During a rare accident involving severe core damage in a Nuclear Power Plant (NPP), if the molten core material can be contained within the boundary of the reactor vessel, the severity of the accident is expected to be greatly reduced. Therefore, the severe accident management strategy based on in-vessel retention (IVR) of molten core debris is highly desirable, and has been adopted by advanced reactor designs such as VVER, AP1000 and APR1400. In these designs, the IVR strategy requires NPP operators to perform specific actions including: a) Opening valves to depressurize the reactor vessel and reduce the stress in the reactor vessel lower head b) Flooding the reactor cavity to a certain high level to ensure the reactor vessel is covered and cooled by water from the outside c) Injecting water into the reactor vessel after the vessel is fully depressurized to increase the probability of IVR success One of advantages of the IVR strategy is that the actions required in this strategy can be performed without AC power.
Success of IVR depends on the heat flux from the molten core material (corium) to the reactor vessel wall. The heat flux must not exceed the mechanical and thermal limits that fail the vessel. The mechanical limit is due to the fact that the reactor vessel wall must be ablated to a very small thickness that allows the heat flux to be conducted through it. However, if the vessel wall is too thin, it is unable to support the corium in the lower head and the dead weight of the lower head wall. In this case, the vessel will creep to failure. The thermal limit is due to the fact that the heat flux must not exceed the critical heat flux (CHF) on the outer surface of the reactor vessel wall. If the heat flux exceeds the CHF, the reactor vessel wall temperature will increase rapidly and lead to failure.
The heat flux from the in-vessel corium to the reactor vessel is non-uniform along the vessel wall. It has been agreed among IVR researchers that the highest heat flux may occur at the top of the corium, where the metallic material in the corium pool is segregated from the heavier oxidic material to form a metal layer. The thinner the metal layer is, the larger the heat flux to the wall, resulting in so-called “focusing effect.” Ideally there would be a sufficient amount of metals including steel and unoxidized Zr to form a thick metal layer at the top. However, in certain conditions, unoxidized Zr in the corium can reduce UO2 in the corium to form U metal. The eutectic U, Zr and steel is heavier than the oxidic material and stays at the bottom of the corium to form a heavy metal layer. The heavy metal is postulated to remove the steel in the top (light) metal layer, making the light metal layer thinner and the “focusing effect” worse.
IVR analysis is challenging and, in many situations, requires simulations using integral a severe accident thermal hydraulic code. An appropriate code is the Modular Accident Analysis Program (MAAP), which is owned by the Electric Power Research Institute (EPRI) and developed and maintained by Fauske & Associates, LLC (FAI). The latest official revision of the MAAP5 code, MAAP5.03, is equipped with comprehensive models of the corium pool in the lower plenum, reactor vessel and in- and ex-vessel heat transfer. The key features of the models are discussed below. Page 1 Technical Bulletin No: N-16-07 In-Vessel Retention (IVR) as a Severe Accident Management Strategy By: Quan Zhou, Ph.D., Sr. Nuclear Engineer, Fauske & Associates, LLC
As shown in Figure 1(a), MAAP5 assumes that metallic material forms a light metal layer once the corium is present in the lower head. If the corium enters a water-flooded lower head, corium can be fragmented due to fuel coolant interaction (FCI). The fragmented frozen corium particles remain as a particle bed on top of the crust separating the metal layer and oxidic layer. As the particle bed is melted, the molten mass is added into the light metal layer and the oxidic layer, eventually leading to a two-layer model as shown in Figure 1(b). At certain conditions, a heavy metal layer can be formed in the lower portion of the oxidic layer, and the corium pool is then modeled as a three-layer structure as shown in Figure 1(c).