Year : 2021 | Volume
: 44 | Issue : 3 | Page : 167--170
Stable isotope analysis in environmental impact assessment of radioactivity releases from nuclear power plants
Mahadeva R Iyer
Former Head Rad. Saf. Systems Division, BARC Mumbai and Former Professional IAEA Vienna, Mumbai, India
Mahadeva R Iyer
Bungalow D4, Raj Kunj Society, Chembur, Mumbai - 400 074
The potential use of stable fission products from the decay of radioactive contaminants in recreating the impact assessment of radioactivity releases from nuclear power stations is examined. With the advances in mass spectrometry, Minimum Detectable Limits (MDLs) comparable to radioactive measurements have been achieved in the case of stable isotopes as well. The applications of stable fission products in safeguards and environmental surveillance by the author are mentioned. The limited application of stable fission products in the analysis of Fukushima environmental samples is reviewed. The article examines the possible candidates for such analysis and points out the enormous possibilities. This is a neglected field in environmental impact assessment till now and calls for standardizing measurement procedures and analysis methodologies. The stable fission product signatures can also be used to study the uptake and transport of radionuclides in the environment.
|How to cite this article:|
Iyer MR. Stable isotope analysis in environmental impact assessment of radioactivity releases from nuclear power plants.Radiat Prot Environ 2021;44:167-170
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Iyer MR. Stable isotope analysis in environmental impact assessment of radioactivity releases from nuclear power plants. Radiat Prot Environ [serial online] 2021 [cited 2022 May 23 ];44:167-170
Available from: https://www.rpe.org.in/text.asp?2021/44/3/167/334783
The possibility of using stable isotope daughter product ratios of natural radioactivity for studying geochemical and geophysical movement of these in the environment with possible application in identifying the source of Thorium deposits in the West coast of India was discussed earlier. In the present article, the potential use of stable fission products from the decay of radioactive contaminants in recreating the impact assessment of radioactivity releases from nuclear power stations is examined.
The radioactive imprints arising from possible releases of radioactive isotopes connected with nuclear fuel cycle operations are subject to radioactive decay but recreating such incidences is possible if one analyses the stable daughter products of fission products that will be still present in the environment. Conventionally, environmental impact assessment of accidental or routine releases of radioactivity from nuclear plants are based on measurements of radioactive imprints from common releases such as 90Sr, 131I, 137Cs, 134Cs, and 106Ru. Although some of these radioactive imprints remain for long time, some like 131I decay soon after the incident. The fact that radioactivity assay particularly by gamma spectrometry is easy and can be carried out at very low levels perhaps was the reason these radioactive signatures are widely used. On the other hand, the analysis of stable isotopes in ultra-trace levels often poses challenges. However, today with the advances in mass spectrometry, MDLs comparable to radioactive measurements have been achieved in the case of stable isotopes as well. Sensitivity of the order of a few femtogram (fg) is possible. This offers the scope for examining the use of such analytical tools for assessing the stable daughter products of radioactive contaminants in the environment and for recreating the radiation dose implications during the time of release. Going through references on the topic the use of stable isotopes analyses using mass spectrometry has found application in Fukushima environmental radioactivity impact analysis., They have used TIMS to assess the long-lived 135Cs. However, they have not analyzed the stable daughter products of other isotopes. The article draws the attention of environmental surveillance groups to start investigate the possibility of using stable products of radioactive releases and to standardise measurement and analysis methodologies to explore the possibilities and scout around the required hardware and develop analytical procedures.
STABLE FISSION PRODUCTS IN SAFEGUARDS
The author had occasion to suggest the use of such methods in IAEA for safeguarding long term repositories of spent fuel where the radioactive signals would have decayed. In 1995, we initiated research programs at IAEA with Prof. Latakhov of Russia to develop on line methods for this purpose. The radioactive signals of emissions from old fuel in repositories are weak, but stable isotopes signals are in plenty. Such stable fission products can appear in the environment in case of any tampering and can be used for safeguarding long-term repositories of irradiated fuel. Now IAEA use these methods for safeguard purposes.
Similarly, we have carried out measurement of long-lived 129I and stable 127I in environmental samples earlier in BARC.
1.6E7 y; fission yield - 0.88%
Stable; fission yield – 0.45% and naturally occurring.
