An experimental investigation on a lower head penetration survivability during a severe accident with external vessel cooling

The integrity of the lower head penetrations may be breached by a melt attack during a severe accident in nuclear power reactors. The failure of the penetrations causes the release of radioactive materials to the containment and then complicate ex-vessel phenomena may occur. The penetration failure processes are two. One is the attack of the supporting welds causing the ejection of the in-core instrumentation nozzles and the other is the flow of high temperature molten debris through the thimble tubes. Although several investigations have been performed on the problem, the penetration failure phenomenon is a plant-specific problem due to the difference of the reactor geometry and materials. So, KAERI (Korea Atomic Energy Research Institute) started the penetration survivability experiments on the KNGR (Korean Next Generation Reactor: Pressurized Water Reactor Type) which would apply external vessel cooling to lessen the probability of lower head failure. The aim of this study is to evaluate the survivability of the lower head penetrations during a melt attack with external vessel cooling. (authors)