Technical Aspects of Nuclear Fuel Cycles

Abstract

After discharge from the reactor core, the fuel elements are stored in a fuel element storage pool onsite for several years to allow radioactivity decay and after heat decrease. Spent fuel elements are shipped then in special fuel transport casks to either intermediate storage facilities or to the storage pool of a reprocessing plant. After a total cooling period of about 7 years LWR spent fuel elements can be chemically reprocessed. The spent fuel elements are moved from the storage pool into the disassembly cell, where they are cut up by large bundle shears into small pieces. These pieces fall into a dissolver basket filled with boiling nitric acid. The PUREX process is used to chemically separate the dissolved spent fuel into uranium, plutonium and higher actinides with fission products. The final products are uranylnitrate, plutonium nitrate and high level waste. The total capacity of commercial reprocessing facilities is currently about 4,500 t\(_\mathrm{ HM}\)/year in France, UK, Russia, Japan and India. The uranium and plutonium products can be converted into oxides and fabricated into Uranium/Plutonium mixed oxide fuel elements. The latter can be loaded into light water reactor or fast breeder reactor cores. Thorium/uranium fuel can be reprocessed using the THOREX process. The thorium/uranium-233 fuel also can be fabricated into mixed oxide fuel elements and loaded into light water reactors or fast breeder reactors. The remaining wastes are classified into high level waste, medium level waste and low level waste. The high level waste after concentration is vitrified by giving it first into a calcinator and then mixing it with borosilicate glass frits and melting this mixture to a glass. The result is a vitrified high level glass in a steel container. The fuel rod hulls and end pieces of fuel elements as well as insoluble residues are compacted by a 250 MPa press into a cylindrical container. Low level organic waste is sent to a medium temperature pyrolysis system and then to a calcination system. The end product is mixed with pastes, grouts or concrete and filled into low level waste containers. The medium and low level waste packages are sent to medium/low level waste repositories which are already in operation in France, Japan, Spain, Sweden, Finland and the USA since the early 1990s. High level waste packages are foreseen to be disposed into deep geological repositories. For the direct disposal concept of spent fuel elements either the fuel elements or only the fuel rods are loaded in high level waste containers and foreseen to be disposed in a deep geological repository. Up to now no deep geological repository is in operation, but test sites are explored and under investigation.