this guideare based on the controlled observation of gamma raysand neutrons which are spontaneously emitted by theplutonium isotopes. Because the gamma rays of interestare emitted by Pu-239, garnma ray assay is the preferredUSAEý REGULATORY GUIDESRegulatory Guides we issued to describe and make avaiille to the publicmethods acceptable to the AEC Regulatory staff of implementing specific parts ofthe Commission's regulations, to delineate techniques .-.ed by the staff inevaluating specific problems or postulated accidents: or to provlde guidance toepplicents. Regulatory Guides we not substitutes for regulations arnd comoliancswith them is not required. Methods and solutions different from those sit out inthe guides will be acceptable if they provide a basls for the findings requisot tothe issuanc or continuance of a pearmil or licemni by the Comnission.* Published guidet will be revised periodically, as appropriate, to accommodateicomments end to reflict new information or experience.Copies of published guides may be obtained by rsquast indicating the divisionsdosircd to the US. Atomic Enrgty Commission, Washington, D.C. 2054'.Attention: Director of Regulatory Standards. Comments and suggestions forinmprovements in thes guides ere encouraged and should be sent to the Secretaryof the Commission, U.S. Atomic Energy Commission. Washington. D.C. 20645.Attention: Chief. Public Promedinga Staff.The guidas ea issued in the following ton broad divisions:1. Power eactors2. Resmrch and Test Reactors3. Fuels and Materials Facilities4. Envwonnmental and SitingS. Materials and Plant ProtectionS. Produects7. TransportationS. Occupational l'slooh9. Antitrust Revow10. General assay method whenever its acceptance criteria aresatisfied. To accomplish either gamma ray, or neutronassay, it is essential to consider the facility in terms of aseries of zones which can be independently assayed.Such zones are designated as "collection zones."1. Delineation of Collection ZonesTypical plutonium process facilities comprise anumber of interconnected glove boxes which containwork areas and most process equipment, in-processstorage areas, and self-contained process equipment.Also, solution processing requires tanks, plumbing, andpumping equipment, which are often located in closeproximity to.the glove box lines. Finally, storage areasfor feed, scrap and waste, and final product are alsooften located in close proximity to the plutoniumprocess area.Each facility can be divided into a series ofcollection zones on the basis of a logical understandingof process activities. Individual glove boxes can besubzoned to improve assay performance, but for mostapplications, individual glove boxes are -examples ofsuitable size areas for discrete collection zones.Gamma ray assay for plutonium holdupmeasurement is practical when a collection zone consistsof a single structure of relatively uniform cross section.When a collection zone contains an item of equipmenthaving significant shielding properties and capable ofcontributing to the holdup, the uncertainty in theholdup prediction based on the observed response maybecome primarily due to attenuating the radiations inthe internal structure. In such cases, neutron assay isapplicable.2. Applicable Methods and InstrumentsTwo ,considerations are critical to the selection ofmethods and instruments. First, to perform an assay, theplutonium radiations must reach the detector, and bedetected. Second, the observed response must beattributable to the collection zone being assayed.Therefore, the assay scheme is developed aroundpenetrating radiations and the detector is collimated toprovide for sufficient directionality in the response toresolve a collection zone from its neighbor zones andfrom the background.2.1 Gamma Ray AssayUnder closely controlled conditions, themeasured plutonium gamma ray spectrum can beinterpreted in terms of the abundance of each gammaray emitter present in the sample. Because of the largenumber of gamma rays',2 present, many regions of theobserved spectrum are characterized by overlappinglines. To accomplish the assay, it is necessary to select anappropriate spectral region and provide a detectionsystem with sufficient resolution to measure the activityfrom one or'two~isolopes o-Thinterest.Gamma ray assay has an ' advantage , overneutron assay in that the emissions are primarily fromthe principal isotopes qf linterest. -Because of the highemission rate of gammna rays, a detection sensitivity ofless than one gram is generally attainable..The most useful portion of the spec trum forholdup assay is the Pu-239 gamma ray complex in the375-440 keV range. The-yields of these lines are given inTable B.l.Table B.1PROMINENT GAMMA',RAYS FROM Pu-239 inENERGY RANGE 375-440 keVEnergy Intensity (- /sec-g Pu-239)375.0 ........................3.59 x J04.l380.2 ...................... 0.70 x 10382.7 ....................... 0.59 x 104392 ..5 ................ ...... 0.26 x,104393.1 .......... .... ..... 1.01 x104.413.7 ...................... 3.43 x I04422.6 ............... ..0.27 x 104Total 9.85 x 1042.1.1 -'Gamma Ray Detection Instruments.Gamma, ray detection-systems consist of ascintillation or -semiconductor detector sensitive togamma rays and .appropriate -.electronics.3 Requiredelectronics include lat least a single-ýchannel analyzer anda timer-scaler unit.- A second :single- channel analyzerused to determine the background radiation correction isa time-saving feature. Battery powered systems arecommercially. available and can provide operationalconvenience, particularly in this application.The detection efficiency and res6lution ofgood Nal(Tl) detectors is'generally adequate for thisapplication. CdTe, Ge(L), and-intrinsic 'Ge:detectorshave better resolution than Nal(TI) but: cost more, aregenerally less available, and are more difficult to operate.-' -The 332.3 keV- gamma-ray from U-237, ashort-lived (6.75 d) daughter -of Pu-241, is usually theprincipal interference for. Pu-239 assay by Nal detectionof the 375-440 keV complex. If the U-237 is inequilibrium with Pu-241, the intensity of this gamma rayis 1.15 x 106 7t/sec-g Pu124l.Since this gamma ray is also emitted inthedecay of Am-241., the. interference from this decaybranch may also be important in case -of preferentialamericium holdups. To avoid this interference whenusing Nal detectors, the assay-energy window is adjustedto span the range from 390 to 440 keV.5.23-2 Detector dimensions are selected toprovide a high probability for detecting the appropriategamma rays. The geometric detection efficiencyincreases as the square of the detector radius; however,the weight of the gamma ray shielding material requiredto collimate the detector also increases ;when largerdetectors are used. The crystal depth is chosen such thatmost of the gamma rays of interest will lose all theirenergy within the crystal;To reduce the pile-up of low energyradiations, the crystal face can be covered with anappropriate shield (e.g., 0.075 cm cadmium). Thisprocedure will reduce counter dead time effects withoutsignificantly affecting assay results.2.1.2 Collimators for Gamma RaysA shaped shield constructed of any densematerial is appropriate for gamma ray collimation. Forcost, availability, and