Methodology for reference accidents definition for ITER

The safety assessment of ITER presented in the Rapport Prémilinaire de Sûreté (RPrS) for the French Regulator is based on the definition and the study of a limited set of postulated incidental and accidental sequences conservatively selected on deterministic grounds. Ultimate safety margins have also been analysed through hypothetical sequences conservatively extrapolated from the more significant accidents. The rationale for event selection consists firstly of the identification of every radiological source and its confinement barriers; failure of one or several of these barriers may then be presumed and a scenario defined, following a standardized grid; furthermore, the calculations and analysis follow a unique logical scheme to assure consistency and exhaustiveness of the report. Nine accident families have been defined: plasma events, loss of power events, in-vessel events, ex-vessel events, cryostat events, magnet events, maintenance events, tritium plant and fuel events, hot cells events. Calculations with qualified computer codes have shown that the consequences of any postulated accident respect limits defined in the safety guidelines. Moreover, no hypothetical sequence shows any cliff effect, illustrating in this way the robustness of the defence in depth approach used for ITER. It is worth to recall that, in the present status of the ITER project, the only systems subject to the analysis are the tokamak, the tritium plant and the hot cells.