The safe management of waste produced from the Nuclear Industry is a vital part of the safety of the industry as a whole. Waste Management covers a wide variety of aspects ranging from waste handling, packaging, transport and eventual storage or disposal. Whilst Serco Assurance has significant experience in all areas of Waste Management, one area of that must be considered to be of particular safety concern is that of nuclear criticality safety.
Nuclear criticality safety is the prevention of an inadvertent self sustaining nuclear chain reaction. It is vital that such a chain reaction is prevented and the undertaking of a criticality safety assessment is part of the overall safety justification process.
Waste, by its very nature, can be varied in composition and the precise knowledge of its form and exact content can be impossible to obtain. It is important that the criticality safety analysis undertaken considers all important aspects of the waste material including:
Any information used as part of the underlying safety justification has to be defendable but in many cases a generic approach has to be undertaken which looks at all of the above aspects, frequently requiring extensive sensitivity studies. Primarily, Serco Assurance will use criticality computer codes such as MONK and WIMS to undertake such criticality safety analysis. These have the ability to assess a wide variety of fissile material systems and geometries that may not be covered by published data or criticality safety handbooks.
Serco Assurance has considerable experience in undertaking criticality safety analysis for waste management issues. Experience particularly includes the safety assessment of current waste storage facilities at UKAEA facilities as well as the possible disposal of waste in a National deep repository undertaken for UK Nirex Ltd. The assessment of the latter poses a particular challenge where criticality safety assurance is required over a period of 1 million years. This includes not only the safety of waste during the emplacement stage and the period of institutional control, but also the longer term issues where waste packaging will have degraded and the fissile material in the waste may have the ability to migrate to other parts of the repository.