PNNL

Abstract

The Hanford Site was originally established in 1943 as part of the World War II Manhattan Project to produce a nuclear weapon. During the Site’s early history, eight single-pass reactors were constructed along the “Hanford Reach” of the Columbia River to produce plutonium. Reactor coolant effluent was held temporarily in retention basins so that short-lived activation products and temperature could dissipate before discharge to the river. Reactor components included valves and pumps constructed with Stellite, an alloy containing high levels of cobalt and other metals. Neutron activation of these components produced cobalt-60. As these components aged, they deteriorated and released radioactive particles into the liquid effluent. Over the 26 years of reactor operations, relatively small numbers of these particles were released to the Columbia River along with the liquid discharges, and the particles were deposited in sediment along the shoreline and on islands. In 1976, portions of the Hanford Reach were opened for public access and the presence of these radioactive cobalt-60 particles became a concern for public exposure. A survey conducted in 1979 determined that the particles were small, with a diameter of approximately 0.1 mm, and their activity level was estimated to be between 63 and 890 GBq. Dose rates from the particles ranged from 1 to 14 µGray/hr. Fourteen particles were collected during the 1979 survey and subsequent monitoring and particle clean-up campaigns continued during the 1980s and 1990s. The presence of radioactive particles in the river environment was a continuing concern as cleanup of the Hanford Site accelerated during the 1990s. Principal issues included: 1) Site management response to the presence of radioactive particles in the Columbia River, 2) methods to monitor this contamination, 3) stakeholder concerns, and 4) anti-nuclear activist intervention. Reducing ecological and human health risk caused by contamination is a major focus of Site cleanup. Because of the 5.3 year half-life of cobalt-60, the radiological risk from these particles is now negligible. Also, at locations where human access is limited, some scientists believe that the reduction in ecological risk gained by cleanup activities is overshadowed by the ecological damage caused by the clean-up activities. Suggestions have been made by scientists and regulatory agencies that it may be economically and environmentally more sound to manage isolated low-level waste sites until the activity decays (i.e., natural attenuation) to levels below health concerns, when the sites can be released.