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N u c l e a r Safeguards—a R e v i e w
Nuclear Safeguards Analysis Downloaded from pubs.acs.org by COLUMBIA UNIV on 12/03/18. For personal use only.
CARLETON D. BINGHAM U.S. Department of Energy, New Brunswick Laboratory, Argonne, IL 60439
The objective of nuclear materials safeguards is the prevention of successful malevolent acts involving nuclear materials and f a c i l i t i e s . Safeguards consist of an integration of physical protective measures, materials control, and materials accountability. This symposium f i t t i n g l y addresses the subject of measurements of nuclear materials for safeguards purposes. Accuracy of measurements is essential to define the quantity of material which is on hand and which must be safeguarded. A facility inventory statement, based on accurately measured values, which agrees with the book inventory is one means by which the absence of diversion is demonstrated. The precision with which measurements are performed becomes the basis for the uncertainty attached to an inventory statement. When measurements can be performed to a higher degree of precision, the overall uncertainty is reduced and the sensitivity for detecting a diversion is enhanced. In order to make authoritative statements about measurement accuracy and precision, a measurement assurance program is necessary. Within the framework of a measurement assurance program, it is relatively simple to demonstrate precision and make statements regarding a random error of measurement. To demonstrate accuracy and make statements regarding bias and/or systemmatic error require that measurements be traceable to accepted reference bases. Safeguards have placed an increased emphasis on nuclear materials measurement assurance during the past decade. This emphasis exists both in the U.S.A. and throughout the international nuclear community. In the U.S.A., measurements on a complex variety of materials from all parts of nuclear fuel cycles are required to be traceable to a national measurement system. (1) Implicit i n such a requirement i s the existence and a v a i l a b i l i t y of appropriate means for demonstrating such traceability. Let This chapter not subject to U.S. copyright. Published 1978 American Chemical Society
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us examine t h i s i m p l i c a t i o n i n l i g h t o f a h y p o t h e t i c a l safeguards measurement. A batch o f m a t e r i a l o f given ( i . e . , observed) mass i s to be measured to confirm an a d d i t i o n to an inventory stratum. The bulk m a t e r i a l i s sampled according to a s t a t i s t i c a l l y designed plan g i v i n g due concern to g a i n or l o s s o f moisture, o x i d a t i o n , e t c . , and the mass o f the i n d i v i d u a l samples i s determined. Each o f the samples i s d i s s o l v e d f o l l o w i n g an accepted procedure and the mass (or volume) of the r e s u l t a n t s o l u t i o n i s determined. From each o f the s o l u t i o n s , a l i q u a n t s ( e i t h e r by weight or volume) are taken upon which the measurements w i l l be performed. I w i l l address measurements l a t e r i n the paper. (At t h i s p o i n t , even before chemical assay measurements have been performed, measurement assurance r e q u i r e s c a l i b r a t e d balances, thermometers, and barometers, the l a t t e r f o r buoyancy c o r r e c t i o n s , to o b t a i n accurate masses or s i m i l a r c a l i b r a t i o n s where volumes are concerned. These c a l i b r a t i o n s f o r mass or volume are every b i t as important to the f i n a l q u a l i t y o f measurement as the measurement i t s e l f , but are o f t e n taken f o r granted. Aside from mentioning these measurements being an i n t e g r a l part o f safeguards measurements, I s h a l l not dwell f u r t h e r on them.) The U.S. N a t i o n a l Bureau o f Standards (NBS) i s charged with the r e s p o n s i b i l i t y f o r e s t a b l i s h i n g and maintaining a n a t i o n a l measurement system. In a d d i t i o n to the b a s i c u n i t s : mass, l e n g t h , time, e t c . , the Bureau e s t a b l i s h e s measurement technology which i s disseminated to users. Reference m a t e r i a l s are one means f o r t r a n s f e r r i n g measurement technology to users inasmuch as instruments and chemical reagents can be c a l i b r a t e d t o reproduce the value assigned to the r e f e r e n c e . Reference m a t e r i a l s form the base f o r d e f i n i n g the q u a l i t y o f a measurement method - i t s accuracy, p r e c i s i o n , and "ruggedness". These m a t e r i a l s are a l s o the means o f r e l a t i n g measurements made at d i f f e r e n t s i t e s to each other. A h i e r a r c h i c a l s t r u c t u r e o f reference m a t e r i a l s e x i s t s i n measurement science such that lower l e v e l s are derived from upper l e v e l s . Primary standards are h i g h - p u r i t y elements or compounds, the v a l i d i t y o f whose r e f e r e n c e values (assigned according to the best a v a i l a b l e s c i e n t i f i c procedures) can be assured when the m a t e r i a l i s t r e a t e d a c c o r d i n g to i n s t r u c t i o n s on the c e r t i f i c a t e . T h i s i m p l i e s a c e r t a i n known chemical s t a b i l i t y or an a b i l i t y to achieve and reproduce a known compositional s t o i c h i o m e t r y . Standard reference m a t e r i a l s (SRM s)issued by NBS c o n s t i t u t e the primary standards f o r the chemical or i s o t o p i c measurement o f uranium and plutonium and provide the l i n k o f t r a c e a b i l i t y to the n a t i o n a l system. Some o f the n u c l e a r ?
