Radiolytic and Thermal Processes Relevant to Dry Storage of Spent Nuclear Fuels

摘要

The scientific and engineering demands of the Department of Energy (DOE) Environmental Restoration and Waste Management tasks are enormous. For example, several thousand metric tons of metallic uranium spent nuclear fuel (SNF) remain in water storage awaiting disposition. Of this inventory, 2300 metric tons are N-Reactor fuel that have been stored for up to 24 years in the Hanford, Washington KBasins. No significant precautions were taken to prevent the fuel from corroding since the fuel rods were intended to be reprocessed. Termination of reprocessing has left these fuels stranded in prolonged water storage and an appreciable quantity of the fuel has corroded. In addition, other defense fuels including the aluminum-clad fuels at the Savannah River Site and Idaho National Engineering Laboratory have corroded during interim storage in water. In 1994, the DOE began to implement a strategy for moving water-stored Hanford fuels into dry interim storage and a Record of Decision 1 ( ROD) documenting this action was put forth by the Department of Energy on March 4, 1996. Several documents 1-4 including this ROD and the final environmental impact statement (FEIS)1, evaluated and documented concerns regarding the potential for releases of radionuclides to the environment. The DOE plans to remove metallic uranium SNF from water storage and seal it in overpack canisters for ''dry'' interim storage, for up to 75 years. Much of the SNF that will be stored will have been severely corroded during water storage. Chemically bound water not removed during proposed drying operations may lead to long-term corrosion and generation of combustible H2 and O2 gas-mixture via radiolysis. No thoroughly tested model is currently available to predict fuel behavior during ''dry'' storage. The PNNL collaborating with the Rutgers University studied the thermo-chemical and radiolytic reactions of actual and prototype SNF materials. The purpose of this project is to deliver pertinent information which can be used to make decisions concerning the safety and treatment issues associated with dry storage of spent nuclear fuel materials. In particular, we set out to establish an understanding of: (1) water interactions with failed-fuel rods and metal-oxide materials; (2) the role of thermal processes and radiolysis (solid-state and interfacial) in the generation of potentially explosive mixtures of gaseous H2 and O2, and (3) the potential role of radiation assisted corrosion during fuel rod storage. The project meets several major DOE/EMSP science needs for the Spent Nuclear Fuel Focus Area: (1) Stabilization of spent nuclear fuel, including mechanism of pyrophoricity and combustion parameters for various fuel types; (2) Characterization of spent nuclear fuel; (3) Development of methods to remove moisture without damage to fuel elements; and (4) Characterization of corrosion, degradation, and radionuclide release mechanisms, kinetics, and rates for fuel matrices.