[en] In 1986 the CEC sponsored a benchmark exercise on aerosol calculation based on the Demona B3 experiment. The results of this exercise were very sensitive to the calculation of energy and mass transfer between the phases. In view of the results of the study mentioned above, it had been decided to carry out a benchmark exercise for severe accident containment thermal-hydraulics codes. This exercise is based on experiment B3 in the Demona programme. The experiment B3 was a simulation of a late overpressure failure scenario in a PWR. The main objective of the benchmark exercise has been to assess the ability of the participating codes to predict atmosphere saturation levels and bulk condensation rates under conditions similar to those predicted to follow a severe accident in a PWR. Several research organizations of the Community member countries have participated at this benchmark exercise. The paper presents the comparison of the experimental results with the following calculated quantities: total system pressure, steam pressure, containment temperature, saturation ratio, steam condensation rate on walls and in the bulk volume, water mass in sump, sump temperature, heat transfer coefficient, steam mass flow rate and structural temperature for a real time period of up to 90 hours. The paper also presents the major conclusions from the exercise in order to identify the status of present codes versus the requirements needed as input and for coupling with aerosol analysis codes