Research on Waterhammer Caused by a Rapid Gas Production in an LWR Severe Accident: 2 — 2D Analysis With RELAP5-3D

Water hammer load induced by ex-vessel steam explosion, which is resulted from molten fuel in water pool, still includes many uncertain factors in safety research against severe accident for design criteria of next generation nuclear power plant. One of the factors is degree of coherency in water bulk accelerated by the explosion. If high-coherent movement in many pieces of water bulk in the pool (as many pieces of the bulk move with coincidence and same direction), an excessive load will be applied to inner structure of the plant. To evaluate the factor, 2 dimensional analysis to simulate the coherent movement induced by pressurized air injection in containment vessel, has been examined with RELAP5-3D, which is one of the most popular codes in nuclear engineering. This 2D analysis simulated glowing single component zone of pressurized air, regarded as large bubble, and cloud-like interface around the zone observed in the experiment where momentum is transferred from the air to the water. This analysis also predicted virtually uniform acceleration of the bulk and detailed 2D flow proceeded to air penetration through the water. And water hammer pressure could also be simulated, with evaluating sonic speed drop through two-component mixed layer around the bulk.