Speaker
Description
At the European Spallation Source (ESS), the Moderator Reflector Plug (MRP) in the target station features a unique geometry integrating cold and thermal moderators to optimise neutron brightness across multiple beam ports. Water is used as a pre-moderator surrounding the moderator piping. This complex configuration introduces design challenges, including closely spaced hydrogen and water pipes, couplings in both vertical and horizontal regions, and local voids along the piping. These features increase the risk of water freezing in the pre-moderator in the absence of sufficient water flow.
As these characteristics are intrinsic to the MRP design, a systematic assessment of the freezing risk was performed. However, in late 2025, an unplanned loss of water flow in the Primary Water Cooling System supplying the thermal moderator—caused by a voltage-drop in the external grid—led to severe damage to the moderator, exceeding the previously evaluated consequences.
In response, several mitigation measures are being implemented to eliminate the risk of water freezing under both normal and off-normal conditions, including scenarios involving loss of complementary power. This contribution presents the required modifications to the Primary Water Cooling System to ensure reliable and continuous water circulation. In particular, a fully passive, gravity-driven solution is described in detail, providing enhanced robustness and safety for MRP operation.