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Japan Atomic Energy Research Institute has been conducting a design study of an advanced marine reactor MRX. MRX employs a water-filled containment vessel. In a case of loss of coolant accident (LOCA), the discharge of water from the reactor vessel almost stops when the pressures of reactor vessel and containment vessel become equal. The core flooding can be maintained passively. The MRX, therefore, does not adopt a safety injection system which the conventional reactors have.The purpose of this study is to investigate the thermal hydraulic behavior in the water-filled containment vessel during the LOCA. This report describes the fundamental experimental study focused on pressure increase of the containment vessel. The experiment was carried out by using a small scale apparatus. The pressures and temperatures in the vessels were measured. According to our observation, the vapor bubbles at nozzle exit contracted or expanded caused by flashing due to pressure reduction or condensation in the containment water. The pressure increase in the containment vessel was revealed to be the sum of compressed nitrogen gas pressure and saturated steam pressure. One-volume thermal equivalent model can predict well the pressure increase in the containment vessel in this experiment.