Cryogenic Hazard at ESS – strategy, safety studies and lessons learned

<!--HTML-->The European Spallation Source (ESS) is building a linear accelerator (linac) aiming at delivering a 2 GeV proton beam on a tungsten target wheel at 5 MW nominal power. The entire accelerator will be housed in an underground tunnel and will be fully operational by 2023. The superconducting section of the linac is composed of 21 High Beta cryomodules, 9 Medium Beta cryomodules and 13 Spoke cryomodules, as well as a Cryogenic Distribution System (CDS) that will be provided with liquid helium. A total of 146 superconducting radio frequency (SRF) cavities operating at 2 K will be housed in those cryomodules. Additionally, cryogenic fluids will also be used for the cold hydrogen moderator surrounding the target as well as for several neutron instruments. In order to achieve a proper cooling, different facilities are being built to house the future cryogenic installation and therefore will be subject to Oxygen Deficiency Hazard (ODH). In order to address cryogenic safety issues ESS wide, a long-term strategy has been prepared based on the selection of calculation models, the implementation of an ODH safety guideline as well as the identification of control measures. At this early stage, some preliminary ODH assessments have been carried out in various areas such as the accelerator tunnel, the compressor building and some laboratories using simple calculation models as well as Computational Fluid Dynamics (CFD) simulations. The presentation will provide an overview of the ODH concept at ESS, examples of safety studies, results and lessons learnt from CFD simulations performed in the accelerator tunnel and finally some upcoming activities.