RADIOIODINE GEOCHEMISTRY IN THE SRS SUBSURFACE ENVIRONMENT

Kaplan, D. ; Emerson, H. ; Powell, B. ; Roberts, K. ; Zhang, S. ; Xu, C. ; Schwer, K. ; Li, H. ; Ho, Y. ; Denham, M. ; Yeager, C. ; Santschi, P.

关键词: {sup 129}I;    radioiodine;    sorption;    natural organic carbon;    Kd value;    distribution coefficient;    performance assessment;    oxidation state;    redox;    anaerobic;    aerobic;   

DOI  :  10.2172/1079900
RP-ID  :  SRNL-STI-2012-00518
PID  :  OSTI ID: 1079900
Others  :  TRN: US1300101

学科分类：核能源与工程

美国|英语

来源: SciTech Connect

【 摘 要 】

Iodine-129 is one of the key risk drivers for several Savannah River Site (SRS) performance assessments (PA), including that for the Low-Level Waste Disposal Facility in E-Area. In an effort to reduce the uncertainty associated with the conceptual model and the input values used in PA, several studies have recently been conducted dealing with radioiodine geochemistry at the SRS. The objective of this report was to review these recent studies and evaluate their implications on SRS PA calculations. For the first time, these studies measured iodine speciation in SRS groundwater and provided technical justification for assuming the presence of more strongly sorbing species (iodate and organo-iodine), and measured greater iodine sediment sorption when experiments included these newly identified species; specifically they measured greater sorption coefficients (K{sub d} values: the concentration ratio of iodine on the solid phase divided by the concentration in the aqueous phase). Based on these recent studies, new best estimates were proposed for future PA calculations. The new K{sub d} values are greater than previous recommended values. These proposed K{sub d} values reflect a better understanding of iodine geochemistry in the SRS subsurface environment, which permits reducing the associated conservatism included in the original estimates to account for uncertainty. Among the key contributing discoveries supporting the contention that the K{sub d} values should be increased are that: 1) not only iodide (I{sup -}), but also the more strongly sorbing iodate (IO{sub 3}{sup -}) species exists in SRS groundwater (average total iodine = 15% iodide, 42% iodate, and 43% organoiodine), 2) when iodine was added as iodate, the measured K{sub d} values were 2 to 6 times greater than when the iodine was added as iodide, and perhaps most importantly, 3) higher desorption (10 to 20 mL/g) than (ad)sorption (all previous studies) K{sub d} values were measured. The implications of this latter point is that the iodine desorption process would be appreciably slower than the (ad)sorption process, and as such would control the rate (and the PA K{sub d} value) that iodine sorbed to and therefore migrated through the subsurface sediment. High desorption K{sub d} values would result in the ???effective K{sub d}??? for a reactive transport model being closer to the desorption K{sub d} value (the rate limiting value) than the (ad)sorption K{sub d} value. In summary, our understanding of {sup 129}I geochemistry has greatly improved, reducing the uncertainty associated with the PA???s conceptual model, thereby permitting us to reduce the conservatism presently incorporated in PA input values to describe {sup 129}I fate and transport in the SRS subsurface environment.