8th International conference
on Radioanalytical &
Nuclear Chemistry
April 5 – 10, 2009
Krypton isotopic signature study of the primary coolant of CANDU nuclear power
plant
Weihua Zhang, Kurt Ungar, Ian Hoffman and Ryan Lawrie
Radiation Protection Bureau, Product Safety Program, HECSB, Health Canada, Ottawa, Ontario
[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
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
respectively; if in the other mode, the activities for these nuclides were 1.15 ± 0.11 and 0.87 ± 0.12 (Ci).
15000 high resolution gamma-spectra files collected from Bruce B reactor
units 5 to 8 from Sept 2002 to Feb 2006 for primary coolant GFP monitoring
purpose
60
50
45
Unisampo/Shaman/Aatami gamma-spectra analysis software
50
40
Kr-85m, half-life 268.8 minutes
Kr-87, half-life 76.3 minutes
Frequency, %
Frequency, %
• Event categorization
• CMC back-trajectory modeling
• Event alarming
• Zero time calculation
• Provide info to decision maker
• Connect to Canadian FNEP
• Scientific study
• Centralized reporting
• Electronic data sharing
30
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25
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15
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10
5
0
0
0
1
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0
0.5
Krypton isotope activity, Ci
During the period of April 2004 to February 2006, total 2542 monitoring spectra from Unit 5 have been processed. The results
indicated that krypton radioisotope observation frequency is positively correlated to its half-life (left); and the relationship could
be fitted with an exponential function (right)
300
120
250
100
25
20
50
0
0
80
60
Kr89/Kr88 half-life ratio 0.019
Kr89/Kr85m, half-life ratio 0.012
10
40
Kr-87
Krypton radioisotope
Kr-89
20
15
10
5
20
4
4.5
5
0
0
Kr-88
25
5
0
Kr-85m
3.5
Five krypton isotopic activity ratio combinations are plotted together in the figure. It illustrates activity
ratios of first column pair in the legend (ordinate) versus second column of legend (abscissa) in
logarithmic scale. The isotopic activity ratio relationship plots provide unique pattern to characterize
the source of the emission, for example discrimination between nuclear reactors and nuclear
explosions. As shown in the figure, most of the ratios show an overlapping region to characterize
CANDU nuclear reactor emissions. However, there are a few outliers have been observed and spread
over several orders of magnitude. In logarithmic scale, decay-induced isotopic activity mixture will
make the point move on a straight diagonal line with progressing time, such as 87Kr/88Kr versus
87Kr/85mKr. In case of krypton precursors present in the coolant, with progressing time, the points will
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
as other sparks in the figure.
Kr89/Kr87 half-life ratio 0.041
Kr87/Kr85m, half-life ratio 0.284
15
Frequency, %
100
30
Kr88/Kr85m, half-life ratio 0.633
Frequency, %
40
Krypton isotope observation frequency, %
150
Half-life, m
Observation frequency %
200
3
35
Kr87/Kr88 half-life ratio 0.448
60
2.5
4. Characterisation of CANDU reactor primary coolant
krypton isotopic activity ratios.
40
20
80
2
Krypton isotope activity, Ci
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
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
activity ratios change is similar to xenon isotope and depends on the half-life as well as on the parent-daughter relationship.
1. Krypton isotope observation frequency in one Unit
100
1.5
3. A more distinctive patterns with histogram analysis of krypton isotopic activity ratios
------------------------ Results ------------------------
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Krypton isotope activity ratio
1
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Krypton isotope activity ratio
1.00E+03
Half-life of krypton radioisotope, m
---------------------------------- Conclusions ---------------------------------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
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
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
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
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
understanding of the noble gas absolute activity concentrations and the isotopic activity ratio at different places worldwide.
Krypton isotopic activity ratio
• Auto energy calibration
• Auto shape calibrations
• Efficiency check
• Peak report
• Nuclide identification
• Activity concentration
• Spectra categorization
Kr-88, half-life 170.4 minutes
35
Kr-89, half-life 3.16 minutes
40
Linssi database
1.00E+01
Kr87/Kr88 vs Kr87/Kr85m
1.00E-01
Kr89/Kr88 vs Kr89/Kr85m
Kr89/Kr87 vs Kr89/Kr85m
Kr89/Kr87 vs Kr89/Kr88
Kr89/Kr88 vs Kr87/Kr85m
1.00E-03
1.00E-03
1.00E-01
1.00E+01
Krypton isotopic activity ratio
1.00E+03