Extent of tritium contamination of cryogenic helium circuit in a fusion reactor: Mechanism and probable scenarios

•Tritium permeation during cryopumps regeneration has been claimed as safety concern for cryogenic helium circuit.•Permeation is predominant over 300 K temperature level.•Level of acceptable inefficiency in 100 K regeneration has been plotted.•Maximum partial pressure of tritium for safe 470 K regeneration has been estimated. The threat to cryogenic plant safety can be dependent on the extent of tritium contamination in the cryogenic helium circuit. There are some scenarios such as regeneration of cryopumps at high temperature e.g. 300 K and 470 K where tritium can handshake with cryogenic helium. The temperature and upstream partial pressure dependency on permeation are predicted in this paper. It has been found that permeation is dominant at temperatures over 300 K. The results also show that for 100 K regeneration of cryopumps there is no possibility of tritium contamination of the cryogenic helium circuit. However, for regeneration cycles at higher temperature i.e. 300 K and 470 K this can be a serious threat, considering partial or incomplete 100 K regeneration. The results predict that 60% or more efficient (which is easily achievable) 100 K regeneration is safe for complete 300 K regeneration cycles. But, to breach the safety cap i.e. 0.2 G Bq/annum or 5.6 × 10-7 gm/annum, even 11.2 mPa of tritium partial pressure at 470 K will take only 8 regeneration cycles to breach the safety limit of tritium in cryogenic helium. The paper also defines the allowable partial pressure of tritium at 300 K and 470 K regenerations required to avoid any tritium contamination to the cryogenic helium circuit.