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Disproportionately High Contributions of 60 Year Old Weapons-137Cs Explain the Persistence of Radioactive Contamination in Bavarian Wild Boars

Environmental Science and Technology, Felix Stäger, Dorian Zok, Anna-Katharina Schiller,    American Chemical Society, ACS Publications 30th Aug 2023

Abstract

Radionuclides released from nuclear accidents or explosions pose long-term threats to ecosystem health. A prominent example is wild boar contamination in central Europe, which is notorious for its persistently high ¹³⁷Cs levels. However, without reliable source identification, the origin of this decades old problem has been uncertain. Here, we target radiocesium contamination in wild boars from Bavaria. Our samples (2019–2021) range from 370 to 15,000 Bq·kg^–1 137Cs, thus exceeding the regulatory limits (600 Bq·kg^–1) by a factor of up to 25. Using an emerging nuclear forensic fingerprint, ¹³⁵Cs/¹³⁷Cs, we distinguished various radiocesium source legacies in their source composition. All samples exhibit signatures of mixing of Chornobyl and nuclear weapons fallout, with ¹³⁵Cs/¹³⁷Cs ratios ranging from 0.67 to 1.97. Although Chornobyl has been widely believed to be the prime source of ¹³⁷Cs in wild boars, we find that “old” ¹³⁷Cs from weapons fallout significantly contributes to the total level (10–68%) in those specimens that exceeded the regulatory limit. In some cases, weapons-¹³⁷Cs alone can lead to exceedances of the regulatory limit, especially in samples with a relatively low total ¹³⁷Cs level. Our findings demonstrate that the superposition of older and newer legacies of ¹³⁷Cs can vastly surpass the impact of any singular yet dominant source and thus highlight the critical role of historical releases of ¹³⁷Cs in current environmental pollution challenges.

Synopsis

Sixty years old ¹³⁷Cs from nuclear weapons fallout contributes significantly to the notorious contamination levels in wild boars in Central Europe that were previously believed to be dominated by Chornobyl.

Introduction

In the face of climate change, nuclear energy is experiencing a renaissance as a low-carbon option to feed humanity’s hunger for energy. (1) However, the release of radionuclides into the environment from nuclear accidents or nuclear weapons fallout poses potential threats to public health and societies and economic activities as some radionuclides are capable of persistently contaminating the food chain, resulting in widespread and long-term risk of radiation exposure. (2,3) The fission product cesium-137 (¹³⁷Cs, half-life T_1/2 = 30.08 y) is a prominent example of such contaminants as it is ubiquitously present in the environment. It originates from the fallout of atmospheric nuclear explosions from the mid-20th century (weapons-¹³⁷Cs) and nuclear accidents, most prominently the Chornobyl (4) and Fukushima (5,6) nuclear accidents (reactor-¹³⁷Cs).

For safety regulations, many countries have employed strict regulatory limits for ¹³⁷Cs levels in general food products (e.g., EU < 600 Bq·kg^–1 and Japan: <100 Bq·kg^–1). (7) However, although routine radiation surveillance provides essential quantitative information on ¹³⁷Cs contamination levels, the attribution of a contamination to its origins remains poorly understood as the ubiquitous weapons-¹³⁷Cs cannot be distinguished from any reactor-¹³⁷Cs. This analytical challenge impedes the comprehensive understanding of the origin of environmental ¹³⁷Cs contamination, which is a critical prerequisite for a quantitative assessment of the responsibilities for certain ¹³⁷Cs legacies and the establishment of more targeted strategies for environmental remediation and protection. More than ever, with threats of nuclear strikes or accidental releases in the course of the Russo-Ukrainian war, it is now imperative to be able to identify the source of any release of ¹³⁷Cs and evaluate their environmental consequences.

Synopsis

Sixty years old ¹³⁷Cs from nuclear weapons fallout contributes significantly to the notorious contamination levels in wild boars in Central Europe that were previously believed to be dominated by Chornobyl.

