Architecture and management of a geological repository - Andra

2 – General DescriptionThis chapter gives an overview of the architecture and management of a reversible disposal system.It describes the main characteristics of the waste to be disposed of and the functions the repositorymust fulfil over time. The main purpose of these functions is to protect the environment and futuregenerations from any risks generated by the waste. They also incorporate a reversibility requirement.The chapter then describes the geological situation for which disposal feasibility has been studied.It also briefly describes the technical options for fulfilling the repository's functions and the principleson which those options are based. These technical options concern the waste disposal containers andthe architecture of the repository.The chapter ends with a summary recapitulation of the role each repository component plays infulfilling the required functions.2.1 Waste studiedThe waste studied (called High Level and Long Lived, or HLLL waste) contains both short-livedradionuclides, usually in large quantities (high level), and long-lived radionuclides 1 in medium to verylarge quantities.The main danger of long-lived radionuclides is from ingestion, exposing living tissue to α radiation;the half-life 2 of some isotopes is more than a hundred thousand years.A high proportion of HLLL waste also has high γ radiation activity, which means that humans must beprotected from external exposure.The β-γ activity in HLLL waste decays fairly quickly. After a few decades, nuclear fuel contains onlya few per cent of the radioactivity it displayed when unloaded from the reactor.The energy generated by radioactivity is mainly converted into heat. The radiation is absorbed by thewaste package's constituent matter and, to a lesser extent, the matter in its immediate vicinity. A fewcenturies later, when the β-γ radioactivity has largely decayed, the residual radioactive energy (fromthe long-lived isotopes) is very low and the amount of heat produced is no longer significant.The decrease in β-γ activity over time suggests that for the most highly radioactive waste there shouldbe an interim period between production and disposal. This can be achieved with temporary storagefacilities. During this period the heat produced by the waste diminishes, and this affects thedimensioning of the disposal installations and their footpint in the host formation.2.1.1 Origin and properties of HLLL wasteHLLL waste is produced by the nuclear power industry, research activities and national defense. Tostudy disposal possibilities, Andra drew up an "inventory model" consolidating data and hypotheseson HLLL waste [3]. It takes future waste into account as well as existing waste.Nuclear power industry waste mainly comes from spent fuel unloaded from nuclear power reactors. Atpresent Cogema reprocesses this fuel in its plants at La Hague. In reprocessing, the uranium andplutonium are separated from the residue – fission products, minor actinides and the mechanicalstructures of the fuel assemblies (cladding sections, end cap parts).For the disposal study, all waste produced by existing nuclear power stations has been taken intoaccount, based on a hypothetical average of forty years in operation for each reactor.12Long-lived isotopes include (i) fission and activation products produced, respectively, by the division of heavy atoms such as uraniumand plutonium during fission in a reactor, and neutron absorption by materials (mainly metals) present in the reactor, (ii) actinides,consisting of uranium and heavier atoms formed when uranium captures neutrons.The half-life of an isotope is the time required for 50% of the amount of the isotope present to decay (i.e. to be spontaneouslytransformed into another element, whether radioactive or stable). An isotope is said to be long-lived if its half-life is strictly longer than30 years.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM30/495