ease of fabrication, lead isrecommended. Less ,than 2% of all 400 keV gamma raysstriking a 1.5-cm-thick sheet of lead will pass throughwithout having suffered an energy loss.The collimator will be most effective whenit is concentric about the crystal and photomultiplierand completely covers the photomultiplier base.Extending the collimator forward of the crystal at least adistance equal to half the diameter of the crystal, andpreferably the full diameter, is recommended.4 Makingthis distance variable to reproducible settings will permitadjustment over a range of collection zone sizes.2.1.3 Check Source for Gamma Ray AssayIt is important to check the operation ofthe detection system prior to each inventory sequence.Either recalibrating one or more collection zones andcomparing the results'to previous analyses or testing theinstrument with an appropriate check source isappropriate. When the performance remains within theexpected value,, the previous calibration data areassumed to be valid. If not, theenergy window may haveshifted, or the unit may be in need of repair andrecalibration.An appropriate check source enables thestability of the assay instrument to be tested at anylocation. Such a source can be prepared by implanting asmall encapsulated plutonium source (containing -0.5 gPu) in the face of a plug of shielding material. The plugis shaped to fit and close the collimator channel, and thesource is positioned to be adjacent to the crystal whenthe plug is in place.The check source is fabricated in a mannerto ensure its internal stability. Other than .radiationsincreasing from the ingrowth of Am-241, the emissionrate of the check source should remain constant.2.1.4 Calibration Source for Gamma RayAssayTo calibrate a collection zone, the observedassay -response is compared to the response obtainedwhen the zone contains a known amount of plutonium.Because of the complexity of the assay, theresponse is assumed to be linear. To be representative oftypical holdup situations, the calibration standard isprepared as an encapsulated disk with a bed thickness ofless than 0.2 cm. Care must be exercised in thepreparation of the calibration standard to ensure thatthe amount encapsulated of total plutonium, Pu-239,and the amount of Amn-241, is known. It is important tomeasure the gamma ray attenuation 'through theencapsulating material and correct the calibrationstandard response to compensate for that attenuation.The amount of plutonium encapsulated in 'the gammaray calibration standard is selected to be representativeof typical accumulations.2.2 Neutron AssayNeutrons are emitted in the spontaneous fissionof Pu-238, Pu-240, and Pu-242 and through theinteraction of emitted a particles with certain lightnuclei. These neutrons suffer little attenuation in passingthrough uranium or plutonium or through moststructural and containment materials. Glove boxwindows may reduce the energy of emerging neutrons,but because of their regular and constant shape, theireffect can generally be factored into the assaycalibration.To be useful for the assay of plutonium holdup,the neutron production rate per gram of plutonium mustbe known. The spontaneous fission contribution to thetotal neutron production can be computed from basicnuclear data, once the isotopic composition of thecontained plutonium has been determined. Computingthe (an) contribution requires a knowledge of thechemical form of the plutonium and the amount and,distribution of certain high (an) yield target materials.The background count rate from neutrondetectors may be a substantial part of the observedactivity, often corresponding to as much as 20 g ofplutonium in typical holdup assays. Thus, neutron assayis primarily applicable to the measurement of significantaccumulations of plutonium.The measured neutron yield from preparedcalibration standards is used to calibrate each neutronassay collection zone. In the Appendix, a method isgiven to calculate the anticipated neutron yield. Thismethod provides the ability to calculate the neutronyield when the isotopic or impurity composition of theplutonium holdup is different from that of the5.23-3 calibration standards. The method can be used tocalculate a ratio of the neutron production rate of theunknown material to the standard material neutronproduction rate. The yield from the holdup material isthen determined by multiplying the measured "known"material yield by the computed ratio.2.2.1 Neutron Detection InstrumentsTo effectively employ the spontaneousneutron yield as a measure of plutonium holdup, it isnecessary to detect the neutrons in the vresence of amore intense gamma ray background and to collimatethe detector so that the only neutrons being counted areemanating from the collection zone under assay._,Holdup assay -is performed under in-plantconditions where ruggedness, high detection efficiency,and high (-y,n) rejection, performance in the detectors isimportant. He-3 has one advantage over BF3 detector*tubes in that the operating voltage for He-3' tubes doesnot increase as rapidly with increased gas pressure.To increase the efficiency of the system,detector gas pressure in the tubes may be increased ormultiple detectors can be connected in parallel to feed acommon preamplifier.He-3 and BF3 detectors have efficiencieswhich increase as 'the energy of the neutrons decrease.To take advantage of this characteristic, the detectorscan be surrounded by a 'neutron moderating material(see Figure B1I). Polyethylene is recommended. Thethickness of the moderator is important. When themoderating distance is short, a fraction of the higherenergy neutrons pass through the gas chamber withoutbeing detected. Conversely, when the moderatingdistance is too long, a substantial number of low-energyneutrons are absorbed by the hydrogen contained in themoderator. A balance -between these, two effects isreached when -the spacing between adjacent tubes isapproximately one-inch of polyethylene, and the -spacingbetween the front of the unit and the detectors and theback of the unit and the detectors is approximately I1%inch when one-inch-diameter tubes are used, andapproximately one inch when two-inch-diameter tubesare used.'To -shield the detector, from low-energyneutrons which may produce a complicated responsepattern, the moderator material is covered .with athermal neutron absorber. Cadmium sheeting approxi-mately .0.075 cm thick can be used for this application.by stopping neutrons coming to the detector from alldirections --except the .-.desired one. The cadmiumsurrounding the detector will...stop essentially allneutrons striking, the, detector with energies below 0.4eV. By adding moderator material around the.outside ofthe,.