RELAP5 Evaluation vs. Marviken Full Scale Containment Experiments

A model of the Marviken Power Station has been built with the analysis code RELAP5. The model has been tested against the measurement data from the experiments run in the facility under the period August 1972 to April 1973 when a series of full-scale blowdown test have been performed. The aim of the work is the employment and the evaluation of the mono-dimensional code RELAP5 for the containment response analysis. Two different blowdown experiments have been selected and used as comparison for the validation test. The simulations have been performed using models that include the main hydraulic and thermal features described in the actual facility. The heat structures and the pressurizers, used to simulate the spray cooling system, have been introduced and treated in separated cases for a better understanding of the response of the model with regards to the presence of these components. In this way it has been possible to distinguish the effects caused by the heat structures from the results induced by the pressurizers. In this way three configurations have been used for each blowdown, one which describes the mere hydraulic system, a second one with the addition of detailed heat structures and a third one including the spray system. The temperature and the pressure initial values for the simulations are not in a steady state but they reproduce the conditions as described at the beginning of each experiments. The discharge rate and the specific enthalpy are accurately set according to the experimental data. The model response for the global fluid pressure and temperature behaviour is, in general, in good agreement with the experimental local data. The difference between the calculations and the experiments are mainly attributed to the inefficient heat transfer between the non-condensable phase (air) and the walls of the structure: the energy is therefore retained in the gas without the possibility to be released to the surroundings causing higher temperatures and pressures in the fluid itself in comparison with the experiments. Furthermore, a one-dimensional model is not able to predict a good mixing of the fluid in the wetwell as it happens in the reality thanks to the natural convection. The pressure, the temperature, the condensation rate as well as the energy in the drywell and in the wetwell are discussed in the paper and compared with the experimental results. In this paper, due to space limitations, only the results for the hydraulic and the heat structure configurations from one blowdown experiment will be discussed.