Pyrolysis and its potential use in nuclear graphite disposal

摘要

Graphite is used as a moderator material in a number of nuclear reactor designs, such as Magnox and advanced gas-cooled reactors in the United Kingdom and the RBMK design in Russia. During construction, the moderator of the reactor is usually installed as an interlocking structure of graphite bricks. At the end of reactor life, the graphite moderator, weighing typically 2000 t, is a radioactive waste which requires eventual management. Radioactive graphite disposal options conventionally include (a) in situ safe-store' for extended periods to permit manual disassembly of the graphite moderator through decay of short-lived radionuclides (b) robotic or manual disassembly of the reactor core followed by disposal of the graphite blocks (c) robotic or manual disassembly of the reactor core followed by incineration of the graphite and release of the resulting carbon dioxide. Studsvik, Inc. is a nuclear waste management and waste processing company organized to serve US nuclear utility and government facilities. Studsvik's management and technical staff have a wealth of experience in processing liquid, slurry and solid low-level radioactive waste using (among others) pyrolysis and steam reforming techniques. Bradtec is a UK company specializing in decontamination and waste management. This paper describes the use of pyrolysis and steam reforming techniques to gasify graphite leading to a low-volume off-gas product. This allows the following options/advantages. (a) Safe release of any stored Wigner energy in the graphite. (b) The process can accept small pieces or a water-slurry of graphite, which enables the graphite to be removed from the reactor core by mechanical machining or water-cutting techniques, applied remotely in the reactor fuel channels. (c) In certain situations the process could be used to gasify the reactor moderator in situ. (d) The low volume of the off-gas product enables non-carbon radioactive impurities to be efficiently separated from the off-gas. (e) The off-gas product can be discharged to atmosphere if permitted, or can be absorbed and converted to a low-activity carbonate solid product, which can be used as infill around other radioactive waste.