Sources and distribution of artificial radionuclides in the oceans: from Fukushima to the Mediterranean Sea

Abstract

This PhD dissertation aimed to understand the sources and distributions of a diverse set of artificial radionuclides (90Sr, 129I, 134Cs, 137Cs, 236U, 237Np, 239Pu and 240Pu) in two contrasting oceanic regions. Firstly, the Pacific Ocean close to Japan, largely impacted by the accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011, which led to the release of radioactive contaminants to the environment immediately after the accident and during the following years. We focused on 90Sr, which was largely understudied despite being of major concern due to its large presence in cooling waters stored in tanks and in the nuclear facility since 2011. We collected seawater, groundwater and surface beach water samples between 0.8 and ~100 km off FDNPP in September 2013, two and a half years after the accident. In this work we also evaluated the temporal evolution of 134Cs, 137Cs and 90Sr concentrations, highlighting the relative importance of various sources in the coast off Japan over time.The radionuclide concentrations in water samples collected in September 2013 were up to 8.9 ± 0.4 Bq·m-3 for 90Sr, 124 ± 3 Bq·m-3 for 137Cs and 54 ± 1 Bq·m-3 for 134Cs in 2013, while their typical concentrations prior to the 2011 accident due to the nuclear tests were ~1 Bq·m-3, 1-2 Bq·m-3 and neglible, respectively. This confirmed the ongoing 90Sr releases from the FDNPP, that we estimated in 2.3-8.5 GB·d-1 in September 2013. Although substantially lower than the rates estimated to have occurred soon after the accident, the continuing releases and the potential larger leaks from the FDNPP evidenced the importance of continuous surveillance of artificial radionuclides in the Pacific Ocean. Secondly, the semi-enclosed Mediterranean Sea, impacted by various sources since the mid-twentieth century: i) on a global scale by atmospheric nuclear weapon tests carried during the 1950s-1960s and the Chernobyl nuclear accident in 1986; ii) at a regional level, by the low level radioactive discharges from the Marcoule nuclear reprocessing plant between 1958 and 1997; iii) and locally by the Palomares accident in 1966. Knowledge on their respective contributions to the presence of 129I, 236U, 237Np and Pu isotopes and on the radionuclide distributions in the Mediterranean Sea was limited or completely unknown. Our data on 129I and 236U showed that their distributions are governed by the thermohaline circulation, and highlighted the importance of constraining each radionuclide source. In particular, we estimated that the contamination derived from the Marcoule nuclear reprocessing plant was likely comparable to that from global fallout for 236U (10-20 kg) and about 2 orders of magnitude larger for 129I (70-90 kg). We also reported for the first time a comprehensive transect of 237Np. The distributions of both 237Np and 137Cs are driven by the circulation of the water masses, as expected for conservative radionuclides. The principal sources of 237Np were found to be global fallout and Marcoule, the later probably contributing to about 30% of the 237Np (~7 kg) present in the Mediterranean Sea in 2013. The distribution of 137Cs showed the influence of inputs and recent changes in circulation. The main source of Pu was confirmed to be global fallout, as shown by the 240Pu/239Pu atom ratios of about 0.18 throughout the Mediterranean Sea. The distribution of 239,240Pu was affected by, in addition to circulation, particle scavenging and remineralization. A notable result of this study was the lower levels of 239,240Pu in the water column of the Eastern Basin than in the Western Basin that were possible due to enchanced boundary scavenging on shelf sediments or the limited scavenging by particles in open waters and enhanced westward transport by water dynamics.

Date of Award	22 Sept 2017
Original language	English
Supervisor	Pere Masque Barri (Director), Jordi Garcia Orellana (Director) & Nuria Casacuberta Arola (Director)