Introduction

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In the face of climate change, nuclear energy is experiencing a renaissance as a low-carbon option to feed humanity’s hunger for energy. (1) However, the release of radionuclides into the environment from nuclear accidents or nuclear weapons fallout poses potential threats to public health and societies and economic activities as some radionuclides are capable of persistently contaminating the food chain, resulting in widespread and long-term risk of radiation exposure. (2,3) The fission product cesium-137 (¹³⁷Cs, half-life T_1/2 = 30.08 y) is a prominent example of such contaminants as it is ubiquitously present in the environment. It originates from the fallout of atmospheric nuclear explosions from the mid-20th century (weapons-¹³⁷Cs) and nuclear accidents, most prominently the Chornobyl (4) and Fukushima (5,6) nuclear accidents (reactor-¹³⁷Cs). For safety regulations, many countries have employed strict regulatory limits for ¹³⁷Cs levels in general food products (e.g., EU < 600 Bq·kg^–1 and Japan: <100 Bq·kg^–1). (7) However, although routine radiation surveillance provides essential quantitative information on ¹³⁷Cs contamination levels, the attribution of a contamination to its origins remains poorly understood as the ubiquitous weapons-¹³⁷Cs cannot be distinguished from any reactor-¹³⁷Cs. This analytical challenge impedes the comprehensive understanding of the origin of environmental ¹³⁷Cs contamination, which is a critical prerequisite for a quantitative assessment of the responsibilities for certain ¹³⁷Cs legacies and the establishment of more targeted strategies for environmental remediation and protection. More than ever, with threats of nuclear strikes or accidental releases in the course of the Russo-Ukrainian war, it is now imperative to be able to identify the source of any release of ¹³⁷Cs and evaluate their environmental consequences.

While isotopic signatures of actinides (e.g., uranium and plutonium) have been used successfully to distinguish the contributions between various sources, (8,9) radiocesium isotopic fingerprints have not yet been applied routinely for source identification. Cesium-135 is an ideal and long-lived candidate (T_1/2 = 2.3 My) after a release, better suited than fast-fading ¹³⁴Cs (T_1/2 = 2.07 y). Also, the production mechanism of ¹³⁵Cs provides more detailed information on the nuclear origin of a contamination, which hence allows attribution of a radiocesium contamination to its source via its distinct ¹³⁵Cs/¹³⁷Cs ratio. Its mother nuclide (¹³⁵Xe) has a large cross-section for thermal neutron capture, resulting in suppressed onset of ¹³⁵Cs under the high neutron flux density of a reactor core. (10) By contrast, despite the intense but short neutron flux at the moment of a nuclear explosion, ¹³⁵Xe mostly “survives” the explosion because most primary fission products of the 135 isobar are ¹³⁵Te and ¹³⁵I, which have yet to decay to ¹³⁵Xe. (11)

A nuclear explosion hence yields a relatively high ¹³⁵Cs/¹³⁷Cs ratio, whereas a reactor yields a low ratio. Nowadays, analytical protocols for commercial triple quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ-MS) as well as thermal ionization mass spectrometry (TIMS) are available for the precise determination of ¹³⁵Cs/¹³⁷Cs, thus allowing the application of the ¹³⁵Cs/¹³⁷Cs ratio as an isotopic fingerprint in nuclear forensics and environmental tracing studies. (12−19) In any case, the application of ¹³⁵Cs/¹³⁷Cs as a forensic fingerprint is still far from routine as it requires meticulous chemical separation and sophisticated analytical procedures.

Synopsis

Sixty years old ¹³⁷Cs from nuclear weapons fallout contributes significantly to the notorious contamination levels in wild boars in Central Europe that were previously believed to be dominated by Chornobyl.

Introduction

ARTICLE SECTIONS

Jump To

---

In the face of climate change, nuclear energy is experiencing a renaissance as a low-carbon option to feed humanity’s hunger for energy. (1) However, the release of radionuclides into the environment from nuclear accidents or nuclear weapons fallout poses potential threats to public health and societies and economic activities as some radionuclides are capable of persistently contaminating the food chain, resulting in widespread and long-term risk of radiation exposure. (2,3) The fission product cesium-137 (¹³⁷Cs, half-life T_1/2 = 30.08 y) is a prominent example of such contaminants as it is ubiquitously present in the environment. It originates from the fallout of atmospheric nuclear explosions from the mid-20th century (weapons-¹³⁷Cs) and nuclear accidents, most prominently the Chornobyl (4) and Fukushima (5,6) nuclear accidents (reactor-¹³⁷Cs). For safety regulations, many countries have employed strict regulatory limits for ¹³⁷Cs levels in general food products (e.g., EU < 600 Bq·kg^–1 and Japan: <100 Bq·kg^–1). (7) However, although routine radiation surveillance provides essential quantitative information on ¹³⁷Cs contamination levels, the attribution of a contamination to its origins remains poorly understood as the ubiquitous weapons-¹³⁷Cs cannot be distinguished from any reactor-¹³⁷Cs. This analytical challenge impedes the comprehensive understanding of the origin of environmental ¹³⁷Cs contamination, which is a critical prerequisite for a quantitative assessment of the responsibilities for certain ¹³⁷Cs legacies and the establishment of more targeted strategies for environmental remediation and protection. More than ever, with threats of nuclear strikes or accidental releases in the course of the Russo-Ukrainian war, it is now imperative to be able to identify the source of any release of ¹³⁷Cs and evaluate their environmental consequences.

