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Krypton Isotopic Signature Study of the Primary Coolant of CANDU Nuclear Power

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Krypton Isotopic Signature Study of the Primary Coolant of CANDU Nuclear Power Krypton isotopic signature study of the primary coolant of CANDU nuclear power 8th International conference plant on Radioanalytical & Weihua Zhang, Kurt Ungar, Ian Hoffman and Ryan Lawrie Nuclear Chemistry Radiation Protection Bureau, Product Safety Program, HECSB, Health Canada, Ottawa, Ontario April 5 – 10, 2009 [email protected] ----------------------------------------- Introduction ---------------------------------------- Nuclear power and fuel reprocessing plants release small quantities krypton radioisotopes into environment, which may be detectable with sophisticated monitoring and analytical equipment. Monitoring of long-lived 85Kr emissions could be used to remotely verify whether plutonium separation was ongoing at a suspect nuclear site. Further, measurements in the vicinity of the site, together with other short-lived krypton isotopic activity ratios as event characterisation, such as 89Kr, 87Kr, 88Kr and 85mKr, will help to distinguish whether observed 85Kr releases are from fuel reprocessing or nuclear power plant operation. Therefore, short half-lived krypton isotopic signature study may also have some importance for nuclear safeguards. To understand nuclear power plant krypton radioisotope background emissions, actual gamma-scanning spectra from the gaseous fission product (GFP) monitoring system of a nuclear power plant were analyzed in this study. The results of nuclide activity were saved into a LINSSI database to build up noble gas inventories for nuclear power plant krypton radioisotope activity ratios analysis. The results may be used to explore the possible krypton isotopic signature of nuclear reactor releases under different operational conditions. 2. Four krypton radioisotopes activity distributions ------- Gamma-spectra automatic analysis platform ------- Histogram analysis was added to the data-mining module to study the activity distributions of 89Kr, 87Kr, 88Kr and 85mKr obtained by platform. It is clear from the figure that the patterns of krypton radioisotope activity distribution are also associated with their half-life. For example, 89Kr and 87Kr with half-life less than 100 minus (left), their activities only show one distribution peak at 0.14 ± 0.02 and 0.29 ± 0.04 (Ci) respectively; while 88Kr and 85mKr with longer half-life, (right) two distribution peaks have been observed. The results seem to indicate that the krypton radioisotope activity data in the primary coolant involves a time dependence associated with two operation modes of nuclear power reactor, i.e. steady-state (equilibrium) normal operation, and shut-down/start-up and on-line refuelling operations, which is a particular 15000 high resolution gamma-spectra files collected from Bruce B reactor characteristic of the CANDU reactor with its ability for on-line refuelling. When the reactor was running in one operation mode, its coolant total activities were 0.41±0.05 and 0.24 ± 0.04 (Ci) for 88Kr and 85mKr units 5 to 8 from Sept 2002 to Feb 2006 for primary coolant GFP monitoring purpose respectively; if in the other mode, the activities for these nuclides were 1.15 ± 0.11 and 0.87 ± 0.12 (Ci). 60 50 45 Unisampo/Shaman/Aatami gamma-spectra analysis software 50 40 35 Kr-88, half-life 170.4 minutes 40 Kr-89, half-life 3.16 minutes Linssi database Kr-85m, half-life 268.8 minutes Kr-87, half-life 76.3 minutes 30 30 25 Frequency, % Frequency, 20 Frequency, % • Event categorization 20 • CMC back-trajectory modeling • Auto energy calibration 15 • Auto shape calibrations • Event alarming • Zero time calculation • Efficiency check 10 • Peak report • Provide info to decision maker 10 • Nuclide identification • Connect to Canadian FNEP • Activity concentration • Scientific study 5 • Spectra categorization • Centralized reporting • Electronic data sharing 0 0 01234567 00.511.522.533.544.55 Krypton isotope activity, Ci Krypton isotope activity, Ci 3. A more distinctive patterns with histogram analysis of krypton isotopic activity ratios ------------------------ Results ------------------------ Six possible krypton isotopic activity ratios were calculated with the shorter lived isotope in the numerator. The activity ratios were also evaluated by histogram analysis. The analysed activity ratio distribution of each isotope pair provided characteristic krypton isotopic signatures in different operation modes of a nuclear power reactor. It is noted from the figures that except 87Kr/88Kr