AbstractFor a number of years, the world fusion safety community has been involved in benchmarking their safety analyses codes against experiment data to support regulatory approval of a next step fusion device. This paper discusses the benchmarking of two prominent fusion safety thermal-hydraulic computer codes. The MELCOR code was developed in the US for fission severe accident safety analyses and has been modified for fusion safety analyses. The ATHENA code is a multifluid version of the US-developed RELAP5 code that is also widely used for fusion safety analyses. The ENEA Fusion Division uses ATHENA in conjunction with the INTRA code for its safety analyses. The INTRA code was developed in Germany and predicts containment building pressures, temperatures and fluid flow. ENEA employs the French-developed ISAS system to couple ATHENA and INTRA. This paper provides a brief introduction of the MELCOR and ATHENA–INTRA codes and presents their modeling results for the following breaches of a water cooling line into the tokamak vacuum vessel, (1) water injection in vacuum with condensation of the flashed steam and (2) water injection in vacuum. The modeling predictions are compared with experimental results from the ingress-of-coolant event (ICE) facility in Japan. It is observed that the codes’ predictions exhibit good agreement with the experiment data, which suggests that the codes include the proper physical mechanisms associated with the chosen accident scenarios. It is thereby concluded that either of the codes can be used to model the defined transient.