<detector in -all -directions except .for the collimatorchannel, neutrons, coming from unwanted directions willlose energy~in 'this shield and will be absorbed in the Cdcover. For each six inches of polyethylene added, thecollimator assembly provides a factor of approximatelyten: in -the directionality of the response. An example ofa collimated ,neutron detector assembly for plutoniumholdup assay is shownin Figure B-I.The weight of the .combined detector andcollimator assembly. can easily exceed requirements for ahand-held detector probe.4 For this reason, and toprovide for reproducible positioning at each assay, asturdy cart housing both the detector/collimator and theassociated-'electronics is recommended; Further, as theitems to be assayed will be at different, heights, theability to raise .and lower. the assembly to reproduciblesettings is ,recommended to expedite the assay andreduce the possibility of errors.2.2.3 Check Source for Neutron AssayTo ensure the proper operation of theneutron assay system prior to making an assay, it isnecessary to test the response of the instrument. Anappropriate , neutron .assay check source can bemeasured, Por one or more :collection zones can berecalibrated and compared to the results of previouscalibrations.An appropriate neutron assay check sourcecan be prepared by implanting a small encapsulatedplutonium source (containing about 5 g Pu).into the faceof a plug. of neutron moderating material .(see FigureB-2). The plug is .fabricated to fit and close thecollimator channel.-2.2.4 Calibration Source for, Neutron AssayTo calibrate a neutron assay collectionzone, the observed response is compared to the responseobtained when the zone contains an additional knownamount of plutonium. Neutron assay is less sensitive toattenuation than.is.,gamma ray assay.. It is important toknow how:..much plutonium is dencapsulated in theneutron assay calibration standard, and the isotopiccomposition of that plutonium.The spontaneous neutron production ratefrom typical reactor plutonium is significantly less thanthe production rate of 375440 keV gamma rays. Toprovide. ,.an adequate response for calibration, it istherefore necessary, to encapsulate a larger amount ofplutonium. in the neutron assay calibration standard.2.2.2 Collimators for Neutron DetectorsTo assay a specific collection zone in thepresence of other distributed sources of plutonium, it isnecessary to collimate the detector. This is accomplished5.23-4 COLLIMATED NEUTRON DETECTOR ASSEMBLY FOR PLUTONIUM HOLDUP ASSAYDETECTOR CABLE ACCESS CHANNEL(TOP SECTION ONLY) --.--T-17.5cmIFRONT VIEW68-5-1cm 1cmI " ITOP VIEW4.5cm.. .. .\NEUTROTUBE CI I 2.6cm DIAI II I I " ,I .I I II I I I I~I I I I !I I I I' I II I I I I t l I.I II I II .1 FRONT¢r 39cm I II I III I I I *I I I I II I II I..1 I I I I I.I I I I I II II -II I I "I I lIl,I I 11I II 1 14.lPOLYETHYLENE BLOCK, COVERED ALL SIDES'WITH 0.0756m CADMIUM SHEETVIEWFIGURE B-1N DETECTORHANNELSMETERý ITYP)DETECTOR TUBE SUBASSEMBLYNEUTRON 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 POLYETHYLENECAN BE ADDED TO IMPROVE DIRECTIONALITY Io.p., POLYETHYLENE PROVIDES~100:1DIRECTIONALITY). COMPONENTS ARE BOLTED OR STRAPPED TO REMAIN IN A FIXED CONFIGURATION.5.23-5 MODERATOR\NEUTRON COLLIMATOR/CHANNEL PLUG1 CHECK SOURCETOP' VIEWCHECK SOURCECOVERCHECK SOURCEFRONT VIEWFIGURE B-2 NEUTRON COLLIMATOR CHANNELPLUG AND CHECK SOURCEWhile the amount needed is best determined through anevaluation of typical accumulations, 100 g Pu isadequate for most applications.The neutron assay calibration standardmay generate more neutrons than directly attributableto the spontaneous fission and (an) reactions. Because arelatively large quantity of PuO2 is encapsulated in theneutron assay calibration standard, some of thespontaneous fission or (an) neutrons may be absorbed inPu-239 or Pu-241 nuclei, producing additional neutronsthrough the induced fission reaction. The amount ofmultiplication depends in a complex manner on theamount and distribution of PuO2 and on the surroundingmedium. The potentially significant calibration errorarising by having too large a neutron yield per gram ofplutonium will be corrected in the long term throughassay verification tests. In the initial phase of assayingholdup, a rough correction for this yield can bemeasured by preparing two additional PuO2 sourcescontaining 1/3 and 2/3 of the neutron assay calibrationstandard mass. These samples need not be encapsulated,as they will be measured only once and can then bereturned to the process stream.The PUO2 used in this test is taken fromthe same batch used to prepare the neutron assaycalibration standard. After weighing out the properquantities, the PuO2is put into containers having closeto the same geometry as found in the neutron assaycalibration standard. Each test sample is transferred toan empty glove box and positioned next to the windowfor measurement. The neutron assay probe is positionedas close as possible to the sample but outside the glovebox. After the measurement is made, that sample istransferred from the glove box and the next sample istransferred in and positioned in the identical location formeasurement. A plot of counts minus background as afunction of PuO2 mass is made and the points visuallyfitted using a French curve. If there is no multiplication,a straight line can be drawn through the. originconnecting all points. Multiplication is indicated whenthe curve turns upward, indicating an increase in countsper gram as the mass of PuO2 increases. A correctionterm is obtained by determining the increase in countsper gram at the mass value corresponding to the neutronassay calibration standard mass. This increase is readilydetermined by plotting the straight line through theorigin and the lowest mass sample response and readingthe difference in counts between the two lines at theabscissa coordinate corresponding to the neutron assaycalibration standard mass. All measurements relating tothis standaid are thereafter reduced by the ratio of thedifference in counts to the observed counts.3. Isolation of Collection ZonesTo ensure that each collection zone isindependently assayed, it is necessary to screen allradiations from the detector except those radiationsemanating from the collection zone being assayed. Thisis principally accomplished through the use of thecollimators described in Sections B.2.1.2 and B.2.2.2.Two additional means exist to further isolate acollection zone.3.1 Detector PositioningAn unobstructed side view of a collection zoneis preferred. When plutonium is located behind the zoneunder assay in another collection *zone or a storagefacility, either consider positioning the detector above orbelow the collection zone, or consider the use of shadowshielding.3.2 Shadow ShieldingIt may not be possible to avoid interferingradiations through the collimator design or throughchoosing the detector position for assay. In such cases, itmay be possible to move a shield panel between thesource of interfering radiations and the collimator zoneunder assay. If the shield panel is very thick and itsdimensions match or exceed the back side of thecollection zone under assay, no interfering radiationswill penetrate through the shadow shield to the detector.While such characteristics are desirable, the size of such ashield would limit its transportability. A rectangular5.23-6 panel containing -5 cm of neutron moderator (e.g.,benelex, WEP, or polyethylene) and -0.5 cm lead sheetis recommended, mounted on wheels as an uprightpanel. To use such a panel, two measurements arerequired.