pair which has only one distribution peak, all isotopic ratios are mostly distributed around two values and the distribution profile gets thinner and shifted to left as the difference between two isotope half-life increases. It should also point out that 87Kr/88Kr and 87Kr/85mKr pair activity ratios exhibit a broad range, which spread over several orders of magnitude from a minimum of 0.038 to a maximum of 5.59 for 87Kr/88Kr pair; a 1. Krypton isotope observation frequency in one Unit minimum of 0.016 to a maximum of 7.25 for 87Kr/85mKr pair. As result of noble gas diffusion from defective solid fuel matrix into the primary coolant, the krypton isotopic During the period of April 2004 to February 2006, total 2542 monitoring spectra from Unit 5 have been processed. The results activity ratios change is similar to xenon isotope and depends on the half-life as well as on the parent-daughter relationship. indicated that krypton radioisotope observation frequency is positively correlated to its half-life (left); and the relationship could 4. Characterisation of CANDU reactor primary coolant be fitted with an exponential function (right) 25 40 krypton isotopic activity ratios. 120 300 120 35 20 Kr87/Kr88 half-life ratio 0.448 Five krypton isotopic activity ratio combinations are plotted together in the figure. It illustrates activity 30 100 250 100 Kr88/Kr85m, half-life ratio 0.633 Kr89/Kr87 half-life ratio 0.041 ratios of first column pair in the legend (ordinate) versus second column of legend (abscissa) in Kr87/Kr85m, half-life ratio 0.284 Kr89/Kr88 half-life ratio 0.019 logarithmic scale. The isotopic activity ratio relationship plots provide unique pattern to characterize 15 Kr89/Kr85m, half-life ratio 0.012 25 the source of the emission, for example discrimination between nuclear reactors and nuclear 80 200 80 explosions. As shown in the figure, most of the ratios show an overlapping region to characterize 20 CANDU nuclear reactor emissions. However, there are a few outliers have been observed and spread 60 150 60 10 Frequency, % Frequency, Frequency, % Frequency, over several orders of magnitude. In logarithmic scale, decay-induced isotopic activity mixture will 15 87 88 Half-life, m Half-life, make the point move on a straight diagonal line with progressing time, such as Kr/ Kr versus 87Kr/85mKr. In case of krypton precursors present in the coolant, with progressing time, the points will 40 100 40 10 Observation frequency % 5 move on a bent curve that converge with the straight diagonal line defined by decay. The orientation of the curve is determined by the decay constant of the three or four isotopes under consideration, such 5 20 50 frequency, isotope observation % Krypton 20 as other sparks in the figure. 0 0 012345678 0123456 0 0 0 Kr-85m Kr-88 Kr-87 Kr-89 0 50 100 150 200 250 300 350 Krypton isotope activity ratio Krypton isotope activity ratio 1.00E+03 Krypton radioisotope Half-life of krypton radioisotope, m ---------------------------------- Conclusions ---------------------------------- 1.00E+01 The krypton radioisotope inventories in the primary coolant of CANDU reactors are a valuable resource in the estimation of atmospheric krypton releases from the reactor site based on more or less conservative release hypotheses from the perspective of their potential confounding impact on the identification of a krypton isotopic signature for nuclear Kr87/Kr88 vs Kr87/Kr85m safeguard monitoring application. The main finding of the study is the krypton isotopic activity ratios in CANDU primary coolant. The results clearly demonstrate the krypton 1.00E-01 Kr89/Kr88 vs Kr89/Kr85m Kr89/Kr87 vs Kr89/Kr85m Krypton isotopic activity ratio isotopic Krypton isotopic activity ratio domain of CANDU nuclear power reactor, as shown in results 4. The observed outliers from this domain can be used to identify the decay-induced isotopic Kr89/Kr87 vs Kr89/Kr88 Kr89/Kr88 vs Kr87/Kr85m activity mixture with and without krypton precursors. This adds one more differentiating characteristic between nuclear power reactor of krypton radioisotope and fuel reprocessing. The background inventory study can also apply to other nuclear facilities’ radionuclide monitoring system. To enhance the efficiency and effectiveness of nuclear 1.00E-03 1.00E-03 1.00E-01 1.00E+01 1.00E+03 safeguards, the krypton radioisotope background inventories will be useful for other regions where there are nuclear facilities, but there is still a requirement for better Krypton isotopic activity ratio understanding of the noble gas absolute activity concentrations and the isotopic activity ratio at different places worldwide. .
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