Radioactive waste derives from all phases of the nuclear fuel cycle and from the use of radioactive materials in industrial, medical, military and research applications; all such wastes must be managed safely. The most hazardous and long-lived wastes, such as spent nuclear fuel and waste from nuclear fuel reprocessing, must be contained and isolated from humans and the environment for many thousands of years. Many Nuclear Energy Agency (NEA) member countries are, therefore, researching plans for the management of long-lived radioactive waste in engineered facilities, or repositories, located deep underground in suitable geological formations.

The main challenge in development of the safety case for deep geological disposal is associated with the long periods of time over which high- and intermediate-level long-lived wastes remain hazardous. A wide range of events and processes may occur over hundreds of thousands of years. These events and processes are characterised by specific timescales. For example, the timescale for heat generation is much shorter than any geological timescale. Therefore, to reach a high level of reliability in the safety case, it is essential to have a thorough understanding of the sequence of events and processes likely to occur over the lifetime of the repository. It then becomes possible to assess the capability of the repository to fulfil its safety functions. However, due to the long periods of time and the complexity of the events and processes likely to occur, uncertainties related to all processes, data, and models need to be understood and addressed. Assessment is required over the lifetime of the radionuclides contained in the radioactive waste.

This paper summarizes the main gas formation mechanisms in deep radioactive waste repositories.Production rates and overall gas volumes were estimated and showed predominance of hydrogen production by anoxic corrosion and radiolysis for French wastes.Gas evolution in the near field has been modeled.First results issued from a sensitivity analysis showed desaturation of the storage cavities for a wide range of parameter values.

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