A two-phase thermo-siphon loop is applied to the Cold Neutron Source (CNS) of China Advanced Research Reactor (CARR). The moderator is liquid hydrogen. The two-phase thermo-siphon consists of the crescent-shape moderator cell, the moderator transfer tube, and the condenser. The hydrogen is supplied from the buffer tank to the condenser. The most characteristic point is that the cold helium gas is introduced into the helium sub-cooling system covering the moderator cell and then flows up through the tube covering the moderator transfer tube into the condenser. The helium sub-cooling system also reduces the void fraction of the liquid hydrogen and takes a role of the helium barrier for preventing air from intruding into the hydrogen system. We call the two-phase thermo-siphon the hydrogen cold system. The main part of this system is installed in the CNS channel made of 6061 aluminum alloy (6061A) of 6 mm in thickness, 270 mm in outer diameter and about 6 m in height. For confirming the safety of the CNS, the combustion tests were carried out using the hydrogen-air mixture under the conditions in which air is introduced into the tube at 1 atmosphere, and then hydrogen gas is supplied from the gas cylinder up to the test pressures. And maximum test pressure is 0.140 MPa Gauge (G). This condition includes the design accident of the CNS. The peak pressure due to combustion is 1.09 MPa, and the design strength of the CNS channel is 3 MPa. The safety of the CNS was thus verified even if the design basis accident occurs. The pressure distribution, the stress, and the displacement of the tube were also measured.

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