Corium, also called fuel-containing material (FCM), is a lava-like material created in the core of a nuclear reactor during a meltdown accident. It consists of:
- mixture of nuclear fuel and oxidized zirconium clad,
- fission products,
- control rods,
- structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam,
- and, in the event that the reactor vessel is breached, molten concrete from the floor of the reactor room.
If the temperature reaches the melting point of UO2, a fuel degrades usually from the center of the core. Due to the formation of the eutectic liquids, the melting temperature may be several hundred degrees below that of the UO2 melting point (3100 K). Zirconium from fuel clad, together with other metals, reacts with water and produces zirconium dioxide and hydrogen. The production of hydrogen is a major danger in reactor accidents. As the eutectic molten mass increases, the corium pool may be formed and expands axially and radially in the core until it reaches either the baffle or the core support plate. At this moment, the corium flows into the lower head. The degradation may ultimately result in very different configurations in the core simultaneously, ranging from intact or barely degraded rods to the formation of a corium pool or a bed of debris.
In all cases, the corium gradually evaporates the water present in the lower head. If there is no additional water supply and the debris configuration is such that it cannot be cooled effectively, the materials’ temperature gradually rises until it reaches the melting point of the steel structures (plates, tubes, etc.) located in the lower head. In the case of adequate cooling of the corium, it can solidify and the damage is limited to the reactor itself. However, in the absence of adequate cooling, corium may melt through the reactor vessel and flow out or be ejected as a molten stream by the pressure inside the reactor vessel.
However, core reflooding may not be beneficial in all conditions. The following pheomena can occur during reflooding:
- massive steam generation, with hydrogen production and an increase in reactor
- coolant system pressure;
- steam explosion through corium-water interaction;
- continuation of core melt, despite water inflow;
- faster release of fission products.
In the event of reactor-vessel failure during a core melt accident, the corium resulting from this core melt and the melting of internal structures will pour onto the reactor pit basemat. The molten core-concrete interaction (MCCI) is treated as one of the important phenomena that may lead to the late containment failure by basemat penetration in a hypothetical severe accident of light water reactors (LWRs). The process is driven by the high initial temperature of the molten corium and the decay heat that is generated inside the melt by the radioactive decay of the fission products. Obviously, the progression of MCCI takes paramount importance and plays a key role to threaten the integrity of the containment, the last barrier of fission products.
With regards to the safety of the Nuclear Power Plants (NPP) in case of a severe nuclear accident, one of the main challenges associated is the retention of the molten nuclear fuel and reactor internals, called corium, within the Reactor Pressure Vessel (RPV). One of the ways of cooling corium with in the RPV is by cooling the vessel from outside. The in-vessel retention can be achieved by full flooding of reactor cavity to cool the external wall of the lower head thereby avoiding structural failure by creep rupture. This strategy is termed as In-Vessel Retention (IVR). In case of the In-Vessel Retention (IVR) strategy, it is expected that the corium pool will be surrounded by an oxide crust, which will be in contact with molten steel from top of the pool as well as from sides of the vessel. Application of this approach to large power reactors is not trivial because of relatively short time between the detection of core melting and the lower head failure.