Regulatory Guide 5.11
| ML13064A124 | |
| Person / Time | |
|---|---|
| Issue date: | 10/31/1973 |
| From: | US Atomic Energy Commission (AEC) |
| To: | |
| References | |
| RG-5.011 | |
| Download: ML13064A124 (16) | |
U.S. ATOMIC ENERGY COMMISSIONREGULATORY UIDEDIRECTORATE OF REGULATORY STANDARDSREGULATORY GUIDE 5.11NONDESTRUCTIVE ASSAY OF SPECIAL NUCLEAR MATERIALCONTAINED IN SCRAP AND WASTEOctober 1973USAEC REGULATORY GUIDES cap- of pabhw guide~s N.Y be oimism by r4 h~i thet divisionsduIlud ms ti US. AsooAti GEmqy Communkilon. V~misgn. D4. 211MG.RagulttwY Guidesn am mimead to dusailm and ntab. mauib to the pubmlic Attntion: DrCOMM Of 88111011100 Sanird Comnutoa sel smtor.on formethtods oamptis to the AEC Ragulatos stall of balanumntim agacifit pets of ifiwOunIMMI In 11um1.dim 879 MMURIwP1a uill i 60001 be Set IDn aam3u1"am Cornutmmon's wulpsabsorn. to delneate mhwtlqm tmd by the staff on of ter Cawrninimon. US. Aftonb Isuqi' Cowmituilan. 10wuhis~on,. C. 2111WOsaklantin med -1ic pro-iss, or postulaibi asIduI. or topn' srwlspdat to A~ttentin: Chief, PbIN smsi utalacwm. RpAgulsor Guklms am ntot usubittuga- for reptifimons an nto swith tins,,is not twosbld. Maimd An solutions diffawn Ior p tmhou. ti Tegud an ouimmmI'u us nud in him tail, Itptng Ubreas duisior:the swuidu will be a 1ap~ if: thw, 0r avdD oami for theIm widis rquuuw toto imuance or cmllim ata @a pas int or licanse by tOm Cormneislon. 1. Pows "umclon a.Prodiuo2. mi~ua5ls usos 7. TrU-WiU*Omi3. ILa andcciepsommeltss .fmdthPublubsad gua wAN he revised paviodially. asamppropruem atocrinufwcudu 4. Enabrommncatl and Skii 9. Anthust Af#osnuunt and so refbot new imiornmtuuon or moalnsL Usawkish ad Pleat PINmmettim Ia. Gae-t
TABLE OF CONTENTSPwg
A. INTRODUCTION
....................................................... 5.11-1
B. DISCUSSION
.......................................................... 5.11.11. Applicable Nondestructive Assay Principles ................................... .11.1 Passive NDA Techniques .............................................. .-11.1.1 NDA Techniques Based on Alpha Particle Decay ....................... -11.1.2 NDA Techniques Based on Gamma Ray Analysis ....................... -I1.1.3 NDA Techniques Based on Spontaneous Fission ..........................-11.2 Active NDA Techniques ............................................... -22. Factors Affecting the Response of NDA Systems ............................... -22.1 Operational Characteristics .............................................. -22.1.1 Operational Stability ............................................ -22.1.2 Geometric Detection Sensitivity ...................................... -22.1.3 Uniformity of StimulatingRadiation ........ ... ............. ........ -32.1.4 Energy of Stimulating Radiation ................................... -32.2 Response Dependence on SNM Isotopic Composition ........................ -32.2.1 Multiple Gamma Ray Sources ...................................... 32.2.2 Multiple Spontaneously Fissioning Pu Isotopes ........................ .32.2.3 Multiple Fissile Isotopes ........................................... 32.3 Response Dependence on Amount and Distribution of SNM in a Container ....... .32.3.1 Self-Absorption of the Emitted Radiation Within the SNM ............... -42.3.2 Multiplication of the Spontaneous or Induced Fission ................... .-42.3.3 Self-Shielding of the Stimulating Radiation ........................ -42.4 Response Dependence on Amount and Distribution of Extraneous Materials Withinthe Container ....................................................... -42.4.1 Interfering Radiations ............................................ -42.4.2 Interference to Stimulating Radiation ................................ -42.4.3 Attenuation of the Emitted Radiation ................................ -42.4.4 Attenuation of the Stimulating Radiation ............................. -42.5 Response Dependence on Container Dimensions and Composition .............. -52.5.1 Container Dimensions ........................................... .52.5.2 Container Structural Composition .................................. .-53. Nondestructive Assay for the Accountability of SNM Contained in Scrap and Waste .... -53.1 NDA Performance Objectives ............................................ -53.2 NDA Technique Selection ............................................. .53.2.1 Plutonium Applications .......................................... -53.2.2 Uranium Applications ............................................ -63.3 Categorization and Segregation of Scrap and Waste for NDA ................... -63.3.1 Calorim etry ................................................... -63.3.2 Neutron Measurements .............................. -63.3.3 Gamma Ray Measurements ......................................... -63.3.4 Fission Measurements ............................................ -73.4 Packaging for Nondestructive Assay ...................................... -83.5 Calibration of NDA Systems for Scrap and Waste ............................ -8iii
C. REGULATORY POSITION
................................................... 5.11-81. Analysis of Scrap and Waste .............................................. ..82. N D A Selection ......................................................... -82.1 Technique ......................................................... -82.2 System Specifications .................................................. -83. Categorization .......................................................... -114. Containers ............................................................. -114.1 Size Constraints ..................................................... -14.2 Structural Features ................................................... 14.3 Container Identification ............. .................................. -15. Packaging ............................................................. -116. Calibration ............................................................ -12REFERENCES ................................................................ 5.11-12iv NONDESTRUCTIVE ASSAY OF SPECIAL NUCLEAR MATERIALCONTAINED IN SCRAP AND WASTE
A. INTRODUCTION
Section 70.51, "Material Balance, Inventory, andRecords Requirements," of 10 CFR Part 70, "SpecialNuclear Material," requires licensees authorized topossess at a-, one time more than one effectivekilogram of special nuclear material to establish andmaintain a system of control and accountability suchthat the limit of error of any material unaccounted for(MUF), ascertained as a result of a measured materialbalance, meets established minimum standards. Theselection and proper application of an adequatemeasurement method for each of the material forms inthe fuel cycle is essential for the maintenance of thesestandards.With proper controls, licensees may select nonde-structive assay (NDA) as an alternative to traditionalmeasurement methods. This guide details proceduresacceptable to the Regulatory staff to provide aframework for the utilization of NDA in themeasurement of scrap and waste inventory componentsgenerated in conjunction with the processing of specialnuclear materials (SNM). Subsequent guides will detailprocedures specific to the application of a selectedtechnique to a particular problem.
