Study on acceptable air concentration limit in helium gas inside the test cell in fusion neutron source A-FNS

Abstract

Fusion Neutron Source A-FNS, which accelerates a deuteron beam on a liquid lithium (Li) target, aims to acquire material properties under neutron irradiation—essential for the realization of the JApan’s DEMOnstration fusion reactor. Neutron irradiation is performed in a Test Cell (TC) surrounded by thick concrete shielding. Since the atmospheric condition inside the TC consists of helium gas at negative pressure, some air in-leakage into the TC is inevitable. In this study, we evaluated acceptable air concentrations in helium gas within the TC and air in-leak rates to prevent Li ignition and atmospheric activation. Based on our recent R&D activity, we established an air concentration limit in helium gas as 5 vol.% to prevent Li ignition in the event of a Li leak. Air activation calculations using the FISPACT code reveal that the tritium production amount (2.8 × 10⁶ Bq/h) is the main contribution to the tritium inventory in the TC. This tritium is primarily produced via the ¹⁴N (n,t) reactions. The air in-leak rates were determined by modeling the helium purification process and developing start-up procedures for the liquid Li target system and TC. Based on these findings, we proposed the design requirements for the TC, interlock (Machine Protection System), and detritiation system in the circulating helium purifier.