* --R1 -Rcz + Rlnterference (1)R2 = Rcz + TRinterference (2)whereR1 is the assay response obtained before the shadowshield is moved into position,R2 is the assay response obtained with the shadowshield in position,Rez is the response component attributable to thecollection zone under assay,Rinterference is the response componentattributable to the interfering radiations, andT is the transmission through the shadow shield.Note that T represents a measured transmission-T.rfor gamma rays or Tn for neutrons. Ty. and Tn aremeasured by counting radiations from any arbitrarysource of plutonium with the shield between thesource and detector and again with the shadowshield removed:T = (R.?,) shield in/(R,,) shield out (3)T, = (Rn) shield in/(Rn) shield out (4)To correct for the interference, subtract R2 from R1,and solve for 'Rlnterference:(R2 -R) ("Rlnterference (1 -T) " (5)To ensure that this correction is sufficiently accurate, itmay be necessary to extend the length of the normalcounting period .to accumulate sufficient countingstatistics (1% statistics are generally adequate for thisapplication).4. Calibration of Collection ZonesEuch collection zone is independently calibrated, ashackground-lfaclors and the of each zonevary widely from zone to zone. A collection zone is bestcalihlaled throngh the in situ measurementi of' knownt'iilih)lU n i lanltdads. When such a program is notpiositlve,. Ihli callibration can it , based on the calculalionof Ithe anticipated response or through measuring amockup ot the collection zone of interest.5*Response tierms refer to neutron or gamma response, asappropriate.The calibration obtained through this procedure isrecommended until a history of comparisons betweenpredicted and recovered holdup quantities is developed,as described in Section B.5 of this~guide.4.1 Detector PositioningTo calibrate each collection zone, the bestposition or series of positions is selected to observe thecollection zone with .the least amount of interferencefrom principal structural components. It is important toview the collection zone with the detector locatedbetween the collection zone and all areas used for Pustorage during inventory. A three-dimensional approachcan be investigated, positioning the detector on top of orbelow the collection zone if it is not possible to have anunobstructed, interference-free side view of thecollection zone. The use of shadow 'shielding can beexplored if it is not possible to get a clear view of eachcollection zone for assay.On the basis of a detailed examination of thephysical layout of the facility, some preliminarymeasurements are made to determine optimum detectorpositions for holdup assay. Once the assay positions forthe detector and shadow shields -are established,permanently marking the assay positions will. facilitatesubsequent measurements.4.2 Calibration SourcesSince this assay is to measure the amount ofplutonium holdup, it is appropriate to use plutonium asthe calibration standard material. Further, as theplutonium holdup will generally be distributed over alarge surface area, it is recommended that the gamma raycalibration standard be fabricated to resemble thischaracteristic, as described in Section B.2 of this guide.4.3 Calibration ProceduresOnce the principal items containing plutoniumhave been removed and the detector located in its assayposition, the response from a calibration standardcombined with the plutonium already held up isobtained. When the collection zone is appropriatelyisolated, two factors influence the observed responsefrom the calibration standard:I. the location of the calibration standard within thecollection zone, and2. the shielding of radiations from the calibrationstandard caused by the items comprising thecollection zone.The gcomelric response variation is measuredby observing lie response from-one calibration standardwith the other standard removed from the collectionzone under investigation. The calibration standardresponse is measured with the standard positioned invarious parts of the collection zone. avoiding internalitems which may attenuate the radiation emanating .fromthe standard.5.23-7 When neutron assay is employed or when thecollection zone consists of a hollow box, pipe, or duct,attenuation is either relatively uniform or negligiblysmall. The calibration of each collection zone thenbecomes a matter of appropriately averaging thegeometric response variations. The average response ofthe entire collection zone is assumed to properlyrepresent that zone. -'If, however, it is known thatplutonium accumulates in one particular location withina collection 'zone, the response of the standard isemphasized when located near the principal collectionsite.If the item to be assayed consists of a largeunit, assay performance may be enhanced by subdividingthe unit into smaller contiguous measurement zones.The repeat dimensions of the subzones are determinedby measuring the rcsponse while moving the standardalong an axis perpendicular to the detector centerline.By studying the response curve, the distance D isselected as the point beyond which sufficient activity isdetected to flatten the response within the subzone.Each subzone will measure 2D across its face. Anexample is illustrated 'in Figure B-3. As the responseabout the centerline is assumed to be symmetrical, onlyhalf of the traverse is indicated. In Figure B-3, D isselected such that the area under the curve to the rightof D is approximately equal to the area above the curveto the leftof D (Area A1 = Area A2). Note: the distancefrom the collection zone to the detector or the distancefrom the crystal face to the end of the collimator, orboth, can be varied to divide the collection zone into anintegral number of subzones.AREA A1100,S A MEASUREMENT DATA POINTSVISUAL FIT TO MEASUREMENT DATAAS RA .To use this relationship, the detector is firstpositioned at point d and a reading is taken. Point d isthe center of the first subzone, selected to coincide withthe physical edge of the calibration' zone. The detector isthen moved a distance 2D along the traverse to thecenter point of the second subzone, and the secondmeasurement taken. The cycle*is repeated to include allof the larger collection zone. The value interpreted forcalibration for each subzone *.corresponds to themaximum of the traverse across each subzone becausethe response has been flattened. The content of theentire collection zone is the sum of the contributionsfrom the subzones.5. Estimation of the Holdup ErrorThe overall uncertainty associated with themeasured plutonium holdup is due to (1) theuncertainty in. the observed response and (2) theuncertainty in the interpretation of that response. Therandom uncertainty components in this application are.frequently negligible in comparison with the geometricuncertainty and the uncertainty in the isotopiccomposition. In this assay application, it is appropriateto estimate the assay error components by assuming themeasured range -(Ri) of the ith fluctuation constitutes aninterval four standard deviations wide. The midpoint ofthe range estimates the mean effect, and the' distancefrom the "midpoint to each extreme comprises anestimated 95% confidence interval. 