B. DISCUSSION
1. Applicable Nondestructive Assay PrinciplesThe nondestructive assay of the SNM content ofheterogeneous material forms is achieved throughobserving either stimulated or spontaneously occurringradiations emitted from the isotopes of either plutoniumor uranium, from their radioactive decay products, orfrom some combination of these materials. The isotopiccomposition must be known to permit a conversion ofthe amount of isotope measured to the amount ofelement present in the container. Assays are performedby isolating the container of interest to permit ameasurement of its contents through a comparison withthe response observed from known calibration standards.This technology permits quantitative assays of the SNMcontent of heterogeneous materials in shortmeasurement times without sample preparation andwithout affecting the form of the material to be assayed.The proper application of this technology requires theunderstanding and control of factors influencing NDAmeasurements.1.1 Passive NDA TechniquesPassive NDA is based on observing spontaneouslyemitted radiations created through the radioactive decayOf plutonium or uranium isotopes or of their radioactivedaughters. Radiations attributable to alpha (a) particledecay, to gamma ray transitions following a and beta (6)particle decay, and to spontaneous fission have served asthe bases for practical passive NDA measurements.1.1.1 NDA Techniques Based on Alpha ParticleDecayAlpha particle decay is indirectly detected incalorimetry measurements. (Note: a small contributionis attributable to the 6 decay of 241Pu in plutoniumcalorimetry applications.) The kinetic energy of theemitted a particle and the recoiling daughter nucleus istransformed into heat, together with some fraction ofthe gamma ray energies which may be emitted by theexcited daughter nucleus in lowering its energy to amore stable nuclear configuration. The calorimetricmeasurement of the heat produced by a sample can beconverted to the amount of a-particle-emitting nuclidespresent through the use of the isotopic abundance andthe specific power [watts gm-f sec1 I of each nuclide.'Plutonium, because of its relatively high specific power,is amenable to calorimetry.The interaction of high-energy a particles with somelight nuclides (e.g., 'Li, 'Be, 1 Oe, 1 1 Be, 1 &O, and 19F)may produce a neutron. When the isotopic compositionof the a-particle-emitting nuclides is known and thecontent of high-yield (an) targets is fixed, theobservation of the neutron yield from a sample can beconverted to the amount of SNM present..1.1.2 NDA Techniques Based on Gamma BayAnalysisThe gamma ray transitions which reduce theexcitation of a daughter nucleus following either a or flparticle emission from an isotope of SNM occur indiscrete energies.2 3 The known a particle decay activityof the SNM parent isotope and the probability that itspecific gamma ray will be emitted following the aparticle decay can be used to convert the measurementof that gamma ray to a measurement of the amount ofthe SNM parent isotope present in the container beingmeasured. High-resolution gamma ray spectroscopy isrequired when the gamma ray(s) being measured isobserved in the presence of other gamma rays or X-rayswhich, without being resolved, would interfere with themeasurement of the desired gamma ray.1.1.3 NDA Techniques Based on SpontaneousFisionA fission event is accompanied by the emission offrom 2 to 3.5 neutrons (depending on the parentnucleus) and an average of about 7.5 gamma rays. A5.11-1 total of about 200 MeV of energy is released, distributedamong the fission fragments, neutrons, gamma rays, betaparticles, and neutrinos. Spontaneous fission occurs withsufficient frequency in 238Pu, 240Pu,242Pu, and 238uto facilitate assay measurements through the observationof this reaction. Systems requiring the coincidentobservation of two or three of the prompt radiationsassociated with the spontaneous fission event providethe basis for available measurement systems.41.2 Active NDA TechniquesActive NDA is based on the observation ofradiations (gamma rays or neutrons) which are emittedfrom the isotope under investigation when that isotopeundergoes a transformation resulting from an interactionwith stimulating radiation provided by an appropriateexternal source. Isotopic' and accelerator4 sources ofstimulating radiation have been investigated.Stimulation with accelerator-generated high-energyneutrons or gamma rays should be considered only afterall other NDA methods have been evaluated and foundto be inadequate. Such systems have been tested to assayvariable mixtures of fissile and fertile materials in largecontainers having a wide range of matrix variability.Operational requirements,. including operatorqualifications, maintenance, radiation shielding, andcalibration considerations, normally require aninordinate level of support in comparison to the benefitsof in-plant application.Fission is readily induced by neutrons in the 113Uand 213U isotopes of uranium and in the 239Pu and24 ' Pu isotopes of plutonium. Active NDA systems havebeen developed using spontaneous fission (e5 2 Cf)neutron sources, as well as (y,n) [Sb-Be) sources and avariety of (an) [Am-Li, Pu-Li, Pu.Be] sources.5 In theassay of scrap and waste, the neutron-induced fissionreactions are separated from background radiationsthrough observing radiations above a predeterminedenergy level or through observing two or three of theradiations emitted in fission in coincidence.The detection of delayed neutrons or gamma rayshas been employed using isotopic neutron sources toinduce fission, then removing either source or containerto observe the delayed emissions.2. Factors Affecting the Response of NDA SystemsRegardless of the technique selected, the observedNDA response depends on (1) the operationalcharacteristics of the system, (2) the isotopiccomposition of the SNM, (3) the amount anddistribution of SNM, (4) the amount and distribution ofother .materials -within the container, and (5) thecomposition and dimensions of the container itself. Eachof these variables contributes to the overall uncertaintyassociated with an NDA measurement.The observed NDA response represents primarycontributions from the different SNM isotopes presentin the container. To determine the amount of SNMpresent, the isotopic composition of the SNM must beknown and the variation in the observed response as afunction of varying isotopic composition must beunderstood. The effects due to items (3), (4), and (5)above on the observed response