Abstract
Radioactive particles originating from nuclear fuel reprocessing at the United Kingdom Atomic Energy Authority's Dounreay Facility were inadvertently released to the environment in the late 1950s to 1970s and have subsequently been found on site grounds and local beaches. Previous assessments of risk associated with encountering a particle have been based on conservative assumptions related to particle composition and speciation. To reduce uncertainties associated with environmental impact assessments from Dounreay particles, further characterization is relevant.Results of particles available for this study showed variation between Dounreay Fast Reactor (DFR) and Materials Test Reactor (MTR) particles, reflecting differences in fuel design, release scenarios, and subsequent environmental influence. Analyses of DFR particles showed they are small (100–300 μm) and contain spatially correlated U and Nb. Molybdenum, part of the DFR fuel, was identified at atomic concentrations below 1%. Based on SR-based micrometer-scale X-ray Absorption Near Edge Structure spectroscopy (μ-XANES), U may be present as U(IV), and, based on a measured Nb/U atom ratio of ~2, stoichiometric considerations are commensurable with the presence of UNb2O7. The MTR particles were larger (740–2000 μm) and contained U and Al inhomogeneously distributed. Neodymium (Nd) was identified in atomic concentrations of around 1–2%, suggesting it was part of the fuel design. The presence of U(IV) in MTR particles, as indicated by μ-XANES analysis, may be related to oxidation of particle surfaces, as could be expected due to corrosion of UAlx fuel particles in air. High 235U/238U atom ratios in individual DFR (3.2 ± 0.8) and MTR (2.6 ± 0.4) particles reflected the presence of highly enriched uranium. The DFR particles featured lower 137Cs activity levels (2.00–9.58 kBq/particle) than the MTR (43.2–641 kBq 137Cs/particle) particles. The activities of the dose contributing radionuclides 90Sr/90Y were proportional to 137Cs (90Sr/137Cs activity ratio ≈ 0.8) and particle activities were roughly proportional to the size. Based on direct beta measurements, gamma spectrometry, and the VARSKIN6 model, contact dose rates were calculated to be approximately 74 mGy/h for the highest activity MTR particle, in agreement with previously published estimates.
Highlights
In nuclear fuel reprocessing, uranium (U) and plutonium (Pu) from spent nuclear fuel are recovered for civil or military uses
Reports on radioactive particles found in the vicinity of reprocessing sites indicate that the dissolution of fuel may be incomplete and that residual fuel fragments and particles in the discharges can give rise to radioactive particle contamination in the environment such as in the case of Krasnoyarsk-26, Sellafield, and Dounreay reprocessing facilities (Bolsunovsky et al, 2017; Dennis et al, 2007; Geckeis et al, 2019; Lind, 2006)
Failure to recognize the presence of radioactive particles, defined by the International Atomic Energy Agency (IAEA) as “a localized aggregation of radioactive atoms that give rise to an inhomogeneous distribution of radionuclides significantly different from that of the matrix background”, may have a number of serious consequences (IAEA, 2011)
Summary
Uranium (U) and plutonium (Pu) from spent nuclear fuel are recovered for civil or military uses. Essential to reprocessing is the dissolution of the spent fuel, a procedure that increases the potential for contaminant release in liquid waste discharges (Choppin et al, 2013). Reports on radioactive particles found in the vicinity of reprocessing sites indicate that the dissolution of fuel may be incomplete and that residual fuel fragments and particles in the discharges can give rise to radioactive particle contamination in the environment such as in the case of Krasnoyarsk-26, Sellafield, and Dounreay reprocessing facilities (Bolsunovsky et al, 2017; Dennis et al, 2007; Geckeis et al, 2019; Lind, 2006). To characterize particle properties of relevance for impact assessments, a combination of advanced technologies are needed (Salbu et al, 1994; Salbu and Lind, 2020)
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