Safety Case Components and Documentation

1. Safety CaseComponents and Documentation David G Bennett 8 April 2014

2. Safety Case Components

3. 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)

4. 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

5. System Description • The system description: • 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”

6. 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

7. Safety Assessment

8. 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

9. 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

10. 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

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