PREDICTING THE FATE OF ¹³⁷CESIUM IN PONDS

Barbara E. Taylor, Philip M. Dixon, Thomas G. Hinton, and Lars Håkanson

Accidental releases of radioactive contaminants are among the costs of nuclear technology. Decisions about remediation plans for these contaminants depend on reliable assessments of their environmental mobility. The experience and data from the Savannah River Site can be used to test and improve our understanding of the fate of radionuclides in lakes and reservoirs, both on the SRS and elsewhere.

In 1963-1964, radioactive material was accidentally released from R-Reactor into the the Par Pond cooling water system, apparently from faulty fuel rods in storage basins at R-Reactor. In 1984, twenty years after contamination occurred, Whicker et al. estimated that about 13% of the original release of ¹³⁷Cs from R-Reactor was retained at Pond B. From a similar inventory study in 1994, Mohler estimated that the total amount of ¹³⁷Cs in the system, including sediments, had declined by 50% since 1984, yielding an effective half-life of 10 yr, which is much shorter than the 30-yr radiological half-life.

Given the current best estimates of export rates, the ¹³⁷Cs budget for Pond B seems not to balance. Similar discrepancies occur in other rates and processes inferred from the inventory data. Whether these imbalances are artifacts of statistical error or symptoms of conceptual error is unclear. We believe that the rich bases of existing data, thoughtfully used with relatively simple models, will enable us to strengthen our inferences about processes controlling ¹³⁷Cs dynamics and to make more robust predictions about its fate in ponds and lakes.