While isotopic signatures of actinides (e.g., uranium and plutonium) have been used successfully to distinguish the contributions between various sources, (8,9) radiocesium isotopic fingerprints have not yet been applied routinely for source identification. Cesium-135 is an ideal and long-lived candidate (T_1/2 = 2.3 My) after a release, better suited than fast-fading ¹³⁴Cs (T_1/2 = 2.07 y). Also, the production mechanism of ¹³⁵Cs provides more detailed information on the nuclear origin of a contamination, which hence allows attribution of a radiocesium contamination to its source via its distinct ¹³⁵Cs/¹³⁷Cs ratio. Its mother nuclide (¹³⁵Xe) has a large cross-section for thermal neutron capture, resulting in suppressed onset of ¹³⁵Cs under the high neutron flux density of a reactor core. (10) By contrast, despite the intense but short neutron flux at the moment of a nuclear explosion, ¹³⁵Xe mostly “survives” the explosion because most primary fission products of the 135 isobar are ¹³⁵Te and ¹³⁵I, which have yet to decay to ¹³⁵Xe. (11) A nuclear explosion hence yields a relatively high ¹³⁵Cs/¹³⁷Cs ratio, whereas a reactor yields a low ratio. Nowadays, analytical protocols for commercial triple quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ-MS) as well as thermal ionization mass spectrometry (TIMS) are available for the precise determination of ¹³⁵Cs/¹³⁷Cs, thus allowing the application of the ¹³⁵Cs/¹³⁷Cs ratio as an isotopic fingerprint in nuclear forensics and environmental tracing studies. (12−19) In any case, the application of ¹³⁵Cs/¹³⁷Cs as a forensic fingerprint is still far from routine as it requires meticulous chemical separation and sophisticated analytical procedures.

Bavaria, southeastern Germany, is notorious for its heavy ¹³⁷Cs contamination following the Chornobyl nuclear accident. (20) It was reported that ¹³⁷Cs inventory in surface soil ranged from 10² to 10⁵ Bq·m^–2 in April 1986 [data from the Federal Office for Radiation Protection (BfS), Germany]. As a potent accumulator of ¹³⁷Cs, (21,22) regional wild boars (Sus scrofa) were subsequently contaminated, and the ¹³⁷Cs activity concentrations in their meat exceeded the regulatory limit by approximately 1–2 orders of magnitude. However, unlike most forest species, which initially also exhibited high ¹³⁷Cs contamination in their bodies followed by a decline with time (i.e., a short ecological half-life), (23,24) ¹³⁷Cs levels in wild boars have not shown a significant decline trend since 1986. (20,25)

In certain locations and instances, the decline in contamination levels is even slower than the physical half-life of ¹³⁷Cs. (26) This phenomenon has been termed “wild boar paradox” and is generally attributed to the ingestion of ¹³⁷Cs accumulating hypogeous fungi (e.g., deer truffle, Elaphomyces) by wild boars. (27,28) Depending on the soil composition, especially clay mineral content, (29) these underground mushrooms are a critical repository of the downward migrating ¹³⁷Cs. They are one major food item for wild boars, particularly during winter when food on the surface is scarce. (30) However, due to the lack of convincing evidence for identifying the sources of ¹³⁷Cs, the origins of the persistent contamination in wild boars remains unclear.

Here, we analyzed the ¹³⁷Cs activities together with ¹³⁵Cs/¹³⁷Cs ratios in wild boar meat samples, collected from 11 Bavarian districts during 2019–2021. Reporting the largest environmental sample set of ¹³⁵Cs/¹³⁷Cs to date (n = 48), we undertook a critical comparison with the published values and validated the feasibility of utilizing ¹³⁵Cs/¹³⁷Cs for source identification. Using a mixing model, we estimated the contribution of weapons-¹³⁷Cs and reactor-¹³⁷Cs, which not only deepens our understanding of the “wild boar paradox” but may also allow a future location-specific prediction of the evolution of the ¹³⁷Cs contamination in wild boars with time. Lastly, our method can be applied for the traceability of ¹³⁷Cs in any environmental samples in the future.

Materials and Methods……………………………………………………..

Results and Discussion………………………………………………………..

……..more https://pubs.acs.org/doi/full/10.1021/acs.est.3c03565

September 8, 2023 - Posted by Christina Macpherson | environment, Germany, Reference