Comments on ''Radiation-damage Resistance in Phyllosilicate Minerals from First Principles and Implications for Radiocesium And Strontium Retention in Soils'' by M. Sassi, K.M. Rosso, M. Okumura, and M. Machida

Sassi et al. (2016) used ab initio molecular dynamics calculations to obtain threshold displacement energy (TDE) values for Mg, Si, Al, O, and H atoms within vermiculite. Then they used the TDE values ‘‘to estimate the probability of Frenkel-pair creation by direct electron ion collision’’ upon b decay of Cs, Sr, and Y. In a context of concern about the long-term effects of fallout radionuclides, they used these probability values to imply substantial radiation damage to phyllosilicates in fallout-contaminated soils and sediments: ‘‘For Cs and Sr, the calculated probability is ~36%, while for Y the probability is much greater at ~89%. The long-term retention picture that emerges is that decay will progressively alter the clay interlayer structure and charge, probably leading to delamination of the clay, and re-release of residual parent isotopes.’’ The TDE values may be useful for understanding the effects of radiation in vermiculite contaminated by highlevel nuclear waste, but in soils and sediments contaminated by radioactive fallout the expected radiation damage owing to Frenkel-pair formation will be so little and so well dispersed that it will have no appreciable effect on retention of fallout radionuclides. That such radiation damage will be little is evident from comparison of the number density of radioactive atoms in fallout-contaminated soil (<2610 cm ) with the typical number density of unit cells (~1610 cm ) in the 2:1 phyllosilicate minerals that hold most of the fallout radioactivity. That such radiation damage will be well dispersed is evident from the typical range in common silicates of b particles having sufficient energy to displace most kinds of atoms in the minerals (from ~0.1 mm to ~1 mm), which is much greater than the dimensions of clay particles. DISCUSSION