Local and global trace plutonium contributions in fast breeder legacy soils

Trace-level plutonium in the environment often comprises local and global contributions, and is usually anthropogenic in origin. Here, we report estimates of local and global contributions to trace-level plutonium in soil from a former, fast-breeder reactor site. The measured ²⁴⁰Pu/²³⁹Pu ratio is anomalously low, as per the reduced ²⁴⁰Pu yield expected in plutonium bred with fast neutrons. Anomalies in plutonium concentration and isotopic ratio suggest forensic insight into specific activities on site, such as clean-up or structural change. Local and global ²³⁹Pu contributions on-site are estimated at (34 ± 1)% and (66 ± 3)%, respectively, with mass concentrations of (183 ± 6) fg g^-1 and (362 ± 13) fg g^-1. The latter is consistent with levels at undisturbed and distant sites, (384 ± 44) fg g^-1, where no local contribution is expected. The ²⁴⁰Pu/²³⁹Pu ratio for site-derived material is estimated at 0.05 ± 0.04. Our study demonstrates the multi-faceted potential of trace plutonium assay to inform clean-up strategies of fast breeder legacies.

Distribution and Evolution of Fukushima Dai-ichi derived 137Cs, 90Sr, and 129I in Surface Seawater off the Coast of Japan

The Fukushima Dai-ichi Nuclear Power Plants (FDNPPs) accident in 2011 led to an unprecedented release of radionuclides into the environment. Particularly important are ⁹⁰Sr and ¹³⁷Cs due to their known health detriments and long half-lives (T_1/2 ≈ 30 y) relative to ecological systems. These radionuclides can be combined with the longer-lived ¹²⁹I (T_1/2 = 15.7 My) to trace hydrologic, atmospheric, oceanic, and geochemical processes. This study seeks to evaluate ¹³⁷Cs, ⁹⁰Sr, and ¹²⁹I concentrations in seawater off the coast of Japan, reconcile the sources of contaminated waters, and assess the application of ¹³⁷Cs/⁹⁰Sr, ¹²⁹I/¹³⁷Cs, and ¹²⁹I/⁹⁰Sr as oceanic tracers. We present new data from October 2015 and November 2016 off the coast of Japan, with observed concentrations reaching up to 198 ± 4 Bq·m^-3 for ¹³⁷Cs, 9.1 ± 0.7 Bq·m^-3 for ⁹⁰Sr, and (114 ± 2) × 10^-5 Bq·m^-3 for ¹²⁹I. The utilization of activity ratios suggests a variety of sources, including sporadic and independent releases of radiocontaminants. Though overall concentrations are decreasing, concentrations are still elevated compared to pre-accident levels. In addition, Japan's Environment Minister has suggested that stored water from the FDNPPs may be released into the environment and thus continued efforts to understand the fate and distribution of these radionuclides is warranted.

Impact of nuclear fuel reprocessing on the temporal evolution of marine radiocarbon

Radiocarbon (¹⁴C) is broadly used in oceanography to determine water ages, trace water circulation, and develop sediment- and sclerochronologies. These applications require an accurate knowledge of marine ¹⁴C levels, which have been largely perturbed by human activities. Globally during the last century the above-ground nuclear weapon testings have been the primary cause of the increased atmospheric and marine ¹⁴C. However, other anthropogenic sources may have caused important regional deviations from the bomb pulse. For the last 70 years European nuclear fuel reprocessing plants have been major contributors of ¹⁴C to air and oceans, yet, their regional impact on surrounding marine ¹⁴C has been largely overlooked. Here we use a collection of bivalve shells of known capture date and age collected from various locations, including the North Sea, the Irish Sea, Norway, and the Bay of Biscay to reconstruct the sea surface ¹⁴C over the last five decades. The measured ¹⁴C values for the period 1969-2019, reported in fraction modern, ranged from 1.1 to 1.6 in coastal waters of the Netherlands and from 1.2 to 3.2 along the coast of the UK, indicating significantly higher levels of ¹⁴C than those expected for the marine bomb pulse (0.950-1.150). The ¹⁴C peaks revealed by the shells coincide with the increase of liquid ¹⁴C releases reported from the reprocessing plants of La Hague into the English Channel, and from Sellafield into the Irish Sea. Conversely, the shells from Norway and Spain showed ¹⁴C values close to the range of the global marine bomb pulse. The observed large spatial and temporal differences in sea surface ¹⁴C show that ¹⁴C dating and tracing studies could become problematic in the English Channel, Irish Sea and North Sea for the time period covering the discharge of liquid ¹⁴C from the reprocessing plants.

Unravelling 5 decades of anthropogenic 236U discharge from nuclear reprocessing plants

Marine biogenic materials such as corals, shells, or seaweed have long been recognized as recorders of environmental conditions. Here, the bivalve Cerastoderma edule is used for the first time as a recorder of past seawater contamination with anthropogenic uranium, specifically ²³⁶U. Several studies have employed the authorized radioactive releases, including ²³⁶U, from nuclear reprocessing plants in La Hague, France, into the English Channel, and Sellafield, England, into the Irish Sea, to trace Atlantic waters and to understand recent climate induced circulation changes in the Arctic Ocean. Anthropogenic ²³⁶U has emerged over recent years as a new transient tracer to track these changes, but its application has been challenged owing to paucity of fundamental data on the input (timing and amount) of ²³⁶U from Sellafield. Here, we present ²³⁶U/²³⁸U data from bivalve shells collected close to La Hague and Sellafield from two unique shell collections that allow the reconstruction of the historical ²³⁶U contamination of seawater since the 1960s, mostly with bi-annual resolution. The novel archive is first validated by comparison with well-documented ²³⁶U discharges from La Hague. Then, shells from the Irish Sea are used to reconstruct the regional ²³⁶U contamination. Apart from defining new, observationally based ²³⁶U input functions that will allow more precise tracer studies in the Arctic Ocean, we find an unexpected peak of ²³⁶U releases to the Irish Sea in the 1970s. Using this peak, we provide evidence for a small, but significant recirculation of Irish Sea water into the English Channel. Tracing the 1970s peak should allow extending ²³⁶U tracer studies into the South Atlantic Ocean.

