Radiological characterization of main cooling reservoir bottom sediments at The South Texas Project Electrical Generating Station

Abstract

The South Texas Project Electrical Generating Station (STPEGS operating license directs that an effective radiological environmental monitoring program be established. Site- specific data should then augment the generation of an accurate dose model. The purpose of this study was to accurately profile the radionuclide distribution of important gamma emitting nuclides and their concentrations in the bottom sediment of the STPEGS main cooling reservoir (MCR). A Loran-C navigation system was used in conjunction with a compass to locate sampling stations. A DietzLafond bottom sampler was used to collect 70 sediment samples from 56 stations with 14 stations being sampled in duplicate. Sample analysis utilized proven standardized procedures and conventional gamma spectroscopy techniques to analyze a typical 0.7 kg sample. Count times were 6-15 hrs depending on the measurability of the radionuclide of interest or the lower limit of detection (approximately 9 pCi/kg dry) for "58CO, '60CO, and 137Cs. An inventory of "58Co, 6OCo, and 137CS in the MCR was estimated from plant effluent release records. Comparisons were made between the release records and the totals derived from the analysis of the bottom sediment. A predictive model of MCR bottom sediment activity was made. The reservoir's '60Co inventory is predicted to increase to 9.2 Ci by the year 2029. The analysis of bottom sediments suggests that 42% of the 60Co is captured in the sediment; however, analysis of other MCR media (e.g., water, biologicals) suggests that the retention percentage is in fact significantly greater. The majority of the '60Co laden sediment is found in the regions of the MCR nearest the effluent release point (MCR circulation discharge facility). The predictive model shows that the 6OCo concentration will decay to current levels approximately 15 yrs after decommissioning. This situation can be induced earlier if the source term is reduced by implementing a successful cobalt reduction program and/or the liquid waste processing system. At no point during projected plant operations or post operation does 60Co in the MCR system result in a significant dose to the public.