Architecture and management of a geological repository - Andra

4 – Waste disposal Packages4.3 Spent fuel disposal packagesTo start with, it should be pointed out that spent fuel is not considered as waste. A repository containeris presented here for exploratory purposes for spent fuel of types CU1, CU2 and CU3, in the case ofthese not being reprocessed.This chapter presents the scientific and technical aspects to be considered in the design of such arepository container, the various technical responses to be envisaged and the option favoured at thisstage of research.It also presents the design basis of this container and manufacturing techniques for feasibilityassessment.4.3.1 Presentation of main issuesFor pressurised water reactor (PWR) fuel assemblies (CU1 and CU2), two initial spent fuelconditioning options are considered: bare or in individual cladding. Both these options offer flexibilityof management upstream of disposal. Individual cladding under consideration is that determined byresearch on long-term storage of spent fuel carried out by the CEA.Other fuel assemblies (of CU3 type) are normally conditioned in cladding in installations upstream.4.3.1.1 Need for watertightness and durabilityWater seepage in fuel assemblies causes corrosion of cladding and metal parts (end caps and grids),dissolving of labile activity and alteration of fuel pellets.Metallic material corrosion speeds vary depending on the type of materials. Corrosion speeds areincreased by temperature and radiolysis.Irradiated uranium oxide pellets (UO 2 ), containing most of the fuel activity, are degraded bydissolving in the presence of water. Water radiolysis which generates localised hyperoxidising agentssuch as oxygenated water (H 2 O 2 ) is likely to initially accelerate dissolvation.In addition, as previously indicated for vitrified waste, the behaviour of radionuclides dissolved inwater is affected by temperature.These factors lead to the use of waterproof containers so that spent fuel does not come into contactwith water at least during the heat transfer phase. A minimum duration of 10,000 years has beenadopted for container watertightness as regards CU1 and CU2 types and a duration of a thousand yearsfor CU3 types.4.3.1.2 Other nuclear safety related issuesAs spent fuel contains a mass of residual fissile material, container design must ensure the eliminationof risks of criticality. When unloaded from the reactor CU1 type fuel (UOX) contains a mass ofresidual fissile material of 10 kg composed of 4 to 5 kg of 235 U and at least 4 kg of 239 Pu whereas CU2type fuel (MOX) contains about 20 kg of residual fissile material including more than 80% plutonium,especially 12 kg of 239 Pu 49 . These fissile material masses require in-depth analysis based on thedisposal package geometry and the characteristics of fuel assembly environment. This analysis mustcover all repository container lifetime phases especially concerning extremely long-term conditions,taking into account changes over time in fissile material masses, of disposal package geometry andcomponent materials and of the environment.49As a reminder, Pu239 critical mass is 510 g.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM144/495