Potential Releases of 129I, 236U, and Pu Isotopes from the Fukushima Dai-ichi Nuclear Power Plants to the Ocean from 2013 to 2015

After the Fukushima Dai-ichi nuclear accident, many efforts were put into the determination of the presence of ¹³⁷Cs, ¹³⁴Cs, ¹³¹I, and other gamma-emitting radionuclides in the ocean, but minor work was done regarding the monitoring of less volatile radionuclides, pure beta-ray emitters or simply radionuclides with very long half-lives. In this study we document the temporal evolution of ¹²⁹I, ²³⁶U, and Pu isotopes (²³⁹Pu and ²⁴⁰Pu) in seawater sampled during four different cruises performed 2, 3, and 4 years after the accident, and we compare the results to ¹³⁷Cs collected at the same stations and depths. Our results show that concentrations of ¹²⁹I are systematically above the nuclear weapon test levels at stations located close to the FDNPP, with a maximum value of 790 × 10⁷ at·kg^-1, that exceeds all previously reported ¹²⁹I concentrations in the Pacific Ocean. Yet, the total amount of ¹²⁹I released after the accident in the time 2011-2015 was calculated from the ¹²⁹I/¹³⁷Cs ratio of the ongoing ¹³⁷Cs releases and estimated to be about 100 g (which adds to the 1 kg released during the accident in 2011). No clear evidence of Fukushima-derived ²³⁶U and Pu isotopes has been found in this study, although further monitoring is encouraged to elucidate the origin of the highest ²⁴⁰Pu/²³⁹Pu atom ratio of 0.293 ± 0.028 we found close to FDNPP.

Anthropogenic 236U and 129I in the Mediterranean Sea: First comprehensive distribution and constrain of their sources

The first basin-wide distribution of ²³⁶U/²³⁸U atom ratios and ¹²⁹I concentrations is presented for the Mediterranean Sea. During the GEOTRACES GA04S-MedSeA expedition in 2013 seawater was collected from 10 vertical profiles covering the principal sub-basins of the Mediterranean Sea. The main objective was to understand the distributions of ²³⁶U and ¹²⁹I in relation to the water masses, and to constrain their sources in this region. The ²³⁶U/²³⁸U atom ratios and the ¹²⁹I concentrations ranged from (710±40)×10^-12 to (2220±60)×10^-12 and from (4.0±0.1)×10⁷ to (13.8±0.3)×10⁷at·kg^-1, respectively. The results show that radionuclide-poor Atlantic Water is entering at the surface through the Strait of Gibraltar whereas comparably radionuclide-enriched Levantine Intermediate Water is sinking in the Eastern Basin and flowing westward at intermediate depths. Low radionuclide levels were found in the oldest water masses at about 1000-2000m depth in the Eastern Basin. At greater depths, waters were relatively enriched in ²³⁶U and ¹²⁹I due to dense water formation occurring in both, the Eastern and Western Basins. The inventories of ²³⁶U and ¹²⁹I cannot be explained only by global fallout from atmospheric nuclear bomb testings carried out in the 1950s and 1960s. We estimate that the liquid input of ²³⁶U from the nuclear reprocessing facility of Marcoule (France), via the Rhône river, was of the same order of magnitude than the contribution from global fallout, whereas liquid and gaseous releases of ¹²⁹I from Marcoule were up to two orders of magnitude higher than global fallout. For both radionuclides, the contribution from the Chernobyl accident is found to be minor.

Reassessment of (90)Sr, (137)Cs, and (134)Cs in the Coast off Japan Derived from the Fukushima Dai-ichi Nuclear Accident

The years following the Fukushima Dai-ichi nuclear power plant (FDNPP) accident, the distribution of (90)Sr in seawater in the coast off Japan has received limited attention. However, (90)Sr is a major contaminant in waters accumulated within the nuclear facility and in the storage tanks. Seawater samples collected off the FDNPP in September 2013 showed radioactive levels significantly higher than pre-Fukushima levels within 6 km off the FDNPP. These samples, with up to 8.9 ± 0.4 Bq·m(-3) for (90)Sr, 124 ± 3 Bq·m(-3) for (137)Cs, and 54 ± 1 Bq·m(-3) for (134)Cs, appear to be influenced by ongoing releases from the FDNPP, with a characteristic (137)Cs/(90)Sr activity ratio of 3.5 ± 0.2. Beach surface water and groundwater collected in Sendai Bay had (137)Cs concentrations of up to 43 ± 1 Bq·m(-3), while (90)Sr was close to pre-Fukushima levels (1-2 Bq·m(-3)). These samples appear to be influenced by freshwater inputs carrying a (137)Cs/(90)Sr activity ratio closer to that of the FDNPP fallout deposited on land in the spring of 2011. Ongoing inputs of (90)Sr from FDNPP releases would be on the order of 2.3-8.5 GBq·d(-1) in September 2013, likely exceeding river inputs by 2-3 orders of magnitude. These results strongly suggest that a continuous surveillance of artificial radionuclides in the Pacific Ocean is still required.