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SRM s are r e l a t e d to other primary chemical standards such as the redox standards ^ Ο ^ Ο γ , As 0o, and Na2C 0i|. Primary standards represent a valued resource and should be used when there are no other s u i t a b l e a l t e r n a t i v e s to demonstrating the t r a c e a b i l i t y l i n k . The p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f primary standards i s expensive i n money and time and r e q u i r e s a h i g h l y t e c h n i c a l e f f o r t by experienced s c i e n t i s t s . Primary standards should not be used r o u t i n e l y to prepare working s o l u t i o n standards or bench c o n t r o l standards. Secondary standards are prepared as elements or compounds o f v a r y i n g l e v e l s o f p u r i t y . T h e i r chemical s t a b i l i t y may be such that large batches cannot be prepared to provide long-term s u p p l i e s . T h e i r s t a b i l i t y , on the other hand, may be s i m i l a r to that o f t h e i r primary counterparts and o n l y the p u r i t y and u n c e r t a i n t y i n t h e i r assigned values may be d i f f e r e n t . Secondary standards, such as those d i s t r i b u t e d by the Department o f Energy's New Brunswick Laboratory (NBL), provide an a l t e r n a t e path to t r a c e a b i l i t y . These standards represent t y p i c a l m a t e r i a l s found i n current nuclear f u e l c y c l e technology. M a t e r i a l s a v a i l a b l e cover the range from ores and counting standards, enrichment plant product, conversion plant intermediates and product, and production plant product. These m a t e r i a l s may be more r e p r e s e n t a t i v e of those i n a c t u a l plant or f a c i l i t y use and, as such, may o f f e r a more simple, but not n e c e s s a r i l y as c e r t a i n , path o f traceability. To what extent i s the implied a v a i l a b i l i t y of reference m a t e r i a l s being met? A group, convened i n November, 1977, to advise the I n t e r n a t i o n a l Atomic Energy Agency (IAEA) o f the current s t a t u s o f chemical and i s o t o p i c reference m a t e r i a l s i n the nuclear f u e l c y c l e , made the observation that the primary nuclear reference m a t e r i a l s g e n e r a l l y a v a i l a b l e to the nuclear community provide the means to demonstrate the t r a c e a b i l i t y o f safeguards measurements. The group recommended that e f f o r t be expended to provide a g r e a t e r v a r i e t y o f secondary r e f e r e n c e m a t e r i a l s and to improve the q u a l i t y o f some e x i s t i n g primary reference m a t e r i a l s . ( 2 ) Where s u i t a b l e r e f e r e n c e m a t e r i a l s are not a v a i l a b l e from e x t e r n a l sources, a f a c i l i t y must r e s o r t to working standards prepared i n t e r n a l l y . These m a t e r i a l s g e n e r a l l y are prepared from a c t u a l m a t e r i a l streams i n the f u e l c y c l e . T h i s path to t r a c e a b i l i t y i s not always s t r a i g h t f o r w a r d - one may need to e x e r c i s e c o n s i d e r a b l e i n g e n u i t y to assure that a l l steps can be traced to the n a t i o n a l system - but t r a c e a b i l i t y can be achieved. I t i s e s s e n t i a l that the measurement method 2
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used i n a given f a c i l i t y undergo thorough and complete t e s t i n g by the developer or the user to document the e f f e c t , i f any, of i m p u r i t i e s , c a t i o n i c and a n i o n i c , and the e f f e c t of s l i g h t u n i n t e n t i o n a l v a r i a t i o n s to the published method ( i . e . , how "rugged" i s the method). Here i s where reference m a t e r i a l s come i n t o play. Reference m a t e r i a l s are e s s e n t i a l for the i n i t i a l s t a t i s t i c a l t e s t i n g o f a method - i . e . , e s t a b l i s h i n g i n i t i a l estimates o f accuracy and p r e c i s i o n . The SRM should then be used to compare the response o f the method to a secondary or working m a t e r i a l of l e s s e r p u r i t y . When c o m p a r a b i l i t y o f observations can be demonstrated, then the l e s s expensive secondary or working m a t e r i a l can be s e l e c t i v e l y doped with i m p u r i t i e s to document the e f f e c t o f impurity l e v e l s and to a s c r i b e accuracy and p r e c i s i o n when t y p i c a l m a t e r i a l s are measured. The "ruggedness" should be studied with these l a t t e r m a t e r i a l s . A commonly heard request i s f o r a reference m a t e r i a l of a given