Characterization of radioactive particles from the Dounreay nuclear reprocessing facility

Ian Byrnes; Ole Christian Lind; Elisabeth Lindbo Hansen; Koen Janssens; Brit Salbu

doi:10.1016/j.scitotenv.2020.138488

Characterization of radioactive particles from the Dounreay nuclear reprocessing facility

Sci Total Environ. 2020 Jul 20:727:138488. doi: 10.1016/j.scitotenv.2020.138488. Epub 2020 Apr 7.

Authors

Ian Byrnes¹, Ole Christian Lind², Elisabeth Lindbo Hansen³, Koen Janssens⁴, Brit Salbu²

Affiliations

¹ Center for Environmental Radioactivity (CERAD CoE), Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences (NMBU), P.O. Box 5003, 1433 Ås, Norway. Electronic address: ian.byrnes@nmbu.no.
² Center for Environmental Radioactivity (CERAD CoE), Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences (NMBU), P.O. Box 5003, 1433 Ås, Norway.
³ Center for Environmental Radioactivity (CERAD CoE), Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences (NMBU), P.O. Box 5003, 1433 Ås, Norway; Norwegian Radiation and Nuclear Safety Authority (DSA), P.O. Box 329, Skøyen, NO-0213 Oslo, Norway.
⁴ AXES, Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.

PMID: 32339828
DOI: 10.1016/j.scitotenv.2020.138488

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 UAl_x 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.

Keywords: Contact dosimetry; Dounreay; Electron microscopy; Micro-XANES; Micro-XRF; Uranium particles.