[en] In a nuclear spent fuel repository, the aqueous rapid release of radio-activity from exposed spent fuel surfaces will depend on the pellet microstructure at the arrival time of water into the disposal container. Research performed on spent fuel evolution in a closed system has shown that the evolution of microstructure under disposal conditions should be governed by the cumulated α-decay damage and the subsequent helium behavior. The evolution of fission gas bubble characteristics under repository conditions has to be assessed. In UO₂ fuels with a burnup of 47.5 GWd/t, the pressure in fission gas bubbles, including the pressure increase from α-decay helium atoms, is not expected to reach the critical bubble pressure that will cause failure, thus micro-cracking in UO₂ spent fuel grains is not expected. (authors)

[en] In a repository, the release of radionuclides from spent fuel rods will strongly depend on the pellet microstructure existing when water comes into contact with the spent fuel surface, i.e. after 10,000 years of disposal. During this period, a large quantity of He atoms is produced by α-disintegrations of actinides in the spent fuel. A conservative model is proposed here to evaluate the consequences of He on the spent fuel microstructure. According to the solubility and diffusion properties of He under repository conditions, two scenarios are considered: He atoms can be trapped in fission gas bubbles or form new bubbles. In spite of the conservative assumptions of the model, the calculated values of bubble or pore pressure are much lower than critical values derived from rupture criteria. No evolution of the microstructure of the spent UO₂ fuel is thus expected before the breaching of the canister.

[en] Very early in its history, nuclear industry has taken care of the future of its wastes. Cementation processes for medium-level activity wastes, vitrification processes for minor actinide solutions and fission products are now proven technologies. The conditioning of wastes is just one ink in the full chain of the waste management process. However, this link is of prime importance because the future of the waste depends on the way it is conditioned. Reciprocally, the storage and disposal largely rely on the confidence given to the behaviour of waste packages with time. The leading role of France in the domain of radioactive wastes conditioning is a strong and valorizable asset at the international industrial plan, but also in terms of social acceptance by showing to the public that technical solutions exist. This monograph takes stock of the conditioning of nuclear wastes and describes the researches in progress, the stakes and the recent results obtained by the CEA (French atomic energy commission). Content: 1 - introduction: waste volumes and fluxes, management strategy, conditioning; 2 - Decontamination and treatment processes for effluent and technological waste; 3 - Glass, a waste conditioning matrix for the long term: Glass packages and manufacturing processes; Nuclear glass formulation, structure and properties; Long-term behavior of glasses; Cold crucible vitrification; 4 - Current conditioning systems for low and intermediate level waste: Cements as confining materials; Bitumens; Metallic structure waste conditioning; 5 - Researching alternative matrices and processes for waste treatment and conditioning: Plasma benefits for incineration / vitrification waste treatment; The Shiva process; Alternative confining matrices; Confinement of waste from pyrochemical processes: status of research; 6 - Spent fuel: a possible confining matrix?: Spent fuel initial characteristics; Spent fuel evolution in a dry storage facility; Modelling the long-term spent fuel behavior; Spent fuel containers in long-term storage and direct disposal concepts; Spent fuel storage and direct disposal: results and outlooks; 7 - general conclusion: Conditioning: a major asset for nuclear waste management; Glossary; Index