compound to match the compound o f i n t e r e s t at a given s i t e . An i n a b i l i t y to t r a c e the measurement through a l l of the a n a l y t i c a l steps i s c i t e d as basis f o r the need. The s t a b i l i t y o f the m a t e r i a l requested notwithstanding, the chemistry o f the measurement process needs to be examined. Where uranium matrices, f o r example, are concerned, once s o l u b i l i z a t i o n i s e f f e c t e d , f o r a l l but unusual cases, converting the s o l u t i o n to a n i t r a t e form, presents the uranium f o r measurement i n the same chemical s t a t e i n s o l u t i o n as i s a c h i e v a b l e with e x i s t i n g metal or oxide primary or secondary standards. Now the p o s s i b l e e f f e c t o f i m p u r i t i e s needs to be considered. I f the measurement method being used e x h i b i t s a response which i s a f f e c t e d by the presence o f i m p u r i t i e s known to be present, the s o l u t i o n s o f standards may r e q u i r e doping to an appropriate c a t i o n i c or a n i o n i c i m p u r i t y l e v e l . A more s a t i s f a c t o r y option would be to change to a method which i s not a f f e c t e d by the i m p u r i t i e s known to be present. I t s u f f i c e s next to demonstrate that t o t a l s o l u b i l i z a t i o n has occurred. F i l t r a t i o n o f the s o l u t i o n , followed by v i s u a l or r a d i o m e t r i c observation f o r a r e s i d u e , i s u s u a l l y an adequate demonstration. I f a r e s i d u e i s present, technology e x i s t s f o r s o l u b i l i z i n g i t , i . e . , f u s i o n s , combustion followed by a c i d treatment, etc.(3) My d i s c u s s i o n thus f a r has been l i m i t e d to the t r a c e a b i l i t y o f d e s t r u c t i v e chemical a n a l y s i s . The p o t e n t i a l advantages inherent i n n o n d e s t r u c t i v e assay (NDA) have a t t r a c t e d much a t t e n t i o n both a t the R&D l e v e l and at the user l e v e l . These advantages equate to near r e a l - t i m e measurements o f process streams and the a b i l i t y to measure heterogeneous composites i n the scrap/waste stream where sampling e r r o r s o v e r r i d e any accuracy and p r e c i s i o n advantages o f d e s t r u c t i v e chemical a n a l y s i s . In the NDA
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area, t r a c e a b i l i t y becomes more d i f f i c u l t , but not impossible as some maintain,(4,5) P r a c t i c a l l y a l l NDA measurements e x h i b i t some form o f matrix dependence which prevents t h e i r being an "absolute" measurement and thus r e q u i r e s that the instrument or system be c a l i b r a t e d by reference to known q u a n t i t i e s o f m a t e r i a l . T h i s known q u a n t i t y o f m a t e r i a l must be contained i n a matrix which has chemical and p h y s i c a l p r o p e r t i e s s i m i l a r to the process m a t e r i a l s to be measured before the c a l i b r a t i o n can be t r a n s f e r r e d to those m a t e r i a l s . Knowledge o f the e f f e c t o f d i f f e r i n g chemical and/or p h y s i c a l p r o p e r t i e s on the measuring system can be used to apply an e m p i r i c a l f a c t o r to the observed system response to o b t a i n an estimate o f the container contents. In the f u e l c y c l e s with which there i s c o n s i d e r a b l e experience, v i z , LWR, LMFBR, and HTGR, there are s i t e s p e c i f i c d i f f e r e n c e s i n f a b r i c a t i o n p r a c t i c e s , e.g., burnable poison vs_ no burnable poison, c o p r e c i p i t a t i o n vs mechanical blending, resin-bead-converted low-density carbide vs h i g h - d e n s i t y mixed c a r b i d e , e t c . , which y i e l d a product with d i f f e r i n g p h y s i c a l p r o p e r t i e s and thus preclude a c e n t r a l i z e d n a t i o n a l source o f a p r a c t i c a l l y managed f i n i t e number o f NDA reference standards. Granted, there are no o f f - t h e - s h e l f primary or secondary standards from a n a t i o n a l l y accepted source; however, v i a the primary and/or secondary m a t e r i a l s that do e x i s t n a t i o n a l l y , a f a c i l i t y can s y n t h e s i z e working standards or analyze r e p r e s e n t a t i v e samples o f a m a t e r i a l p r e p a r a t i o n i n a manner that t r a c e a b i l i t y can be demonstrated. With the c u r r e n t s t a t e o f - t h e - a r t o f NDA measurement technology, which i s h i g h l y matrix dependent or m a t e r i a l s p e c i f i c , t r a c e a b i l i t y o f NDA measurements must be through