Detailed Design of the Vacuum System for Cold Neutron Source Systems in HANARO

The cold neutron produces by making thermal neutron passing through a volume of moderator installed into the reflector tank of the reactor. The HANARO CNS has adopted the liquid hydrogen as a moderator. The liquid hydrogen contained in the moderator cell evaporates due to the gamma heating. The hydrogen vapor goes up to the condenser, where it is re-liquefied then returns down to the moderator cell. This thermo-siphon loop can be only established under the very low temperature environment, which requires a method of a thermal insulation. The major performance of the vacuum system is to serve the normal operation vacuum level for thermal insulation of cryogenic components. The vacuum for the cryogenic insulation shall be at least lower than 10-5 torr during the CNS normal operation. This report describes the results of detailed design of the vacuum system according to its design requirements. The summary derived from the detailed design are followings. - Non-safety class and Seismic II class (except for the equipment installed in the vacuum box) - Quality class is classified in T or S depending on the its design criteria - The vacuum system should be surrounded by the blanketing gas to isolate them from the air - The discharged gas from the vacuum system should be guided to the discharged gas collection tank and check whether the volume of hydrogen gas is less than 4% or not and then the discharged gas is released to the reactor hall. - In case, the hydrogen gas in the discharged gas collection tank is over than 4%, the outlet valve of the vacuum system should be automatically closed. - The pneumatic valves installed in the vacuum system should be actuated by the nitrogen gas and solenoid valves for operating the pneumatic valves should be placed to outside of the vacuum box. - It should be always required to measure the hydrogen leak from the vacuum box, which be filled with the blanketing gas, by using the proper hydrogen detector. - The design pressure of the vacuum box, the outer thing of double wall pipe, and the valve box is 3 MPa (a)