129I/127I ratio is a very sensitive index of the impact of weapon testing on the environment globally. As compared to a steady state natural atom ratio of 2.2 × 10−12, the samples from post weapon testing era samples show ratios of the order of 10−8. Thus, the ratio is a very sensitive index of nuclear operations. Very sensitive gamma spectrometric analysis along with neutron activation analysis was standardized for this purpose. The potential use of this to evaluate impact of any accidental release need to be further explored.
85Kr has been traditionally used by IAEA to monitor the reprocessing operations in reprocessing plants.
Stable Fission Products for Recreating Nuclear Incidents
We traditionally use the ratio of 134Cs/137Cs ratio for identifying the source of radioactive depositions – to find if these are from fall out or from releases from nuclear plants and further identify the burn up of the fuel from which it has emanated in order to finger print the source of emission. Historically, we had used these ratios in India to identify the source soon after Chernobyl accident from the samples deposited in civil aircrafts which came through the region. Now the stable products of these two isotopes are 134Ba and 137Ba which remain in the environment in good quantities. It is possible to use the ratios of these two isotopes to recreate the events and can lead to information on when these were deposited and if it is from weapon fall out, Chernobyl accident release or from a nearby nuclear plant.
85Kr is a long-lived isotope which is of concern for the public dose from releases in reprocessing plants is difficult to sample and analyzes, but its daughter 85Rb is stable and amenable for the easy detection. A lot of such possibilities may exist and strategies need to be worked out making use of these stable products to recreate the dose impact from releases from nuclear facilities.
Karunakara et al. have presented the results of extensive analysis of 137Cs radioactivity measurements in the environmental samples around Kaiga Nuclear Power Station and compared with the fallout levels and reported a value of 65 Bq/kg. Using the atom ratio of 135Cs/137Cs to be 0.36 (Fukushima value) the corresponding 135Cs atom concentration would be around 108. This concentration is sufficient to employ the ICPMS mass spectrometry, as the sensitivity is reported to be much better than this. However, for employing this technique to stable fission products possible isobaric interference from other elements need to be taken care of. As such potential analysis of stable fission product daughter products of radio-toxically relevant fission products provide many possibilities.
Measurement of the fission product cesium isotopes 135Cs and 137Cs at low fg 10–15 levels in ground water by Inductively Coupled Plasma-Mass Spectrometry ICP-MS is reported by Liezers et al. They further used in line chromatographic methods to reduce isobaric interferences and achieved sensitivity of 2 fg/ml (0.1 uBq/ml) for 135Cs.
Lot of fission products is trapped in the environment due to weapon testing and from the various accidents notably the Chernobyl accident. Most of the radioactivity has died out, but the stable products are still there and may serve as a useful tool in studying the history of depositions. This will also serve as a tool for studying the movement and deposition of these nuclides in the environment. However, atmospheric dilutions need to be taken into account to make use of these signals.
The first requirement is to standardize methods and to establish measurement strategies for analysis of stable nuclides. There is also a need to generate limited database to develop methodology for interpretations. My firm belief is once such database is generated it can lead to useful applications. It will lead to new dimensions in environmental monitoring. However, someone has to take a plunge to become a pioneer!
Some Specific Examples of Use of Stable Fission Products
Kubota et al. analyzed the environmental samples from the Fukushima accident and derived isotopic ratios of cesium isotopes by gamma spectrometry and thermal ionization mass spectrometry (TIMS). The 134Cs/137Cs ratio was found to be 0.07 and the ratio of 135Cs/137Cs was found to be 0.36 which differed from the results of atmospheric nuclear tests 0 and 2.7. They conclude that the ratio are useful to assess the origin of radioactive contamination and foresee that the difference in the “ratio of 135Cs/137Cs will contribute to estimations of the origin of radioactive contamination in future.”
135Cs/137Cs was estimated by Shibahara et al., using TIMS. They report a value of 0.38 for 135Cs/137Cs and 0.031 for 134Cs/137Cs. However, these references do not mention the use of ratios with stable daughter products such as Barium. 137Ba stable product daughter of 137Cs and 134Ba daughter of 134Cs are worth analyzing using TIMS.