can be reduced throughappropriate selection of containers, compatiblesegregation of scrap and waste categories, and consistentuse of packaging procedures designed to improve theuniformity of container loadings.2.1 Operational CharacteristicsThe operational characteristics of the NDA system,together with the ability of the system to resolve thedesired response from a composite signal, determine theultimate usefulness of the system. These operationalcharacteristics include (I) operational stability, (2)geometric detection sensitivity, (3) stimulating radiationuniformity, and (4) energy of the stimulating radiation.The impact of the operational characteristics notedabove on the uncertainty of the measured response canbe reduced through the design of the system and the useof radiation shielding (where required).2.1.1 Operational StabilityThe ability of an NDA system to reproduce a givenmeasurement may be sensitive to fluctuations in theoperational environment. Temperature, humidity, andline voltage variations affect NDA systems to someextent. These effects may be manifested through theintroduction of spurious electronic noise or changes inthe high voltage applied to the detector(s) or amplifiers,thereby changing the detection efficiency. Theenvironment can be controlled if such fluctuations resultin severe NDA response variations which cannot beeliminated through, calibration and operationalprocedures.The sensitivity to background radiations can bemonitored and controlled through proper location of thesystem and the utilization of radiation shielding, ifrequired.2.1.2 Geometric Detection SensitivityThe NDA system should be designed to have auniform response throughout the detection chamber.The residual geometric response dependence can bemeasured using an appropriate source which emitsradiation of the type being measured. The source shouldbe small with respect to the dimensions of the detectionchamber. The system response can then be measuredwith the source positioned in different locations todetermine the volume of the detection chamber whichcan be reliably used.5.11-2 An encapsulated Pu source can be used to testgamma ray spectroscopic systems, active or passive NDAsystems detecting neutrons or gamma rays, orcalorimetry systems. Active NDA systems can beoperated in a passive mode (stimulating source removed)to evaluate the magnitude of this effect. Rotating andScanning containers during assay is a recommendedmeans of reducing the response uncertaintiesattributable to residual nonuniform geometric detectionsensitivity.2.1.3 Uniformity of Stimulating RadiationThe stimulating radiation field (i.e.,. interrogatingneutron or gamma ray flux) in active NDA systmnsshould be designed to be uniform in intensity and energyspectrum throughout the volume of the irradiationchamber. The residual effect can be measured using anSNM sample which is small with respect to thedimensions of the irradiation chamber. The response canthen be measured with the SNM sample positioned indifferent locations within the irradiation chamber. If thesame chamber is employed for irradiation and detection,a single test for the combined geometric nonuniformityis recommended.Various methods have been investigated to reducethe response uncertainty attributable to a nonuniformstimulating radiation field, including rotating andscanning the container, source scanning, distributedsources, and combinations of these methods. Scanning arotating container with the detector and source positionsfixed appears to offer an advantage in responseuniformity and is therefore recommended.2.1.4 Energy of Stimulating RadiationIf the energy of the stimulating radiation is as highas practicable but below the threshold of any interferingreactions such as the neutron-induced fission in 238U,the penetration of the stimulating radiation will beenhanced throughout the volume of the irradiationchamber. A high-energy source providing neutrons abovethe energy of the fission threshold for a fertileconstituent such as 2a3U or 23 2Th can be employed toassay the fertile content of a container.The presence of extraneous materials, particularlythose of low atomic number, lowers the energy spectrumof the interrogating neutron flux in active neutron NDAsystems. Incorporating a thermal neutron detector tomonitor this effect and thereby provide a basis for acorrection to reduce the response uncertainty caused bythis variable effect is recommended.Active neutron NDA systems with the capability tomoderate the interrogating neutron spectrum canprovide increased assay sensitivity for samples containingsmall amounts of fissile material (<100 grams). Thismoderation capability should be removable to enhancethe range of usefulness of the system.2.2 Response Dependence on SNM IsotopicCompositionThe observed NDA response may be a composite ofcontributions from more than a single isotope ofuranium or plutonium. Observed effects are generallyattributable to one of the three sources described below.2.2.1 Multiple Gamma Ray SourcesPlutonium contains the isotopes 23.Pu through242pU in varying quantities. With the exception of242P.u, these isotopes emit many gamma rays.2 3 Theobserved Pu gamma ray spectrum represents thecontribution of all gamma rays from each isotope,together with the gamma rays emitted in the decay of24 'Am, which may also be present.Uranium gamma rays are generally lower in energythan Pu gamma rays. Uranium-232, occurring incombination with 233U, has a series of prolificgamma-ray-emitting daughter products which include228Th, with the result that daughter products of 232Uand 232Th are identical beyond 2281%.2.2.2 Multiple Spontaneously Fissioning PuIsotopesIn addition to the spontaneous fission observedfrom 240Pu, the minor isotopes 238Pu and 242Putypically contribute a few percent to the total rateobserved.6 In mixtures of uranium and plutoniumblended for reactor fuel applications, the spontaneousfission yield from 238U may approach one percent ofthe 24OPu yield.2.2.3 Multiple Fissile IsotopesIn active systems, the observed fission response mayconsist of contributions from more than one isotope.For enriched uranium, if the energy spectrum of thestimulating radiation extends above the threshold for238U fission, that response contribution will be inaddition to the induced 23"U fission response.In plutonium, the observed response will be the sumof contributions from the variable content of 239pu and24 1 Pu.When elements (e.g., plutonium and uranium) aremixed for reactor utilization, the uncertainty in