PNNL

Abstract

Accidental discharges of the hazardous nuclear fission products 137Cs+ and 90Sr2+ into the environment, such as during the Fukushima Dai-ichi nuclear accident, have repeatedly occurred throughout the nuclear age. Numerous studies of their fate and transport in soils and sediments have demonstrated their strong and selective binding to phyllosilicates such as clay minerals, primarily via cation exchange into interlayer sites. The locally concentrated amounts of these radioactive beta-emitters that can be found in these host minerals raises important questions regarding the long-term interplay and durability of radioisotope-clay association, which is not well known. This study goes beyond the usual short-term focus to address the permanence of radioisotope retention in clay minerals, by developing a general theoretical understanding of their resistance to defect creation. We report ab initio molecular dynamics (AIMD) calculations of the threshold displacement energy (TDE) of each symmetry-unique atomic specie comprising the unit cell of model vermiculite. The determined TDE values are material specific and radiation independent. We use them to estimate the probability of Frenkel pair creation via direct electron-ion collision, as could be induced by the passage of a high energy electron emitted during the beta-decay of 137Cs, 90Sr and daughter 90Y. For 137Cs and 90Sr, we found that the probability is about 36%, while for 90Y the probability is much higher with about 89%. The long-term retention picture that emerges is that decay will progressively alter the clay interlayer structure and charge, likely leading to delamination of the clay and re-release of residual parent isotopes. Future work examining the effect of Frenkel defect accumulation on the binding energy of parent and daughter radionuclides in the interlayer is thus justifiable, and potentially important for accurate long-term forecasting of radionuclide transport in the environment.