'The errorattributable to this effect is then approximately2=(R) 2(6)If a.severe effect is~noted, the response can often becorrected for the variation in the correspondingparameter by measuring the. value of that particularparameter at the time of the assay. Using a measuredrelationship between the response and the value of thatparameter, the observed response is corrected.5.1 Response Uncertainties'5.1.1 Counting StatisticsThe magnitude of the uncertaintiesattributable to variations in the geometric distributionand in the attenuation of the radiations are expected todominate the total 'response uncertainty. 'The relativestandard deviation due to counting statistics can usuallybe made as small as desired through '(I) using moreefficient detectors or (2) extending the counting period.Having 1000 to't0,000 net counts is generally sufficientfor most holdup assay applications.5.1.2 Instrument Instabilities-Fluctuations in ambient temperature,humidity, electronic noise, and line voltage (forAREA A20 25 5D 75 100DISTANCE FROM DETECTOR CENTERLINE TO POINT SOURCE, CENTIMETERSFIGURE B-3 EQUIVALENT DIAMETER SUSZONE TO ACHtEVE A FLATPLANAR RESPONSE. SELECT D SUCH THAT AREA A1= A2.5.23-8 non-battery-powered electronic units) generally affectthe stability of electronic systems. The magnitude of thisuncertainty can be estimated by monitoring the checkstandard response and determining the range ofvariability as described in Section B.5 of this guide,5.1.3 Geometric UncertaintyThe geometrical variation in the observedresponse is measured by moving the calibration sourcewithin the bounds of each collection .zone. Two -cases aredescribed below.5.1.3.1 Isolated Collection ZonesWhen a single unit comprises acollection zone, the standard is moved .to all .sites withinthe zone at which an accumulation of plutonium mightoccur. With sufficient collimation, the response for. thecollection zone under investigation is independent of itsneighbor zones. The average of the response, weighted toreflect 'prejudgments on the likelihood of accumulationsites, is then used as the calibration point. As shown inSection B.5, the range of values can be assumed tocomprise an expectation interval four standarddeviations wide. The geometric error is then estimatedusing Equation 6.5.1.3.2 Overlapping Collection ZonesWhen a collection zone is subdividedinto overlapping subzones, the geometric uncertaintydue to the dimension perpendicular to the detectorcollection zone centerline is eliminated through thearea-averaging calibration method described in Section4.3.The uncertainty in the depthdimension in each subzone can be determined throughthe procedure outlined 'for isolated collection zones.Judgment can be used to weight the calibrationdata toemphasize principal accumulation sites.5.1.4 Attenuation UncertaintyIf the attenuation is not extreme, it can bemeasured in situ, mocked up, or computed for thedifferent conditions encountered. The worst and bestcases can be assumed 'to determine the range ofpermissible effects. Using Equation 6, the magnitude ofIhis uncertainty component can then be estimated.Again, judgmaent is appropriate to weight the correctionfactor.5.2 Interpretation UncertaintiesTwo factors are central to the issue here,assuming that the calibration standard material is similarto the held-up material.5.2.1 Interfering Radiations5.2.1.1 Gamma Ray AssayAn uncertainty in the observed gammaray response may arise due to the presence of extraneousgamma ray emitters or due to fluctuations in thebackground from the' Compmon scattering ofhigher-energy gamma rays. The shape of the backgroundgamma ray spectrum may change in such cases to suchan extent that even with the energy windows stabilized.the background correction is irregular and uncertain.The magnitude of this effect is generally small. It can be.monitored by observing the spectrum with amultichannel analyzer, but unless the data onperiodically recovered 'holdup accumulations are inerror, this contribution can be ignored.5.2.1.2 Neutron AssayA change in the neutron yield for aplutonium sample of fixed isotopic content is primarilyattributable to the fluctuation in the concentration ofhigh (an) yield impurities.* Judgment can be used todetermine the range *of permissible impurityconcentrations. The variation in a typical neutron yieldcan then be predicted using the methods discussed in theAppendix of this guide. Again, the range of permissiblevariations is assumed to constitute an acceptance intervalfrom which the component error is computed usingEquation 6.5.2.2 Isotopic UncertaintiesIf the process equipment is cleaned eachtime the isotopic composition of the plutonium feed isvaried, the holdup will consist primarily of the currentmaterial. New calibration standards can be prepared orthe previous yield data can be normalized using themethods presented in tht Appendix to correct tor effect. When mixing occurs, use of the stream-averagedisotopic composition is appropriate. The uncertaintybounds are estimated by considering the highest .,idlowest fissile isotopic batches and computing thecorresponding range.5.3 Holdup and Its Associated ErrorThe amount of Pu holdup can be ,measuredthrough the systematic application of the programdeveloped in conjunction with the principles and pitfallsdiscussed herein. For each collection zone, measuredholdup and its error can be determined.*Over a long period of time the a-particle production ratcincreases due to the ingrowth of Am-24 1.5.23-9 5.3.1 Initial OperationsDuring the initial phase of operations, theerror associated with the in situ assay of plutoniumholdup is estimated by combining the component errorsdetermined in the preceding sections of this guide (B3.5.1and B.5.2).5.3.2 Routine OperationsTo ensure the validity of assay predictionsand to more realistically estimate the uncertainty inthose predictions, it is necessary to establish a programto measure, the amount of plutonium recovered when acollection zone is cleaned out. By comparing the aniountof plutonium recovered to the recovery amountpredicted, the collection zone calibration can be updatedand the assay error can be based on relevant verificationtests.The update data is computed as thedifference in the assays before and after cleanout:(PU)assay = Rbefore -Rafter , (7)The difference.(A) in assay and recovery,A = (PII)assay -(Pu)recovery (8)is then computed.The standard deviation in the A values (s.)is computed separately for" each collection zone,including no more than the twelve precedingmeasurement tests:sA (K- 1 (9)When a value of A is determined, it is usedto update the estimate sb. The -standard, deviationestimate s. can be used to estimate the~error in. the assayprediction for the collection zone for which it has beenestablished.The amount of plutonium collected duringthe cleanout of a specific collection zone can be assayedthrough sampling and chemical analysis, throughcalorimetry, or through other applicable nondestructiveassay methods (eg.,. spontaneous fission coincidencedetection or gamma ray assay). Each of these topics isthe subject of a Regulatory Guide.

C. REGULATORY POSITION