theresponse is compounded by introducing additional fssilecomponents in variable combinations.2.3 Response Dependence on Amount andDistribution of SNM in a ContainerIf a system has a geometrically uniform detectionsensitivity and a uniform field of stimulating radiation(where applicable), a variation in the response per grainof the isotope(s) being measured is generally attributableto one of the three causes described below.5.11-3 2.3.1 Self-Absorption of the EmittedRadiation Within the SNMFor a fixed amount of SNM in a container, theprobability that radiation emitted by the SNM nucleiwill interact with other SNM atoms increases as thelocalized density of the SNM increases within thecontainer. This is a primary source of uncertainty ingamma ray spectroscopy applications. It becomesincreasingly important as the SNM aggregates into lumpsand is more pronounced for low-energy gamma rays.2.3.2 Multiplication of Spontaneous orInduced FissionThe neutrons given off in either a spontaneous or aninduced fission reaction can be absorbed in a fissilenucleus and subsequently induce that nucleus to fission,resulting in the emission of two or more neutrons. Thismultiplication results in an increased response from agiven quantity of SNM. Multiplication affects theresponse of all active NDA systems and passivecoincidence neutron or gamma ray detection systemsused to observe spontaneous fission. This effect becomesincreasingly pronounced as the energy of the neutronstraversing the container becomes lower or as the densityof SNM increases within the container.2.3.3 Self-Shielding of the StimulatingRadiationThis effect is particularly pronounced in activesystems incorporating a neutron source to stimulate thefissile isotopes of the SNM to fission. More of theincident low-energy neutrons will be absorbed near thesurface of a high-density lump of SNM, and fewer willpenetrate deeper into the lump. Thus, the fissile nucleilocated deep in the lump will not be stimulated tofission at the same rate as the fissile nuclei located nearthe surface, and a low assay content will be indicated.This effect is dependent on the energy spectrum of theincident neutrons and the density of fissile nuclei. Itbecomes increasingly pronounced as the energy of theincident neutrons is decreased or as the density of theSNM fissile content is increased. The density of fissilenuclei is increased when the SNM is lumped in aggregatesor when the fissile enrichment of the SNM is increased.2.4 Response Dependence on Amount andDistribution of Extraneous Materials within theContainerThe presence of materials other than SNM within acontainer can affect the emitted radiations in passive andactive NDA systems and can also aff.ct the stimulatingradiation in active assay systems. The presence ofextraneous materials can result in either an increase or adecrease in the observed response.Effects on the observed NDA response are gener.llyattributable to one of the four causes described below.2.4.1 Interfering RadiationsThis problem arises when the material emits aiadiation which cannot be separated from the desiredsignal. This problem is generally encountered in gammaray spectroscopy and calorimetry applications as thedaughters of241Pu, 23 U, and 2 3 2 U grow in. In gammaray applications, the problem is manifested in the formof additional gamma rays which must be separated fromthe desired radiations. In calorimetry, the daughterscontribute additional heat.2.4.2 Interference to Stimulating RadiationMaterial lowers the energy of neutrons traversing acontainer giving rise to an increase in the probability ofinducing fissions. This problem becomes increasinglypronounced with low-atomic-number materials.Hydrogenous materials (e.g., water, plastics) have thestrongest capability to produce this effect.2.4.3 Attenuation of the Emitted RadiationThis effect may include the partial or complete lossof the energy of the emitted radiation. The detection ofa reduced-energy radiation may mean that the radiationcannot be correctly assigned to its source. This effectcan be severe for gamma ray systems. The effectincreases with atomic number and the material densitywithin the container. Also, systems which detectneutrons above a given energy will observe fewerneutrons above the given energy whenlow-atomic-number material is added to the containerand thus produce a low assay indication.The attenuation of the emitted radiation may becomplete, as in the case of the absorption of neutrons inthe nuclei of extraneous material. The probability forthis absorption generally increases as the energy of theincident neutrons decreases. Hence, this effect is furtheraggravated when low-atomic-number materials arepresent to reduce the energy of the emitted neutrons.2.4.4 Attenuation of the Stimulating RadiationThis phenomenon is similar to that of the precedingsection. In this instance, the stimulating radiation doesnot penetrate to the SNM within the container and thusdoes not have the opportunity to induce fission. Thepresence of neutron poisons (e.g., Li, B, Cd, Gd) mayattenuate the stimulating radiation to the extent that theresponse is independent of the SNM fissile content. Mostmaterials absorb neutrons. The severity of thisabsorption effect is dependent on the type of material,its distribution, and the energy of the stimulatingneutrons.The presence of extraneous material can thus alterthe observed response, providing either a high or a lowSNM content indication. This effect is fuirther aggravatedby nonuniformiry within the container of either the5.11-4 SN:.' or the matrix in which it is contained. Thisdependence is severe. Failure to attend to itsramifications through the segregation of scrap and wastecategories and the utilization of representativecalibration standards may produce gross inaccuracies inNDA measurements.2.5 Response Dependence on ContainerDimensi..j and CompositionThe items identified as potential sources ofuncertainty in the observed response of an NDA systemin Sections 2.1, 2.3, and 2.4 above can be minimized oraggravated through the selection of containers to beemployed when assaying SNM contained in scrap orwaste.2.5.1 Container DimensionsThe practical limitation on container size for scrapand waste to be nondestructively assayed represents acompromise of throughput requirements and theincreasing uncertainties in the observed NDA responseincurred as a penalty for assaying large containers.Radiations emitted deep within the container musttravel a greater distance to escape the confines of thecontainer. Therefore, with