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Stable Isotope Amount Ratio Measurements of Noble Gases, a Potential Method to Identify Reprocessing Activities

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Stable Isotope Amount Ratio Measurements of Noble Gases, a Potential Method to Identify Reprocessing Activities XA9949416 IAEA-SM-351/101 STABLE ISOTOPE AMOUNT RATIO MEASUREMENTS OF NOBLE GASES, A POTENTIAL METHOD TO IDENTIFY REPROCESSING ACTIVITIES Y. Aregbe, K. Mayer*, S. Valkiers, P. De Bievre, J. Poths** Institute for Reference Materials and Measurements European Commission - JRC B-2440 GEEL (Belgium) •present address: European Commiision-JRC, Institute for Transuranium Elements, D-7612J Karlsruhe, Germany **Los Alamos National Laboratory, CST-7, l.os Alamos, New Mexico 87545 New approaches need to be developed and implemented, aiming to strengthen the safeguards system. One particularly interesting area is the application of environmental sampling for safeguards purposes. High precision isotopic analysis of samples collected within or away from declared nuclear facilities enables to read characteristic signatures e.g. measurements of some natural krypton and xenon abundances may indicate reprocessing activities. As a result of the reprocessing process of nuclear fuel, the radioactive fission products wKr and the short lived IMXe as well as the stable isotopes of krypton and xenon are released into the atmosphere. Blending calculations of fission xenon and krypton released in air, were carried out in order to obtain a reliable approximation of the expected changes in isotope amount ratios and the respective detection limit thereof. Applying the KORIGEN code [1] two different scenarios were used as input data: In Scenario A a PWR operated for electricity generation for commercial purposes is considered whereas in Scenario B Plutonium production was suspected to be due to low burn-up. The isotope amount ratio «(l36Xe)/w(13lXe) shows the most significant alteration due to the dilution and therefore it is the most interesting indicator for "added" fission to natural xenon. In Fig. 1 the solid line and the dashed line indicate the natural «(t3SXe)/«(ll2Xe) with its uncertainty, the detection limit for «(13SXe)/«('"Xe) was calculated to have a value of 1:100000, which corresponds to a dilution of lg fission xenon in 2-10' m3 air. The two scenarios could be distinguished down to a dilution factor of lg fission xenon in 106 m1 air. Solving the set of (i-1) equations (Eq.l), as used in isotope dilution mass spectrometry, for the unknown isotope amount ratios of the released fission noble gas (Rj,) provides the tool to recalculate its initial isotopic composition. Nx j,: number of atoms of fission noble gas or natural noble gas R^y: isotope abundance ratio considered in fission noble gas or natural noble gas ^ R,.xy the sum of isotope abundance ratios in fission noble gas or natural noble gas Ru: the same isotope amount ratio in the blend Due to the fact that the lightest isotopes (l24Xe and 78Kx) are not produced through nuclear fission (U,M(7Wt=0) the number of unknowns {NJN<y and Y,K,x) do not exceed the number of equations, and the equation system can be solved. However, 175 successful application of the model strongly depends on the accuracy of the measurements of Ry and Rn. Calculated isotapic alterations in natural xenon tlragiidease of fission wnon 032968- V? Vf Q3SB7. ACER- £L Scenario A a Scenario B 'S? O32SB5. 0015 aftn Xenon Uttrt. (film Xsion, Is) QOK) >,,032963- Q329K- 00» 032*0- irao The development of highly accurate gas mass spectrometry at the IRMM has reached a level that measurements on field samples of noble gases might indeed become feasible: a relative measurement precision and accuracy of 10s has become available from a gas isotopic measurement procedure which has lead to an improved value of the Avogadro constant [2]. It opens the possibility to conceive stable isotope measurements for the purpose described. Since fission noble gases are more enriched in the heavier isotopes as compared to atmospheric noble gases, a set of synthetic xenon and krypton mixtures have been prepared, respectively. Gravimetric and volumetric blends were prepared from enriched l3SXe, IJ'Xe, l34Xe and 8SKr with xenon and krypton of natural isotopic composition. The slight, intended, differences of the isotopic composition, i.e. the dilution factor, in those mixtures correspond to the expected differences of the isotopic composition of xenon (krypton) in air samples taken in the vicinity of a reprocessing plant. The procedure, for high accuracy measurement of ratios of gas isotope amounts, contains verifying the ideal gas behaviour during the measurement by checking the observed ratio values for quantitative conformity towards kinetic gas theory [3]. The thus measured values were in excellent agreement, 510"* or better (relative) to the isotope amount ratios calculated from the preparation values of the mixtures. In Fig 2. the plotted values indicate the prepared /j(1MXe)/n('"Xs) isotope amount ratio in the set of synthetic mixtures as a dilution of ""'"Xe in natural xenon. 176 Isotopic alterations in natural Xenon due to mixing with 61% enriched l3*Xe 03*3. 03SBM0- prepared values Q3SS12S- -natural xenon $ Uxwt (ratals) COWED- .-O3S8S7S- •^"0388750 Q388S2S- Q388SCO- 2uD 6X0 10000 MOOD 22M0 dilution fictor Rg2 Up to now the detection of released fission noble gases in air has been performed by radiometric measurements of "Kr. Due to its rather long half live it is available for measurements even after longer atmospheric transport. Although stable noble gas measurements are less sensitive than the radiometric measurements oa "Kr [4], highly accurate differential measurements on the stable isotopes of xenon and krypton, sampled in the vicinity of reprocessing plants, can serve as a tool complementary to common radiometric measurements. They reveal a source of information on the plant's activities which has been widely neglected. References [1] U. Fischer, H. W. Wiese: Verbesserte konsistente Berechnung des nuklearen Inventars abgebrannter DWR-Brennstoffe auf der Basis von Zell-Abbrand- Verfahren mit KORIGEN, KfK 3014, Januar 1983 [2] P. De Bievre, S. Valkiers, F. Schaefer, H. S. Peiser, P. Seyftied, High- accuracy isotope abundance measurements for metrology, PTB Mitteil. 104 (1994) 225-236 [3] Y.Aregbe, S. Valkiers, K.Mayer, P. De Bievre, R. M. Wessel and A, Alink, Measuring Amount Ratios of Gas Isotopes by two Primary Methods of Measurement, Metrotogia, submitted [4] Y. Aregbe, K. Mayer, S. Valkiers, P. De Bievre: Detection of Reprocessing Avtivities through Stable Isotope Measurements of atmospheric Noble gases, Fresenius'J.AnaLChem., in press 177.
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