Simulation of the cool-down process for the ESS cryogenic moderator system

The ESS cryogenic Moderator System (CMS) has been designed to supply a subcooled liquid hydrogen (17.5 K and 1.0 MPa) to two hydrogen moderators where the nuclear heating is estimated to be 6.7 kW (to be increased to four in the future). The CMS is cooled by a helium refrigerator system with a cooling power of 30.3 kW at 15 K. In this study, a one-dimensional simulation code has been developed to understand temperature behaviors during the cool-down process. The operational methods and parameters have been optimized. The CMS cool-down process is divided into three phases (I: gaseous hydrogen above 36 K, II: condensation from 36 to 31 K, and III: liquid hydrogen below 31 K). It turns out from the simulation result that the CMS would be able to be cooled down to the nominal condition of 17 K within 27 hours.