increasing container size, theprobability that radiations emitted near the center of thecontainer will escape the container to the detectors-decreases with respect to the radiations emitted near thesurface of the container.In active NDA systems, a relatively uniform field ofstimulating radiation must be provided throughout thatvolume of the container which is observed by thedetection system. This criterion is required to obtain auniform response from a lump of SNM positionedanywhere within a container. It becomes increasinglydifficult to satisfy this criterion and maintain a compact,geometrically efficient system with increasing containersize. For this reason, the assay of small-size containers isrecommended.To facilitate loading into larger containers forstorage or offsite shipmen following assay, the size andshape of the inner and outer containers should be chosento be compatible.Packaging in small containers will produce morecontainers to be assayed for the same scrap and wastegeneration rates. An offsetting benefit, however, is thatthe assay accuracy of an individual container should besignificantly improved over that of large containers. Inaddition, the total scrap and waste assay uncertaintyshould be reduced through statistically propagating alarger number of random component uncertainties todetermine the total uncertainty.2.5.2 Container Structural CompositionThe structural composition of containers will affectthe penetration of the incident or the emergingradiation. Provided all containers are uniform, theireffect on the observed response can be factored into thecalibration of the tvstem. The attainable assa" accr:will be reduced w en containers with poor penetraýor varying composition or dimensions are selected.3. Nondestructive Assay for the Accountabilit) io.SNM Contained in Scrap and Waste3.1 NDA Performance ObjectivesThe measurement accuracy objectives for anyinventory component can be estimated by consideringthe amount of material typically contained in thatinventory category. The measurement performancerequired is such that, when the uncertaintycorresponding to the scrap and waste inventorycomponent is combined with the uncertaintiescorresponding to the other inventory components, thequality constraints on the total limit of error of thematerial unaccounted for (LEMUF) will be satisfied.3.2 NDA Technique SelectionNDA technique selection should reflect aconsideration of the accuracy requirements for the assayand the type and range of scrap and waste categories tobe encountered. No single technique appears capable ofmeeting all requirements. When more tharl one type ofinformation is required to separate a compositeresponse, more than one NDA technique may berecquired to provide that information.3.2.1 Plutonium ApplicationsCalorimetry determinations are the least sensitive tomatrix effects, but rely on a detailed knowledge of the2"1 Am content and the plutonium isotopic compositionto transform the measured heat -flux to grams ofplutonium.'Gamma ray spectroscopy systems complement thepotential of other assay methods by providing thecapability to nondestructively determine, or verify, the241 Am content and the piutonium isotopic composition(except 2142 Pu). High-resolution gamma ray systems arecapable of extracting the maximum amount ofinformation (isotopic composition, isotopic content,presence of extraneous gamma ray sources) from anassay, but content density severely affects the accuracyof quantitative predictions based upon that assaymethod.Passive coincidence detection of the spontaneousfission yield of Pu-bearing systems provides an indicationof the combined 238Pu, 240Pu, and 242Pu samplecontent. With known isotopic composition, the Pucontent can be computed.' Neutron multiplicationeffects become severe at high Pu sample loadings."5.11-5 Plastic scintillation coincidence detection systems areoften designed in conjunction with active neutroninterrogation source systems. Operated in passive andactive modes, such systems are able to provide an assayof both the spontaneously fissioning and the fissilecontent of the sample. The spontaneous background canbe subtracted from an active NDA response to provide ayield attributable to the fissile SNM content of thecontainer.Active NDA can be considered for plutonium scrapand waste applications after the potentialimplementation of the passive techniques has beenevaluated. With the wide range of isotopic compositionsencountered, together with the mixture with variousenrichments of urax-um, the requirements to convert anobserved composite response into an accurate assay ofthe plutonium and uranium fissile content becomeincreasingly severe.The application of these methods to the assay ofplutonium-bearing solids and solutions are the subjectsof other Regulatory Guides.3.2.2 Uranium ApplicationsActive neutron systems can provide for bothhigh-energy and moderated interrogation spectrumcapabilities. Operation with the high-energy neutronsource will decrease the density dependence and neutronsel f-shielding effects, significantly enhancing theuniqueness of the observed response. To extend theapplicability of such a system to small fissile loadings, awell-moderated interrogating spectrum can be. used totake advantage of the increased 2 ' sU fission probabilityfor neutrons of low energy. In highly enriched uraniumscrap and waste (>20% 23sU), active NDA featuring ahigh-energy stimulating neutron flux is recommended.The number and energy of the gamma rays emittedfrom the uranium isotopes (with the exceptions of theminor isotopes 2321 U and 23 'U) are generally lower thanfor the plutonium case. The 185-keV transition observedin the decay of 23 sU is frequently employed in uraniumapplications. The penetration of this 23 'U primarygamma ray is so poor that the gamma ray NDAtechnique is not applicable with high-density,nonhomogeneous matrices.There arise occasions when a passive enrichmentdetermination is practical through the measurement ofthe 185-keV gamma ray. One criterion required for thisapplication is that the contents be relativelyhomogeneous. This information can then be combinedwith an assay of the 38U content of the sample tocompute the total uranium and 23sU sample content.The 238U sample content can be obtained eitherthrough the detection of the 23SU spontaneous fissionneutron yield or through the assay of the 234Padaughter gamma activity, provided either the 234Pa is inequilibrium or its content is known. Enrichment meterapplications for uranium will be the subject