Burnup Credit in Spent-Fuel Transport to COGEMA La Hague Reprocessing Plant

As consignor on behalf of COG EM A, Nuclear Transport Limited (NTL) is in charge of spent-fuel transports from European power plants (excluding Elcctricite de France) to the COGEMA reprocessing plant at La Hague. NTL has carried out the transport operations since the early 1970s; this now represents more than 2,380 movements to COGEMA (about 20,400 fuel elements and 5,516 t of uranium) from 29 reactors spread throughout Europe. For this purpose, NTL operates a wide range of transport casks of several designs with a capacity varying from 3 PWR fuel assemblies to 12 PWR or 32 BWR fuel assemblies for the larger cask designs. Spent-fuel transport casks are designed always to be subcritical throughout the transport cycle, including the unloading operations at the reprocessing plant. Subcriticality is accomplished by the fuel support frame also known as \"basket\" which fits into the cask cavity. The role of the basket in tem1s of criticality is to ensure a well defined geometry as well as the removal of neutrons. This is achieved by a neutron poison distributed in the walls of the basket compartments. Casks are always loaded under water, but for transport two options arc possiblc:\"wct\" casks have reactor pond water in direct contact witl1 the fuel during transport while \"dry\" casks travel with a gas-filled cavity. Unloading at La Hague takes place in wet or dry conditions: however, in the first case the water is not poisoned by the addition of boron. During the unloading operations, the fuel assemblies are placed into racks which are transferred into a storage pond for a period of time, awaiting reprocessing. Storage racks are similar in design to cask baskets and perfom1 the san1e function using the same means. The use of transport casks is subjected to package approvals issued by competent authorities. To obtain a package approval, a designer must demonstrate the subcriticality of the package under the most reactive conditions. The same applies to storage racks, which also require operating licenses. The most reactive conditions are achieved during the cask unloading, when the cavity is flooded with demineralised water and fuel elements are considered \"fresh,\" that is, still having their initial enrichment in fissile material. However, this assumption is very conservative, as, during the irradiation process, the fuel elements use the major part of the initial fissile isotopes for power generation and therefore lose part of their reactivity.