Speaker
Description
The European Spallation Source ERIC (ESS) will provide long-pulsed cold and thermal neutron fluxes at very high brightness to the research community. Spallation neutrons are produced by a linear proton accelerator with an average beam power of ultimately 5 MW. These neutrons are moderated to cold and thermal energies by two hydrogen moderators and a thermal water pre-moderator. The nuclear heating for the 5 MW proton beam operation is estimated to be 6.7 kW. The ESS cryogenic moderator system (CMS) is designed to circulate subcooled liquid hydrogen at 17 K and 1 MPa with a flow rate of 0.5 kg/s to remove the nuclear heating at the moderators. Heat load is efficiently removed through a plate-fine type He-H2 heat exchanger housed in the CMS cold box by a large-scale 20 K helium refrigeration system, the Target Moderator Cryoplant (TMCP), with a cooling capacity of 30.3 kW at 15 K. High-pressure helium flow at 16 K is transported from the TMCP cold box to a valve box adjacent to the CMS cold box through a 300 m-vacuum insulated cryogenic helium transfer line (CTL). When proton beams are injected, a heat load of 6.7 kW is suddenly applied to the CMS. The feed helium flow rate to the heat exchanger is adjusted by the feed control valve in the valve box to compensate for the heat load, while the bypass valve is automatically adjusted to maintain a pressure drop through the bypass line. Additionally, a mixing valve in the valve box maintains the return temperature at 21.2 K by blending in a helium flow from the TMCP cold end that has subsequently warmed up by an ambient heater at ambient temperature. This mechanism allows the available cooling capacity to be regulated without applying any thermal disturbance to the TMCP cold box. A one-dimensional simulation model of the heat exchanger was developed to study the propagation of thermal fluctuations from the CMS to the TMCP induced by proton beam injection or trip. The simulation results provided insights into the required ramping-up speed of the feed helium flow rate and the timing to initiate the ramp-up mode. In this study, the cooling capacity control approach was experimentally evaluated using the valve box during preliminary CMS commissioning in 2024, where helium was used instead of hydrogen, and the CMS was not connected to the moderators. In the experiment, compressor discharge pressures were varied between 1.0 MPa to 1.7 MPa during the test. The experimental results identified the optimal operational conditions for each proton beam power.