of anotherRegulatory Guide.Calorimetry is not applicable to the assay oturanium enriched in the 2 'U isotope because of thelow specific a activity. In 233U applications, the intenseactivity of the daughter products of 232U imposes asevere complication on the use of calorimetry.3.3 Categorization and Segregation of Scrap andWaste for NDAThe range of variations in the observed response ofan NDA system attributable to the effects noted inSections 2.3 and 2.4 above can be reduced or controlled.Following an analysis of the types of scrap and wastegenerated in conjunction with SNM processing, a plan tosegregate scrap and waste at the generation points can beformulated. Recovery or disposal compatibility isimportant in determining the limits of each category.Limiting the range in variability in those extraneousNDA interference parameters discussed in Sections 2.3and 2.4 is a primary means of improving the accuracy ofthe scrap and waste assay. Once the categories areestablished, it is important that steps be taken to assurethat segregation into separate, uniquely identifiedcontainers occurs at the generation point.Category limits can be established on the basis ofmeasured variations observed in the NDA response ofcontainer loaded with a known amount of SNM. T1,variation in extraneous parameters can then be mockedup and the resultant effect measured. In establishingcategories, the following specific items are significantsources of error.3.3.1 CalorimetryThe presence of extraneous materials capable ofabsorbing (endothermic) heat or emitting (exothermic)heat will cause the observed response to be less orgreater than the correct response for the Pu in thesample.3.3.2 Neutron MeasurementsThe presence of high-yield (an) target material willincrease the number of neutrons present in the sample.A fraction of these neutrons will induce fission in thefissile SNM isotopes and add another error to themeasurement.3.3.3 Gamma Ray MeasurementsGamma rays are severely attenuated in interactionswith heavy materials. Mixing contaminated combustibleswith heavy, dense materials complicates the attenuationproblem. Mixing of isotopic batches or mixing wi'radioactive non-SNM can also add to the complexitythe response.5.11-6 3.3.4 Fission MeasurementswhereScrap or waste having low-atomic-number materialswill reduce the energy of the neutrons present in thecontainer, significantly affecting the probability ofstimulating fission reactions.Neutron-absorbing materials present in SNM scrapor waste may significantly affect the operation of NDAsystems. Table B-I of this guide identifies neutronabsorbers in the order of decreasing probability ofabsorption of thermal neutrons. An estimate of thesignificance of the presence of one of these materialsmay be obtained from the ratio of its absorption crosssection to the absorption cross section of the SNMpresent in the container:R = N, GalNSNM~aSNMN, = the number of atoms per cubiccentimeter of material,Gal = absorption cross section of theextraneous material (Table B-I),NSNM = numbetiof atoms of SNM present percubic centimeter,OaSNM = absorption cross section of the SNM.233U oa = 573 barns23Su oa = 678 barns239Pu oa = 1015 barns24'Pu oa = 1375 barns(Thermal neutron values)TABLE B-1NATURALLY OCCURRING NEUTRON ABSORBERS8NaturallyOccurringElementAbsorptionCross Section(barns) *NaturallyOccurringElementAbsorptionCross SctionIberns)*SymbolSymbolGadolinium ..........Samarium. ...........Europium ............Cadmium ............Dysprosium ..........Boron ...............Actinium ............Iridium ..............Mercury .............Protactinium .........Indium ..............Erbium ..............Rhodium ............Thulium .............Lutetium ............Hafnium .............Rhenium ............Lithium .............Holmium ............Neodymium ..........GdSmEuCdDyBAcIrHgPaInErRhTmLuHfReLiHoNd46,0005,6004,3002,45095075551044038020019117314912711210586716546Terbium ............Cobalt .............Ytterbium ..........Chlorine ............Cesium .............Scandium ...........Tantalum ...........Radium ............Tungsten ...........Osmium ............Manganese ..........Selenium ......... .Promethium .........Lanthanum ..........Thorium ............Iodine .............Antimony ..........Vanadium ..........Tellurium ...........Nickel .............TbCoYbaCsScTaRaWOsMnSePinLaThISbVTeNi463837342824212019151312119876555*Cross section for thermal neutrons5.11-7 The magnitude of this effect is dependent on thedistribution of the materials and the energy of theneutrons present within the container. The relationshipabove is a gross approximation, and for convenience incalculation, including only the primary fissile isotope issufficient to determine which materials may constitute aproblem requiring separate categorization for assay. Inextreme cases, either methods should be sought tomeasure the content of the neutron absorber to providea correction for the NDA response or a different methodshould be sought for the assay of that category.3.4 Packaging for Nondestructive AssayNondestructive assay provides optimal accuracypotential when the packages to be assayed are essentiallyidentical and when the calibration standards representthose packages in content and form. Containers for mostscrap and waste can be loaded using procedures whichwill enhance the uniformity of the loading within eachcontainer and from container to container. Compactionand vibration are two means to accomplish thisobjective.3.5 Calibration of NDA Systems ior Scrap andWasteTo obtain an assay value on SNM in a container ofscrap or waste with an associated limit of error, theobserved NDA response or the predicted content mustbe corrected for background and for significant effectsattributable to the factors described in the precedingparts of this discussion.The calibration of radiometric nondestructive assaysystems is the subject of another Regulatory Guide.*One procedure for referencing NDA results toprimary standards is the periodic selection of a containerat random from a lot submitted for assay. Thatcontainer should then be assayed a sufficient number oftimes to reduce the random uncertainty of themeasurement to a negligible value. The SNM content ofthat container can then be determined through adifferent technique having an accuracy sufficient toverify the stated performance of the NDA system. Thisreference method. should be traceable to primarystandards. High-integrity "recovery of the contents,followed by sampling and chemical analysis is onerecommended technique.
