Architecture and management of a geological repository - Andra

3 – High Level Long-Lived Wastesupport systemGuide thimblescontaining cluster rodsFigure 3.2.1PWR fuel assembly with its rodsThe neutronic poison rods are used during reactor start-up (first operating cycle) to control excessreactivity due to the use of entirely new fuel; they are removed during the following cycles andtherefore become waste. The control rods are used to control the reactor power level and its immediateshut-down if necessary. They are replaced after several cycles in the reactor and also become waste.Some rods contain neutron-absorbing materials: boron, in the form of PYREX glass for the neutronicpoison rods, boron carbide (B 4 C) and/or an alloy of silver, indium and cadmium (SIC) for control rods.The number of rods containing these materials depends on the reactor.Other activated wastes from PWRs include metal waste, mainly dead-end tubes, known as coreinstrumentation system (CIS) thimbles fitted on the underside of the reactor vessel. These tubes areused to insert the neutron probes required to control the nuclear reaction. They are replaced, ifnecessary, after a certain period of use and then become waste.These various types of reactor operation waste have specific chemical and radiological characteristicsdistinguishing them from other types of B waste. Their specific chemical nature is due to the nature ofthe materials making up some of the waste. More specifically, control rods add significant quantitiesof SIC alloys and B 4 C, whereas other types of B waste generally contain none of these. Theradiological activity is due solely to activation products formed by neutronic activation of the elementsand impurities contained in the waste materials during their time in the reactor. These activationproducts are located inside the materials and are therefore unlikely to be dispersed. Among them, theactivation products that contribute most to the radioactivity of the waste are, in decreasing order,nickel 63 ( 63 Ni) which is a long-lived isotope (half-life 100 years), then iron 55 ( 55 Fe) and cobalt 60( 60 Co) which are short-lived isotopes.The high radioactivity of 60 Co has thermal consequences. This waste therefore belongs to category B,with the highest heat rating in relative terms (roughly 20 Watts per package when the packages areproduced, taking the conditioning hypotheses set out below). Since this is largely due to cobalt, theresidual heat rating of the package drops rapidly as the cobalt decays. As an illustration, the heat ratingis divided by 2 after 5 years of cooling, by 3.5 after 10 years of cooling and by 6 after 15 years ofcooling. Another consequence of this radiological inventory is the high level of radiation of thepackages. Thus, the equivalent ß-γ dose rate in pseudo-contact with the package (i.e. at a distance of5 centimetres) is roughly 50 sieverts per hour (Sv/h), at the time of its production. This is mainlyattributable to 60 Co, but also to silver-108m ( 108m Ag) whose half-life is 420 years, and remains at arelatively high level even after 10 years' cooling (around 15 Sv/h).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM77/495