Gas issues in safety assessment of deep repositories for radioactive waste
In many European countries, deep geological disposal is the preferred option for the long-term management of intermediate and high-level radioactive waste, and spent nuclear fuel. It is well known that radioactive gases may be generated in deep geological repositories for radioactive wastes, and that the type and quantity of gas generation varies with waste type and repository concept. Differences of opinion arise concerning the treatment of gas issues in safety cases for such deep repositories. Organisations from seven different EU Member States and Switzerland gathered to investigate this issue in an effort to reach some common ground. The first step of GASNET was for participants to put down on paper their current approaches to the treatment of gas issues in safety assessment. Building on these, key issues were identified, and an international workshop was held to evaluate how these issues can be most effectively treated. The final report of the GASNET project documents the current approaches to the treatment of gas issues in safety cases, and discusses the strengths and weaknesses of these approaches. Gases can be produced in a number of ways, for example from the corrosion of metals or from the decay of organic matter in a repository. As the repository evolves, the type and rate of gas generation will change. Gas production can also affect the repository environment, for example the movement of groundwater. These and other aspects have to be considered in a safety case for a deep repository, assessing for example the possible consequences of repository gas production on human health. Mathematical modelling is commonly used as a tool in such safety cases, to provide insights into the issue of repository-produced gases. Different rock types have different chemical and physical characteristics that facilitate or restrict the movement of gas. Therefore the importance of gas issues in a safety case is affected by the choice of 'natural barrier' - the rock type for repository construction and the overlying geology. The report describes additional design measures that can be implemented to reduce the effects of gas generation and movement. The report also considers the human element. The way in which the repository is created and, in particular, how it is sealed can strongly impact gas production and movement. The possibility of unintentional intervention at a later time was also considered. The GASNET report represents a comprehensive look at gas safety issues for deep geological repositories. It is publicly available and should be considered key reading for governmental institutions considering such an option. This work could also help respond to public issues concerning deep geological disposal.