C. REGULATORY POSITION
In the development of an acceptable framework forthe incorporation of nondestructive assay for themeasurement of SNM-bearing scrap and waste, strongconsideration should be given to technique selection,*To be based on ANSI N15.20, which is currently indevelopment.calibration, and operational procedures; to thesegregation of scrap and waste categories; and to theselection and packaging of containers. The guidelinespresented below are generally acceptable to theRegulatory staff for use in developing such a frameworkthat can serve to improve materials accountability.1. Analysis of Scrap and WasteThe origin of scrap and waste generated inconjunction with SNM processing activities should bedetermined as follows:a. Identify those operations which generateSNM-bearing scrap or waste as a normhal adjunct of aprocess.b. Identify those operations which occasionallygenerate SNM-bearing scrap or waste as the result of anabnormal operation which renders the productunacceptable for further processing or utilizationwithout treatment.c. Identify those scrap and waste items generated inconjunction with equipment cleanup, maintenance, orreplacement.The quantities of scrap and waste generated duringnormal operations in each category in terms of the totalvolume and SNM content should be estimated. Bulkmeasurement throughput requirements should bedetermined to assure that such assay will not constitutean operational bottleneck.2. NDA Selection2.1 TechniqueThe performance objectives for the NDA systemshould be derived as discussed in Section B.3.1.Techniques should be considered for implementation inthe order of precedence established in Table C-I of thisguide.Selection should be based on attainable accuracy,factoring into consideration the characteristics of thescrap and waste categories. The application of suchtechniques will be the subjects of other RegulatoryGuides.2.2 System SpecificationsNDA systems for SNM accountability should bedesigned and shielding should be provided to meet .thefollowing objectives:a. Performance characteristics should be essentiallyindependent of fluctuations in the ambient operationalenvironment, including:(!) External background radiations,(2) Temperature,(3) Humidity, and(4) Electric power.b. Response should b~e essentially independent cpositioning of SNM within the scrap or waste containeincluding effects attributable to:5.11-8 TABLE C-1NDA TECHNIOUE SELECTIONTECHNIQUE Pu S"SU ;20% "'aU <20% asU(1) ]st (1+2)* 3rd NA NACALORIMETRYNR NR NA NA(2) 3rd 2nd 2nd Ist (2+5)GAMMA RAY1st lIt 1st Ist(3) 2nd (3+2) NA NR 3rd (3+2)0*SPONTANEOUSFISSION 2nd (3+2) NA NR MR(4) 4th 1st 1st 2adSTIMULATEDFISSION 3rd 2nd 2nd 2nd(5) NR NM mR (5+2) MR (S42)GROSS NEUTRONNR Mt MR Mt*Above wommeadation reten to h0hdinty, m m rns. Loweremmmnmntion rfas to ow4enmsty, M ."Spontaneous fuson of " 'OU.NR-NOT RECOMMENDED-Technique =maima for dd allimtimb.NA-NOT APPLICABLE.MN-NOT INDEPENDENTLY bea.,.- a* o m a i dowith a cmplmeatury amy metho TABLE C-2NDA INTERFERENCE CONTROLPresnce ofHeat Producing Mixted High.Yield Ganne Neutron Lumped vs. Lumped vs.or Absorbing Mixed Isotopic Miscellaneous (a,ni Target Ray Neutron Moderating Distributed DistriburedNDA Technique Process SNM Retches Radletions Material Absorbers Absorbers Materials SNM Matrix Mat0Calorimetry xxx xxx -Gamma RaySpectroscopy -x x- xxx -xxx xxSpontaneousFission Detection -xx xxx .... xb xxc xx xx XStimulatedFission Detection -x x xb xxt.xxxC xxd d a" jXXe Xe :.0hKey: -No apparent sensitivity.x Some sensitivity. Evaluate effect in extreme cases.xx Marked sensitivity. Categohize and calibrate accordingto magnitude of observed effect.xxx Strong sensitivity. Requires correction to imy. Mayrender technique unacceptable in extreme cases ifcorrection not possibleNotes: a -Effect depends on type and nature of radiation detected.b -Effect less pronounced in coincidence detection systems.c -Same as a, additional effect due to neutron multiplication.d -Moderated-neutron stimulating source.e -High-energy stimulating sourc (1) Detector geometrical efficiency, and(2) Stimulating source intensity and energy.Techniques to achieve these objectives are discussed inSection B of this guide.3. CategorizationScrap and waste categories should be developed onthe basis of NDA interference control, recovery oordisposal compatibility,9 and relevant safetyconsiderations. Categorization for NDA interferencecontrol should be directed to limiting the range ofvariability in an interference. Items to be considereddepend upon the sensitivity of the specific NDAtechnique, as shown in Table C-2.The means through which these interferences aremanifested are detailed in Section B. When such effectsor contents are noted, separate categories should beestablished wherein the materials are isolated.4. Containers4.1 Size ConstraintsScrap and waste should be packaged for assay incontainers as small as practicable, consistent with thecapability and sensitivity of the NDA system.To enhance the penetration of stimulating oremitted radiations containers should be cylindrical. The:diameter should be less than five inches to provide forsignificant loading capability, ease in loading, reasonablepenetrability characteristics, and compatibility withcriticality-safe geometry requirements for individualcontainers, where applicable.Containers having an outside diameter of 4-3/8inches will permit nineteen such containers to bearranged in a cross section of a 55-gallon drum, even.when that drum contains a plastic liner. Containershaving an overall. length equal to. some integral fractionof -the length of a 5.5-gallon drum -are furtherrecommended when shipment or storage within suchcontainers is to be. considered. For normal operations, anoverall length. of either 1.6-1,/2 inches (two layers or 38containers per drum) or 