Design of a hydrogen vent line for ESS cryogenic moderator system

The ESS cryogenic moderator system (CMS) circulates subcooled liquid hydrogen at 17 K and 1 MPa to remove nuclear heating at two hydrogen moderators. All the hydrogen will be safely released to the atmosphere on the roof top of the Target building through a hydrogen vent line (HVL) with a total length of 36 m. Two-phase hydrogen would flow through the HVL because the hydrogen expands by isenthalpic process. The HVL has been designed to avoid decreasing the wall penetration temperature below 253 K and to be sized big enough to limit the backpressure below the design pressure. One-dimensional transient thermohydraulic analysis code that can treat two-phase flow heat transport behavior has been developed. A natural convection heat transfer from air to the cold surface is considered. In this work, the wall temperature reductions and the pressure drop along the HVL during release of cryogenic hydrogen are analyzed. The size and thickness of the HVL have been verified based on the analysis results.