Three-dimensional CFD analysis of in-vessel LOCA induced by a highly under-expanded helium jet in the vacuum vessel

Helium is widely used as a coolant in conceptual fusion reactor designs. Following an In-vessel Loss of Coolant Accident (LOCA), high-temperature, high-pressure helium is discharged into the vacuum vessel (VV). Under extremely high pressure ratios, the release forms a highly under-expanded jet and produces strongly non-uniform pressure and temperature loads before activation of the Vacuum Vessel Pressure Suppression System (VVPSS). Traditional lumped-parameter approaches can estimate global VV pressurization but cannot resolve localized peak wall pressure and temperature, which are directly relevant to structural failure criteria. In this work, a three-dimensional CFD model is developed to simulate the early in-vessel LOCA process and to capture the transient structural evolution of the under-expanded helium jet. The model credibility is examined through comparisons with free-jet experimental data, impinging-jet pressure experiments and empirical correlations. The results show that the jet impingement region can experience pressure excursions much higher than the vessel-averaged pressure. For extreme breach conditions, the local pressure may exceed the VV design limit before the pressure at the burst disk location reaches its activation threshold. These findings provide actionable insight for identifying high-risk regions, evaluating local structural tolerance, and optimizing the placement and response logic of pressure suppression systems of fusion reactors.