THE MOVEMENT OF RADIONUCLIDES IN CRYSTALLINE BEDROCK AT THREE NUCLEAR POWER PLANT SITES IN NEW ENGLAND

The author has developed groundwater models at three nuclear power plant decommissioning sites in New England over the past eight years in support of obtaining License Termination from the Nuclear Regulatory Commission. All three sites have released radionuclides into the groundwater through various plant components that stored or moved radioactive materials in liquid form, or from diffusive release of isotopes from “activated” plant structures such as the concrete containment systems surrounding the plant reactor vessel. The primary radionuclide of interest at each site has been tritium, which is a non-reactive and non-retarded contaminant in groundwater. However, at two of the sites there were locally high concentrations of other nuclides found in groundwater such as strontium-90, cesium-137, and cobalt-60, all of which exhibit some degree of retardation in soil and bedrock.

Groundwater modeling was used first to integrate the site characterization work developed from field mapping, borings and monitoring wells, testpits, remote sensing, and geophysics. Where possible, groundwater modeling was used to reproduce measured water levels on the site in both transient and steady-state conditions, and reproduce known radioactive plume development. Modeling was used to predict the future radioactive plume fate and transport under various scenarios such as pumping on the far side of major water bodies that separate the plant site from residential areas. Modeling was used to decide how much contaminated soil and rock had to be removed to result in a plume that did not exceed regulatory standards. Modeling has been relied upon heavily in the design of the long-term groundwater monitoring systems.

We used 3-D porous media models incorporating both soil and bedrock and adapted them for use in modeling both generalized plume movement in bedrock as well as discrete fracture zones that were identified or inferred. This presentation will describe the nature and extent of several radionuclide plumes in bedrock, how the bedrock fracture system was characterized as to both its flow system and its geochemical transport system, and how remediation has conducted to reduce source terms.