R&D on support to ITER safety assessment

Abstract

After performing its first ITER safety assessment in 2002 on behalf of the French “Autorité de Sûreté Nucléaire (ASN)”, the French “Institut de Radioprotection et de Sûreté Nucléaire (IRSN)” is now analysing the new ITER Fusion facility safety file. The operator delivered this file to the ASN as part of its request for a creation decree, legally necessary before building works can begin on the site. The IRSN first task in following ITER throughout its lifetime is to study the safety approach adopted by the operator and the associated issues. Such a challenging new technology calls for further in-house expertise and so in parallel a R&D program has been set up to support this safety assessment process, now and in the next years. Its main objectives are to identify the key parameters for mastering some risks (that would have been insufficiently justified by the operator) and to perform some verifications with methods and codes independent from the operator's ones. Priority has been given to four technical issues (others could be investigated in the future, like the behaviour of activated corrosion products). The first issue concerns the simulation of accident sequences with the help of the ASTEC European system code, developed by IRSN (jointly with its German counterpart, the GRS) for severe accidents in Pressurised Water Reactors. A preliminary analysis showed that most of its physical models are already applicable, e.g., for thermal-hydraulics in accidents caused by water or air ingress into the vacuum vessel (VV) or dust transport. Work has started in 2008 on some model adaptations, for instance oxidation of VV first wall materials by steam or air, and on validation on the ITER-specific ICE and LOVA experiments. Other model improvements are planned in the next years, as feedback from the work done for the other technical issues and from the code validation. The second issue concerns the risk of gas explosion due to concentrations of hydrogen and carbon monoxide in the ITER main volumes (VV but also the neighbouring volumes), produced by wall materials oxidation. A consistent program of modelling in the Computational Fluid Dynamics (CFD) TONUS IRSN code and of experiments in the TOSQAN (IRSN) and ENACEFF (CNRS/Orléans, France) facilities has been defined: inertising as means of mitigation, flammability limits, flame acceleration and transition to detonation, gas combustion. The third issue concerns the risk of dust explosion due to concentrations in the ITER volumes after remobilisation of dust deposits on walls. A consistent program of modelling in the CAST3M code (CEA), in collaboration with the Carthagene University (Spain), and of experiments in the BISE and TOSQAN IRSN facilities has been defined. The concerned phenomena are dust spatial distribution, remobilisation and entrainment, and explosion. The studies will also address the characterisation of dust and the control and mitigation processes. The fourth issue concerns the tritium behaviour. The theoretical analysis addresses its retention in the VV first walls, its chemistry in the gas phase during transport in the cooling circuits, and in the liquid phases during trapping in the neighbouring volumes or buildings. The efficiency of the diverse processes foreseen for detritiation, either in the VV or in the other rooms, in relation with the ventilation systems, is being also investigated.