Helium-cooled test blanket module box behaviour under accidental pressurisation

Abstract

The helium-cooled lithium–lead (HCLL) breeder blanket concept is one of the two breeder blanket lines presently developed by the EU for DEMO reactor. In the short-term so-called DEMO relevant test blanket modules (TBMs) of these breeder blanket concepts shall be designed, manufactured, tested, installed, commissioned and operated in ITER for first tests in a fusion environment. For the purpose of licensing such test module in the ITER facility, a safety assessment of the different possible accidental conditions has to be performed. This paper presents the results of the thermo-mechanical calculations that have been obtained in case of internal module leak occurring between the 8.0 MPa pressurised helium circuit and the lithium–lead circuit. In a conservative manner, it is also assumed that helium cooling of the TBM box is stopped at the accident initiation. In such situation the TBM box will be entirely pressurised at 8.0 MPa and the different challenges are related to the thermo-mechanical behaviour, the plasma power hold up and the heat removal capacity of the module. An assessment to estimate the time when the TBM box can withstand the different loads induced by the situation is presented. Particularly the available time span to trigger an emergency plasma shutdown is estimated in regards of the maximal allowable stress for the EUROFER which is the test blanket module material. A 3D finite element model has been developed with CAST3M computer code taking into account the beryllium layer, the decay heat after shutdown and the thermal radiation phenomena. After a description of the model, the paper presents and explains the results and particularly the methodology followed to determine the maximal allowable stress location which must combine the thermal transient calculations and the pure mechanical calculation under the 8.0 MPa loading. The results show that the hottest spot of the first wall (FW) was the most challenged location and that the structure can withstand such accidental conditions without plasma shutdown up to 15 s. Afterwards, the structural design criteria defined by ITER for in-vessel components (SDC-IC) is not any more fulfilled. This does not necessary mean that a lithium–lead leakage can occur in the vacuum vessel. More research and development are needed to have a clear understanding of crack propagation and break size in a material like EUROFER where few data are available, particularly at the temperature reached in this accident. Finally as a conclusion of the studies presented in this paper, it can be stated that, although very conservative assumptions were taken, the time span is large enough to trigger a plasma shutdown in order to avoid more severe operational and safety consequences of this accidental situation.