In the severe accident of an LWR (Light Water Reactor), a steam explosion caused by an injection of molten core into a water pool is one of the factors whose possibility still cannot be neglected completely. According to a rapid bubble growth by a steam explosion, if the water mass is pushed up having a coherency in time and direction in its movement, it would give a severe waterhammer to the structure. In this research, we conducted an experiment using a cylindrical model containment vessel with 1 m diameter and 6 m height, and investigated the behavior of water mass pushed up by a growing bubble. In addition, we also conducted an experiment using a small-scale vessel with 0.428 m diameter to know the scale effects of this phenomenon. In these experiments, a rapid bubble growth of a steam explosion was simulated by injecting high-pressure air into a water pool. According to a rapid bubble growth at the bottom of the water pool, it was observed that the water mass was pushed up without an air penetration until the water level reached a certain elevation. The elevation became considerably high in the case where the initial water level was almost equivalent to the diameter of the vessel. Water surface velocities at the air penetration reached 10–35 m/s. After the air penetration, the water having these high velocities continued to move up and hit the orifice plate installed at the middle of the vessel. The maximum waterhammer pressure measured at the orifice exceeded 10 MPa. In that case, the total load to the vessel measured at the foot of the vessel reached about 540 kN, which corresponds to about 55 ton. The results of this study suggest that the possibility of waterhammer as well as the direct impact of pressure wave should be paid attention in the assessment of a steam explosion.

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