Decline of Radionuclides in the Nearshore Environment Following Nuclear Reactor Closure: A U.K. Case Study

Abstract

Radioactive discharges from nuclear facilities are frequently made into the marine environment and their fate during and after cessation of discharges is a matter of interest and concern. This study examines the decline of the radionuclides 60Co and 65Zn along the southern U.K. coast, over the period 1988−1998, following the closure of the steam-generating heavy water (SGHW) reactor at AEA Winfrith, Dorset, UK. 60Co and 65Zn (and other activation products such as 63Ni and 55Fe) were widely dispersed in the marine environment off the central south coast of England, due to authorized releases from AEA Winfrith. Significant interaction occurred with clay-rich sediments and biota. A general exponential decline in 60Co activities (and in 65Zn activity) is found in intertidal mudflat sediments, seaweed and marine fauna in different areas along the south coast following closure of the reactor in 1990. Effective half-lives are determined which vary from 1 to 4 years in surface sediments (60Co only), 1−4 years in seaweed and 0.5−2.5 years in crustaceans, bivalves and molluscs. Physical mixing and bioturbation largely control the rate at which 60Co declines in surface sediments. Both 60Co and 65Zn show a relatively slow rate of decline in seaweed and in marine fauna, showing that even after the virtual cessation of discharge from nuclear facilities, contamination of these organisms may persist for a number of years, albeit at reduced activities. Reasons for this persistence are likely to include absorption of radionuclides from sediment, and release and recycling of radionuclides via breakdown of contaminated organic material.