working standards which have been prepared and/or c h a r a c t e r i z e d by measurement technology t r a c e a b l e to the n a t i o n a l system. NBS i s c o l l a b o r a t i n g with the European Economic Community (EEC) i n the c h a r a c t e r i z a t i o n o f a j o i n t US-EEC reference f o r the NDA of product low-enriched uranium oxide. This may enable more t i m e l y measurements o f product to be performed to increase the confidence i n the inventory statement. NBL i s preparing two prototype NDA secondary standards c o n t a i n i n g enriched uranium based upon the response by NDA users to a questionnaire attempting to define common areas o f a p p l i c a t i o n and need. These are expected to be a v a i l a b l e f o r e v a l u a t i o n by comparative measurements i n l a t e 1978. To what extent does the q u a l i t y o f e x i s t i n g reference m a t e r i a l s meet the needs o f the nuclear community, where are improvements r e q u i r e d , and how are these requirements implemented? In the IAEA Advisory Group
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meeting mentioned e a r l i e r , i t was acknowledged that there were s e v e r a l areas i n which improvements t o the q u a l i t y o f e x i s t i n g reference m a t e r i a l s should be made. E x i s t i n g plutonium i s o t o p i c standards were o r i g i n a l l y measured and c e r t i f i e d using mass d i s c r i m i n a t i o n data obtained using separated uranium i s o t o p e s , not plutonium. Plans already e x i s t t o remeasure plutonium isotope r a t i o s and c o r r e c t the observed r a t i o s on the b a s i s o f mass d i s c r i m i n a t i o n data from separated plutonium i s o t o p e s . C e r t a i n uranium i s o t o p i c standards should a l s o be c e r t i f i e d f o r uranium content so that systemmatic e r r o r s are not introduced i n t o isotope d i l u t i o n measurements when a "UoOg." s t o i c h i o m e t r y i s assumed. C e r t a i n uranium and plutonium assay standards should a l s o be c e r t i f i e d f o r isotope abundance d i s t r i b u t i o n thereby a l l o w i n g one standard t o s u f f i c e where two are now r e q u i r e d . More accurate plutonium and americium h a l f - l i v e s are necessary i n order t o transform c a l o r i m e t r i c measurements i n t o measurements o f a q u a n t i t y o f plutonium. NBS and other n a t i o n a l standards o r g a n i z a t i o n s are moving to implement these recommendations. As the s t a t e o f r o u t i n e measurement a r t improves, there needs t o be a corresponding r e d u c t i o n i n the u n c e r t a i n t y assigned t o reference m a t e r i a l s used t o c a l i b r a t e such measurements. T h i s f u r t h e r r e q u i r e s the development o f improved methods f o r c h a r a c t e r i z i n g primary r e f e r e n c e m a t e r i a l s . A reference m a t e r i a l c e r t i f i e d t o 0-1% i s o f l i t t l e use t o make accuracy statements f o r measurement methods r o u t i n e l y capable o f 0.05$ RSD p r e c i s i o n . M a t e r i a l s which a r e now c e r t i f i e d t o +0.02$ o f the assigned value may r e q u i r e i n the near future a r e c e r t i f i c i a t i o n t o 0.002$ so that the u n c e r t a i n t i e s i n secondary or working standards are l e s s s u b j e c t t o the e f f e c t o f the u n c e r t a i n t y i n the r e f e r e n c e value o f the primary standard. This a d d i t i o n a l decade o f c e r t a i n t y w i l l be expensive t o provide, but i f a t t a i n e d , can represent a strengthening o f nuclear m a t e r i a l s safeguards i n that reduced p r e c i s i o n and increased accuracy t r a n s l a t e into a greater s e n s i t i v i t y f o r detecting a d i v e r s i o n which, i n p a r t , meets the o b j e c t i v e o f safeguards. Literature Cited 1. Code of Federal Regulations, T i t l e 10, Part 70, Section 70.57. 2. Report of the Advisory Group to the International Atomic Energy Agency on Chemical and Isotopic Reference Materials i n the Nuclear Fuel Cycle, held November 8-10, 1977, Vienna, Austria. To be published. 3. Bingham, C. D . , Scarborough, J . Μ., and P i e t r i , C. E., "Safeguarding Nuclear Material", Proceedings of Symposium, Volume I I , pp 107-115, International Atomic Energy Agency, Vienna, 1976.
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4. 5.
Bishop, D. Μ., Nuclear Materials Management (1976), Volume V (No. 1), pp 16-27. Bingham, C. D . , Yolken, H. T., and Reed, W. P . , Nuclear Materials Management (1976), Volume V (No. 2), pp 32-35.
RECEIVED MAY 1, 1978.
