AbstractThe scope of this benchmark is to assess the capabilities of the best estimate thermal hydraulic codes to simulate the main physical phenomena occurring during an in-vessel break transient within a water-cooled fusion-type reactor: pressurisation of a volume at low initial pressure, critical flow, counter pressure effect, relief into an expansion volume. The results, which are given by the code are of the same order of magnitude. Discrepancies are observed which are mainly due to the different ways the codes simulate the break mass flow rate and the pressurisation of the vacuum vessel.