To develop a program for the periodic in situ assayof plutonium residual holdup as an acceptablemeasurement method for this inventory component, it isnecessary to consider -the -factors' in -the -followingsections.Note: Care must be exercised during the fabrication anduse of check sources and-calibration. standards to ensuretheir continued integrity and to prevent contamination.4. Delineation of Assay Collection ZonesA plan of each plutonium processing facility shouldbe examined.,to establish, independent collection zones.Individual glove boxes and similar containmentstructures should be so-identified.. Using the layout andtouring the facility, -an. assay. site(s) for. each collectionzone should be selected:1. Assay site(s) -should afford a clear, unobstructedview ,9f-the collection zone with no other collection or....storage. areas in_ the line- of sight of the .collimatorassembly. Location of the. detector probe above orbelow, the -collection zone- should be considered if anunobstructed side , view is not possible. If an-unobstructed view is not, possible, shadow, shieldingshould be used to isolate the collection zone, for assay.2. The assay site should be set back as far as possiblefrom each collection zone to reach a compromisebetween interference from neighbor zones and efficientcounting..3.' Gamma ray assay should ,be applied to measure theplutonium- held up in all collection. zones containing lessthan the neutron- detection limit- and for' singlecontainment structures which do not contain irregularlyshaped structural components capable of significantlythe emerging gamma rays. Neutron assayshould: be applied to measure the accumulation ofplutonium holdup in all structures not suitable, forgamma ray assay. -.4. Each collection zone should be uniquely numbered.(Neutron collection zones could be preceded by an "N",gamma ray collection zones by a "G". Subzones shouldbe identified by an alphabetic suffix to the collection-zone identification.) ..5. Each assay site should be' marked with paint orcolored tape on the floor. (To be consistent, blue tapeshould be used for neutron assay sites, orange for gammaray sites.) The height setting for midpoint assay shouldbe recorded in the measurement log corresponding toeach assay site....2.., Assay InstrumentsNeutron and gamma ray assay capability should beprovided using separate or compatible' electronics withinterchangeable detector probes. Compatible electronics5.23-10 should provide for both He-3 or BF3 neutron detectionand Nal(TI) gamma ray detection. The electronics unitshould have a temperature coefficient of less than 0.1%per 'C. Battery-powered electronics should be providedto expedite assays.2.1 Gamma Ray AssayGamma ray assay should be 'based on theactivity observed in the energy range from 375 keV to440 keV, excluding the composite gamma ray complexcentered at 333 keV. Yield data for appropriate gammarays are presented in Section B.2.1 of this guide.2.1.1 Detector SelectionGamma ray detectors should have FWHMresolution equal to or better than 7.5% at 662 keV(Cs- 137 gamma ray). NaI(TI) can meet suchspecifications and is suitable for this application. Thecrystal depth should be sufficient to detect a significantpercentage of 400-keV gamma rays. For NaI(TI), theminimum depth should be one inch. A two-inch depth isrecommended.The crystal should be stabilized with asuitable radioactive source. An'ý internal Cs] seedcontaining Am-241 is recommended for this application.The electronics should be capable of stabilizing on thereference radiation emitted by the seed. The crystal face(external to the cover) should be covered with 0.075 to0.150 cm cadmium sheet to filter low-energy radiations.Two single-channel analyzers should be.provided with lock-set energy windows. One channelshould be set to admit gamma rays from 390 keV to 440keV unless equilibrium of the U-237 and Pu-241 can beassured. The 333-keV region of the gamma ray spectrumshould be excluded. With Nal detectors, it is necessaryto exclude the 375 keV gamma ray to ensure that thetail from the 333 keV complex is not added. The secondchannel should be set above the first window to providea background correction for the assay window. Thissecond window should be set from approximately 450keV to 600 keV.2.1.2 Gamma Ray CollimatorA cylinder of shielding material such aslead should be made c(ncentric with the gamma raydetector. The end of the cylinder opposite the crystalshould be blocked with the shielding material. Thethickness of the collimator should -be chosen to providesufficient directionality for the specific facility (1.5 cmof lead thickness should be sufficient for mostapplications). The collimator sleeve should be extendibleover the end of the crystal to reproducible settings tovary the degree of collimation for different collectionzones.2.1.3 Gamma Ray Check SourceTo ensure the continued normal operationof each system, an encapsulated plutonium check sourceshould be provided. The source should be small enoughto be implanted in a section of shielding material soshaped as to close off the collimator opening. The checksource should be positioned adjacent to the detector.The source should contain an amount of plutoniumsufficient to provide a gross count rate of 1000 to10,000 counts per second.2.1.4 Gamma Ray Calibration SourceTo permit the calibration of gamma .rayassay collection zones, a calibration standard should befabricated by encapsulating plutonium oxide in a disk.The isotopic composition of the plutonium and theabundance of Am-241 should be measured and bechosen to be nominally representative of the plutoniumbeing processed. The total amount of plutoniumencapsulated should be closely monitored. Attenuationlosses within the bed of PuO2 and through theencapsulating material should be measured and thecalibration standard response normalized to counts pergram incorporating these corrections.2.2 Neutron Assay2.2.1 Neutron Detector SelectionNeutron detectors should have highdetection efficiency and be capable of operating in thepresence of gamma radiation. He-3 and BF3 neutrondetectors are recommended for this application. Multipledetector tubes with matdhed operating performanceshould be connected in parallel to a single preamplifierto increase the overall detection efficiency obtainablefrom a single detector tube. Neutron detectors should besurrounded by a layer of neutron moderator material toenhance their detection efficiency. The neutronmoderator layer should be covered with a low-energyneutron absorber to filter out extraneous neutrons fromthe desired signal. A recommended configuration isdiagrammed in Figure B-I.2.2.2 Neutron CollimatorA slab collimator or concentric cylindercollimator of a suitable neutron moderator material(e.g., polyethylene) should be constructed to completelysurround the detector with its associated moderator andfilter assembly, 'leaving open orly the collimatorchannel. A recommended 'configuration is shown inFigure B-1.The moderator thickness should beselected to provide. the directionality required for eachfacility. A directionality profile providing a 10:15.23-11 response ratio (six inches of polyethylene) should beadequate for most applications; however, each situationshould be evaluated as discussed in Part B of this guide.2.2.3 !NeutronCheck SourceAny neutron source which emitsapproximately 100-10,000 neutrons/second isacceptable for this application. The source should besmall enough to be contained within a section of,neutron moderator material so shaped as to completelyfill the collimator channel of the