Extensive tabulation of fission product core inventory of all radioactive and sable fission products obtained using ORIGEN code for the MIT Research Rector (MITR) is reported by Kennedy. As comparison, for a core of 3 years in MITR, the fission product atom ratio of 135Cs/137Cs works out to 0.23. These are only typical values and vary from reactor to reactor. ORIGEN code can be used to establish the finger print of the ratios in a reactor and compare with the results of environmental samples around the reactor. This gives a powerful tool to identify the source of the isotopes in the environment and to assess the impact from the operation of the reactors.
There were no 135Cs data available for Japanese environmental samples before the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Maintaining a data bank of such ratios would help to assess the environmental impact from the operation of the reactors. 135Cs/137Cs database will be useful for the application as a tracer in the future.
There are several such ratios using stable fission products are possible some of which are indicated below
Cs 134 ---- Ba 134
2.065 y stable
Xe 135----Cs 135---- Ba 135; Fission Yield - 6.52%
9.14 hrs 2E6 yrs stable
1.6E7 y; Fission Yield - 0.88%
Recreating of public dose due to abnormal releases from nuclear facilities using stable fission products.
Nearly 30% of the fissions lead to stables fission products of Xe and Kr. Gaseous fission products provide earlier signal of radioactivity escape from NPPs easily if there is a leak and appear in the environment but these being gases are not easy to be analyzed. Kr decays to Rb and Xe decays to Cs and Ba. Most of the gaseous fission products are short lived but their stable daughter products signals are forever present in the environment and provide a potential tool for the assessment of environmental impact from the operation of NPPs. Many of these do not occur in nature otherwise and are unique signature of nuclear events. However, a strategy for the analysis has to be worked out. Some potential candidate fission product decay chains with half-lives for such fingerprinting are suggested below:
Kr87--- Rb87----Sr87; Fission Yield - 2.51%
Kr88---Rb88---Sr88; Fission Yield - 3.59%
2.8h 18m stable
Kr92----Rb92----Sr92---Y92----Zr92 Fission Yield - 5.98%
Xe 133 ---- Cs 133 Fission Yield - 6.71%
5.27 d stable
Xe 135----Cs 135---- Ba 135 Fission Yield - 6.52%
9.14 hrs 2 × E6 yrs stable
Xe 137 ----Cs 137--- Ba137 Fission Yield - 6.17%
4.2 m 30 y stable
Xe 138---- Cs138-----Ba 138 Fission Yield - 6.69
14.1m 32.2 m Stable
Similarly, 89,90Sr are important radioactive fission products which has environmental dose implications but since they are pure beta emitters, calls for elaborate radiochemical separations and the easy route of gamma spectrometry cannot be used in this case. These decay to stable 89Y and 90Zr which could be used as indicators of their precursors. The ratios would be clear fingerprints of the source and would help in recreating the incidences.
Sr89---Y89 Fission Yield - 4.73%
Sr90----Y90--- Zr90 Fission Yield - 5.82%
28.1y 64.5h stable
Takeshi Ohno et al. have used triple quadruple ICP for the analysis of Fukuhsima soil samples for 90Sr and methods to reduce isobaric interference from 90Zr and reported the detection limit of equivalent of 0.6 Bq/kg in 1 g environmental samples.
Such ratios and concentrations of nuclides would be useful in tracing the source of the depositions whether they are from operating reactors or fuel storage facilities or from fallout since the ratios carry tell-tale finger prints of the sources.
Only the possibilities are indicated by specific cases above. A more exhaustive examination need to be undertaken for identifying potential candidates. Moreover, accordingly, measurement and analysis strategies need to be standardized. However, the first requirement is to establish the methodology of analysis of stable isotopes and their ratios. There are potential methods but evidently people concentrated on radioactivity measurements because of easy detection methods. However, with advanced developments in atom level elemental analysis such as mass spectrometry, spectrophotometry, and gas chromatography, it has become possible to achieve required specificity and sensitivity. This is a neglected field in environmental impact assessment till now.
The stable fission product signatures can also be used to study the uptake and transport of radionuclides in the environment. Making use of these as tracers such analyses may also find applications in meteorology.
The article is dedicated to the memory of my Guru Dr. A K Ganguly, Doyen of Environmental Sciences.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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