11 inches (three layers or 57containers per drum) is therefore recommended.4.2 Structural FeaturesContainers should be selected in accordance withnormal safety considerations and should be:a. Structurally identical for all samples to be assayedwithin each category,b. Structurally identical for as many categories as.practicable to facilitate loading into larger containers orstorage facilities,c. Uniform in wall thickness and material composition,d. Fabricated of materials that do not significantlyinterfere' with the radiations entering or leaving thesample,e. Capable of being sealed to verify post-assayinitegrity, andf. Compatible with subsequent recovery, storage, anddisposal requirements, as applicable.In most NDA applications, uniformity, ofconposition is .more important than the specification of,- particular material. Table C-3 gives generalrecommendations for container structural materials.TABLE C-3SCRAP AND WASTECONTAINER COMPOSITIONNDA Technique Container CompositionCalorimetry metal (aluminum, brass)Gamma Ray Analysis cardboard, polyethylenebottle, thin metalSpontaneous or thin metal, cardboard,Stimulated Fission polyethylene bottleGross Neutron thin metal, cardboard,polyethylene bottle4.3 Container IdentificationTo facilitate loading and assay within thesegregation categories, containers should either beuniquely color-coded or carry unique color-codedidentification labels. Identification of categories shouldbe documented and operating personnel instructed toassure compliance with established segregationobjectives.5. PackagingContainers, where practical, should be packagedwith a quantity of material containing sufficient SNM toassure that the measurement is not being made at theextremes of the performance .bounds for that system.Packaging procedures should be consistent with relevantsafety practices.Containers should be packaged in as reproducible amanner as possible. Low-density items should becompacted to reduce bulk volume and to increase thecontainer SNM loading. Lowering the bulk volumereduces the number of containers to be assayed andgenerally improves the assay precision.5.11-1l If assay predictions are significantly affected by thevariability in the distribution of the container contents,compacting or vibrating the container on a shake tableto settle the contents should be used to enhance theassay accuracy in conjunction with rotating and scanningthe container during assay.6. CalibraionThe NDA system(s) should be independentlycalibrated for each category of scrap or waste to beassayed.Within each category, the variation of interferenceeffects should be measured within the boundariesdefining the limits of that category. Calibrationstandards should employ containers identical to those tobe employed for the scrap or waste. Their contentsshould be mocked up to represent the range of variationsin the interferences to be encountered. To minimize thenumber of standards required, the calibration standardsshould permit the range of interference variations to be.simulated over a range of SNM loadings.Calibration relationships should be verified atintervals sufficiently frequent to detect deviations fromthe expected response in time to make correctionsbefore the containers are processed or shipped.Assay values should be periodically verified throughan independent measurement using a techniquesufficiently accurate to resolve NDA uncertainty.Periodically, a container of scrap or waste should berandomly seleted for verification. Once selected, theNDA analysis should be repeated a minimum number of.five times to determine the precision characteristics ofthe system. The contents of that container should thenbe independently measured using one of the followingtechniques:a. Recovery of the contents, followed by sampling andchemical analysis,b. High-accuracy calorimetry (Pu only) with isotopicsample taken from contents and determined throughstandard techniques.c. Small-sample screening followed by selectivechemical analyses. This technique is applicable to casesin which the contents consist of a collection of similaritems. Each item should be assayed in a small-samplesystem capable of an accuracy greater than or equal tothat of the system being calibrated. No less than fiveitems should then be selected for chemical analysis.Those items should be chosen to span the range ofobserved responses in the screening assay.Verification measurements -should be used toperiodically update calibration data when thecomparison with predicted quantities is satisfactory.Calibration of the system is not acceptable when theNDA predicted value does not agree with the measuredvalue to within the value of the combined limits oferror:I NDA-VER 14 (LEIDA + LEER)1/2Calibration data and hypotheses should bereinvestigated when this criterion is not satisfied.The calibration of NDA systems will be the subjectof another Regulatory Guide.REFERENCES1. F. A. O'Hra et al., Calorbmetry for SafieswdPwposes, MLM-l 798 (January 1972).2. R. Gunnink and R. J. Morrow, Gwnma RayE .ies ad AbaWue Awnhft intemitni for224,23,240,.261Pu .d "'Am, ECRL-SIO7(July 1971).3. J. E. Cline, R. .. Gehrke, and L. D. Mcsuac, GwnnvRays Emitted by the Ftosonable Nudlda andAssciated Isotopes. ANCR-1069 (July 1972).4. L A. Kull, Catalogue of Nucw Maerial SafieguardIstrument, BNL-17165 (August 1972).5. J. R. Deyster and L. A. Kull, SqaudsApplications for Isotopic Neutron Sources,BNL-50267 (T-596) (June 1970).6. R. Sher, Opeiting Oanclmtfics of Neutron WellCobsedmee Countat, BNL-50332 (January 1972).7. R. B. Perry, R. W. Brandenburg, N. S. Beyer, TheEffect of Induced Fmion on Plutonium Asay witha Neutron Coiddumce Well Coutmer, Trans. Am.Nucl. Soc., 15 674 (1972).g. Reactor Physics Constants, ANL-580D (1963).9. Regulatory Guide 5.2, Classjcation of UnibndiatedPlutonium wad 1wisum 5.11-12