detector assembly. Thesource should be implanted ,directly adjacent to theneutron detectors, outside the cadmium thermal neutronfilter. A recommended configuration for this assembly isdiagrammed in Figure B.2.2.2.4 Neutron Assay Calibration StandardTo permit the, calibration of neutron assay* collection zones, a calibration standard should be-fabricated by encapsulating PuO2.The PuO2 should benominally representative of the plutonium beingprocessed in isotopic composition, in Am-241 content,"and in the content of high (a,n) yield target materials.The amount of plutonium to be encapsulated should bechosen to be representative of the amounts of plutoniumestimated to be held up in typical neutron assaycollection zones.'The' neutron yield of the calibrationstandard should be measured and also computed usingthe. method described in the Appendix. The observedneutron. count rate should be normalized.6 If the,predicted response differs by more than 10%76, theresponse should be normalized as discussed in SectionB.2.2.4.2.3 Service CartA cart carrying electronics and both detectorprobes should be provided. The capability to raise orlower the probes to reproducible settings should beincluded.2.4 Notation of Operating ParametersWhen compatible electronics are used tofacilitate neutron and gamma ray assay, a notation ofathe respective settings should be affixed to theelectronics unit. To decrease the likelihood of incorrectsettings, the neutron probe and the -appropriateelectronics settings should be color-coded blue; thegamma ray probe and :corresponding electronics settingsshould be coded orange.3. CalibrationEach collection zone should be independentlycalibrated when all in-process material has been locatedso that the response from the calibration standards willnot be influenced by the in-process material.3.1 Instrument CheckThe stability of the neutron and gamma raydetection systems should be tested prior to eachinventory by comparing the observed counts obtainedfrom the check source, minus the counts with theshaped shield in place but without the check source, tothe readings obtained prior to previous inventories. Ifthe measurement is consistent with previous data (i.e., iswithin plus or minus two single-measurement standarddeviations of the mean value of previous data), allpreviously established calibrations using this detectionsystem should be considered valid. If the measurement isnot consistent, the operation of the ..unit should bechecked against the manufacturer's recommendationsand repaired or recalibrated, as required.3.2 Zone CalibrationThe geometric response profile for eachcollection zone should be determined by measuring thevariation in the response as a calibration standard ismoved within the defined limits of the collection zone.The. response variation should then be averaged todetermine the response per gram of plutonium for thatcollection zone. The averaging should be weighted toreflect known local accumulation sites within eachcollection-zone. The response per gram should be usedto directly translate the observed response to grams oiplutonium, after the response is corrected forbackground.3.2.1 Subzone CalibrationWhen a collection zone is too large to beaccurately measured in a single assay, the collection zoneshould be divided into overlapping subzones. The repeatdimensions of each subzone perpendicular to thedetector-to-collection-zone line should be determined sothat the response variation across that distance is nulled.Using this procedure, the residual geometric uncertaintyshould be determined by measuring the response as acalibration standard is moved along the depthcoordinate. The calibrated response should then reflectthe average of the depth response, weighted to reflectknown accumulation sites.4. Asmy Procedures4.1 Ammy LoAAn assay log should be maintained. Eachcollection zone or subzone should have a separate pagein the amy log, with the corresponding calibrationderived on the page facing the assay data sheet.Recording space should be provided for the date of5.23-112 measurement, gross counts, corrected counts, and thecorresponding grams plutonium from the calibration inaddition to position and instrument electronic settingverification.4.2 Preassay ProceduresPrior to inventory, the isotopic composition ofthe plutonium processed during the current operationalperiod should be determined. Variations in the neutronand gamma ray yield data from the calibration standardshould be calculated. Either the calibration data or thepredicted holdup should then be corrected to reflect thisdifference.Prior to each inventory, the operation of theneutron and gamma ray assay detection systems shouldbe checked.Prior to any assay measurements, feed into theprocess line should be stopped. All in-process materialshould be processed through to forms amenable toaccurate accountability. All process, scrap, and wasteitems containing plutonium should be removed from theprocess areas to approved storage areas to minimizebackground radiations.4.3 MeasurementsThe assay cart should:be moved in sequence tothe assay site(s) corresponding to each collection zone.Assaying all gamma ray sites before assaying neutronsites (or Vice versa) is recommended.Before assaying each collection zone, theoperator should verify the floor location, probeselection, probe height, and electronics settings. Allcheck and calibration sources should be sufficientlyremoved so as not to interfere with the measurement.Prior to taking a measurement, a visual check of thezone and the line of sight of the detector probe shouldbe made to assure that no obvious changes have beenmade to the process area and that no unintendedaccumulations of plutonium remain within thecollection zone. The operator should initial themeasurement log to assure conmpliance for eachcollection zone.Having met all preceding requirements, themeasurement at each site should be taken, recorded, andconverted to grams plutonium. If each value is within anexpected or permissible range, -the cart should be movedto the next site and the cycle repeated. If a highresponse is noted, the cause should be investigated. Ifthe collection zone contains an unexpectedly largecontent of plutonium, that collection zone should becleaned to remove the accumulation for conversion to amore accurately accountable material category. Afterthe cleanout has been completed, the zone should bereassayed and the recovered material quantity used totest the validity of the zone calibration.5. Estimation of the Holdup ErrorDuring the initial implementation of this program,the error quoted for the holdup. assay should becomputed on the basis of estimating the errorcomponents, as described in SectionsB.5.1 and B.5.2.Prior to the cleanout of any collection zone forwhatever purpose, that zone should be prepared forassay and measured as described in:Section C.4 of thisguide. Following this assay, the collection zone shouldbe cleaned out and the collected plutonium should thenbe assayed using an appropriately accurate assaymethod. When the collection zone has been cleaned andthe collected plutonium removed,, the collection zoneshould be reassayed. The recovered plutonium should beused to update the calibration and,. from the. sixth