Using CFD as Preventative Maintenance Tool for the Cold Neutron Source Thermosiphon System

The cold neutron source (CNS) system of the Open Pool Australian Light-Water (OPAL) reactor is a 20 L cryogenically cooled liquid deuterium thermosiphon system. The CNS is cooled by forced convective helium which is held at room temperature during stand-by (SO) mode and at ~20 K during normal operation (NO) mode. When helium cooling stops, the reactor is shut down to prevent the moderator cell from overheating. This computational fluid dynamics (CFD) study aims to determine whether the combined effects of conduction and natural convection would provide sufficient cooling for the moderator cell under the influence of reactor decay heat after reactor shutdown. To achieve this, two commercial CFD software packages using an identical CFD mesh were first assessed against an experimental heat transfer test of the CNS. It was found that both numerical models were valid within the bounds of experimental uncertainty. After this, one CFD model was used to simulate the thermosiphon transient condition after the reactor is shut down. Two independent shutdown conditions of different decay-heat power profiles were simulated. It was found that the natural convection and conduction cooling in the thermosiphon were sufficient for dissipating both decay-heat profiles, with the moderator cell remaining below the safe temperature of 200°C.

Design and Analysis of a Moderator Cell Structure for CARR CNS

A new moderator cell structure based on the single-phase thermo-siphon loop is proposed for the Cold Neutron Source (CNS) of China Advanced Research Reactor (CARR). Different from previous single-phase moderator cells, the design scheme of a crescent-shape hydrogen layer together with a helium cooling jacket is adopted in this paper. Theoretical analyses on cold neutron gain factor, heat transfer performance and structural strength of the moderator cell are carried out aiming at optimizing the preliminary design scheme. Results show that the modified moderator cell has high cold neutron gain factor, good heat transfer capacity and low stress, which could meet the function requirements of the CNS of CARR.

Safety Design and Mock-Up Tests on the Combustion of Hydrogen-Air Mixture in the Vertical CNS Channel of the CARR-CNS

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.