Safety Case Components and Documentation David G Bennett 8 April 2014

Safety Case Components

Safety Case Context The safety case context comprises: Regulatory requirements and criteria for the safety case The particular decision step in the lifecycle of the disposal facility Key disposal system characteristics e.g. the nature of the waste and the site The purpose of the safety assessment The assessment timeframes Assessment philosophy (e.g. conservative, realistic) Assessment end-points (e.g. dose, risk, fluxes, others)

Safety Strategy The safety strategy comprises: The high-level approach for achieving safe disposal The overall management strategy for the activities required in planning, operation and closure of a disposal facility The preferred strategy for the management of all radioactive waste is containment and isolation from the accessible biosphere [SSR-5] The set of intended safety functions the timeframes over which they will be available explanation of system robustness and defence in depth

System Description The system description: Related terms: Provides information on the disposal system Demonstrates system understanding Provides the basis for safety assessment Helps to determine needs for further system characterisation and facility design work Related terms: The system description includes much of what is sometimes called the “assessment basis” or the “phenomenological basis” Also closely related to the “site descriptive model”

System Description The system description should provide information on: The facility design and the reasons for its selection The “near-field”: The wastes (e.g., origin, quantities, properties, radionuclide inventory), The engineering (e.g., waste conditioning and packaging, disposal units, engineered barriers, disposal facility cap, drainage) The zone disturbed by excavations The “far-field” - e.g., geology, hydrogeology, geochemistry, tectonic and seismic conditions, erosion rates The “biosphere” - e.g., climate and atmosphere, water bodies, human activities, biota, surface geology, topography

Safety Assessment

Limits, Controls and Conditions The safety case should be used to assist in the establishment of limits, controls and conditions, e.g: Site-specific limits on the total waste inventory, on acceptable concentration levels for specific radionuclides in the waste, and other waste acceptance criteria (WAC) Particularly relevant for near-surface disposal facilities Controls and conditions on repository construction and on the manufacture, materials and quality of engineered barriers and their emplacement Conditions for a monitoring and surveillance programme

Integration of Safety Arguments Showing that safety assessment results comply with regulatory criteria is not sufficient Multiple lines of reasoning should be used, including discussion of: The use of best available techniques The history of design optimisation Waste isolation and containment Passive safety Robustness and defence in depth QA and peer review Conservatisms in safety assessment Natural (and other) analogue information Application of limits, controls and conditions

Integration of Safety Arguments In summary, the safety case should: synthesise the available evidence, arguments and analyses highlight the principal reasons why planning, development and use of the disposal system should continue acknowledge any limitations of currently available evidence describe the approach that will be used to manage any open questions and uncertainties

Safety case documentation Clear Logical structure Transparent Traceable Should always explain why? Need to include accurate summary documents Written for their intended audience(s)