teston, should serve as the assay error estimate. Separaterecords should be maintained for each collection zone toestimate the error in assaying the plutonium holdup.To ensure that error predictions remain current,only data of the twelve preceding independent testsshould be used to estimate the assay error. Collectionzones not cleaned for other purposes should be cleanedfor assay verification at intervals not to exceed twomonths.REFERENCES1. R. Gunnink and R. J. Morrow, "Gamma RayEnergies and Absolute Branching Intensities for238,239,240,241Pu and 241Am," UCRL,51087(July 1971).2. J. E. Cline, R. J. Gehrke, and L. D. Mclsaac,"Gamma Rays Emitted by the Fissionable Nuclidesand Associated Isotopes," ANCR-1069 (July 1972).3. L. A. Kull, "Catalogue of Nuclear MaterialSafeguards Instruments," BNL-17165 (August1972).4. An example of a collimator for uranium gamma rayassay is found in R. B. Walton, et al, "Measurementsof UF6 Cylinders with Portable Instruments," Nucl.Technol., 21, 133 (1974).5. W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "AMethod for Surveying for Uranium-235 with Limitof Error Analysis," Gulf-GA-A12641 (June 1973).5.23-13 APPENDIXNEUTRON YIELD COMPUTATIONSThe following model for the calculation of the totalspontaneous neutron yield from plutonium-bearingmaterials assumes that the plutonium is widelydispersed. With this condition, there will be nosignificant neutron production created through inducedfission of Pu-239 or Pu-241. The total neutron yield pergram of plutonium holdup will then be the sum of thespontaneous fission and (an) contributions:Yn = YSF + Y(,t,n) (1)1. Spontaneous Fission NeutronsTo determine the spontaneous neutron yield pergram of plutonium held up within a collection zone, theisotopic composition of the plutonium and uranium (ifpresent) must be known. The contribution fromspontaneous fission can generally be calculated byneglecting the contribution from U-238:YSF = W238Q238 + W240Q240 + W242Q242(2)whereWi = weight fraction of the ith plutoniumisotope. For reactor fuel applications, W238 + W239+ IQi = spontaneous fission neutron yield per gram ofthe ith plutonium isotope (see Table 1).2. (a,n) NeutronsThe maior contribution to the total neutronproduction from (ax) reactions will typically be due tothe 048 (an) Ne-21 reaction when the plutonium existsas the oxide. The yield from this reaction per gram ofplutonium can be calculated using the isotopicweight fractions (Wj) and the Yi yield data given inTable 1.Y(an) Oxy WiYi(3)The yield per gram of Put2 is calculated bymultiplying the yield per gram of plutonium by thegravimetric dilution factor (Pu/PuO2 -0.882).The presence of certain impurities can contributesubstantially to the total (atn) production rate.Approximate values of (a,n) impurity yields for thehighest yield (an) target materials are given in Table 2.To compute the impurity (an) contribution, the total aparticle production is determined. Production rates of aparticles per gram of the principal nuclides of interestare shown in Table 1. This contribution to the totalneutron yield can be computed using the relationship:Y(a,n) Impurity = Y0 TPjlji(4)TABLE 1a Particle and Spontaneous Fission Neutron YieldsHalf-life Alpha Activity PuO2(mn) Yield8 Spontaneous FissionNuclide (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 103Pu-239 24,150 2.30 x 109 54.5 2.22 x 10-2Pu-240 6,529 8.43 x 109 202.1 1.03,x 103Pu-241 14.35b 9.39 x 10' 2.03 2.43 x 10-2Pu-242 379,000 1.44 x 108 3.13 1.75 x 103Am-241 433.8 1.27 x 1011 3.46 x 103 6.05 x 10-1U-234 2.47 x 105 2.29 x 108 4.65 5.67 x 10-3U-235 7.1 x 108 7.93 x 104 1.37 x 10-3 5.96 x 10-4U-238 4.51 x 109 1.23 x 104 1.93 x 10--4 !.12 x 10-2a -Oxygen yield from PuO2 form only.b -&-branching ratio -2.46 x 10-55.23-14 whereY,, = total a production= WWiai + WArnm'AmiWi = Pu isotopic weight fractionsWAm = Am weight fraction = Am/Puai = a yield per gram of nuclide i (see Table 1)TABLE 2(Q,n) Yield Rats of Low-Z Impurities in Pu02aP.I mpurity (n/a-ppm)Li ......................... 6.29 x 10-12Be ......................... 2.00 x 10 -'0B .... ...................... 4.63 x 10-11C .......................... 2.77 x 10-130b ....... .................. 1.56 x b0-"13F ........................... 2 .44 x 1O-IlNa ......................... 3.00 x 10-12g........................... 2.67 x 10-12i f ......................... 1.45 x 10-12Si ......................... 3.25 x 10-13aAssumnes zero yield from all other impurities.bOxygen not contained in oxide.Pj = (an) yield per ppm of the impurity j (seeTable 2)Ii = impurity j content, expressed in ppm(weight) of plutonium.3. Sample Calculation (PuO2-UO2)Consider the case of recycle plutonium blended t63 wt %Pu in a normal U02 matrix, Where the isotopiccomposition is Pu-238 (.25%). Pu-239 (75.65%), Pu-240(18.48%), Pu-241 (4.5%), Pu-242 (1.13%), and Am-241(.28% of Pu).For mixed oxides, the oxygen density isapproximately the same for the case ofPuO.. This fact,together with the atomic similarity of uranium andplutonium, justifies the assumption that the oxygen(a,n) yield per gram of mixed oxide is the yield per gramof PuO2, further reduced by the blending ratio,Pu/(Pu + U).Using the values given in Table I, the spontaneousfission yield and total a production per gram ofplutonium can be computed. Results are shown in Table3.The a particle yield of plutonium is constant in timefor all intents. However, the Am-241 a production in-creases at a rate which results in approximately a 0.3%;.increase per month in the total a production, for therange of plutonium isotopic compositions intended forreactor fuel application.In the present example, the impurity levels of theprincipal (a.n) target materials are shown in Table 4. Theneutron yields attributable to (an) interactions on thoseTABLE 3Sample CalculationSpontaneous Fission Alpha Production PuO21a,n)aNuclide W (nsec-g Pu) (cx/sec-g Pu) (n/sec.- Pu)Pu7238 .0025 6.4 1.58 x 199 42.6Pu-239 .7565 <.05 1.74 x 109 41.3Pu-240 .1848 189.4 1.56 x l09 37.3Pu-241 .0450 <.05 4.23 x 106 0. iPu-242 .0113 19.8 1.63 x 106 <0.05Ain-241 .0028 <.05 3.56 x 108 9.7rotal Yields 215.6 5.26 x 109 131.0-- oxygen yield onl impurities are also shown in Table 4, calculated using thea particle production rate of 5.3 x 109 a/sec-g Pu,computed above. In this example, the mixed oxides arecomposed of blended PuO2 and U02 particlesapproximately 40 microns in diameter. If the particlesize were smaller or the mixed oxide was createdthrough coprecipitation, the uranium impurity contentwould also contribute to~the plutonium(an) yield. Thiscontribution can be ignored for large particles andestimated by combining the impurities for small particlesand coprecipitatedoxides.The total neutron yield in this example is 380n/sec-g Pu. In this example, the percentage of plutoniumtq the total Pu + 0 is 0.8835. Using this gravimetricdilutign factor, the neutron yield is 336 n/sec-g PuO2.Ifthe PuO2 is blended with U02 to 3%, i.e., PuO2/PuO2 +U02 = 0.03, the neutron yield. from the blend will be10.1 n/sec-g MO.TABLE 4Impurity (.,n) YieldArbitraryConcentration (a,n) YieldImpurity (ppm) Wnisec-g. Pu)Li 9 0.30Be 8 8.42B 10 2.44C 200 .30F 125 16.0OR ... 4600 3.77Na 120 1.90Total 33.1aOxygen present in moisture, not as oxide.5.23-16