The lower part of the reactor pressure vessel is normally housed in a lower chamber (a reactor cavity) in a position in the basemat of the reactor containment building.
If the core melts down, as a consequence of a severe accident, the resultant molten material could perforate the bottom of the pressure vessel and pour into the underlying lower chamber and over the basemat. The basemat is usually made of concrete with a steel liner embedded therein to ensure a leak-tight seal. In the case of an accident, molten material erodes the basemat then penetrates into the ground where it cools, releasing radioactivity and contaminating the environment around the power station.
Nuclear reactors which are used for commercial power stations can be of different types, depending upon
the coolant used (light water, heavy water, sodium, helium, carbon dioxide, etc.); PA1 the neutron energy spectrum (thermal i.e slow neutrons, or fast neutrons); or PA1 the fuel used (uranium dioxide, uranium metal, uranium alloy, etc.) and its enrichment in Uranium-235. PA1 Pressurized Water Reactors (PWR) PA1 Boiling Water Reactors (BWR) PA1 Heavy Water Reactors (HWR). PA1 1) Containment Slow Overpressurization. This problem can be overcome either by the operation of a containment heat removal system (if the system is passive or if power is available for active systems) or by the existence of containment filtered venting. Several nuclear power plants have been equipped with filtered venting systems. PA1 2) Hydrogen Deflagration or Detonation. Hydrogen is produced during the accident evolution by the oxidation of superheated metal. To avoid this problem there are various means: a) containment inerting with nitrogen (if the containment is small), b) burning of the hydrogen by means of ignitors before its concentration reaches too high a value, c) hydrogen dilution in large containment buildings. Solutions to the hydrogen problem have been introduced into several nuclear power plants although further progress is needed in this area. In any case, any means capable of reducing the quantity of hydrogen produced is a desirable feature. PA1 3) attack of the containment concrete basemat by the molten corium. PA1 "core catchers", such as that described in U.S. Pat. No. 4,036,688 (Golden) in which a bed of refractory material is used to protect the containment basemat; PA1 solutions such as that of U.S. Pat. No. 3,503,849 (West) which require (i) a considerable enlargement of the cavity cross section below the vessel to spread the corium into a thin layer, and (ii) a set of cooling pipes below the bottom of the cavity, to remove the heat from the corium.
Among the water cooled reactors the more common are:
They are all thermal neutron reactors and use, as fuel, low enrichment uranium dioxide; the fuel pellets are stacked and sheathed in metal cans (usually Zircalloy or stainless steel); clusters of these fuel pins make up the fuel element; and hundreds of fuel elements make up the reactor core.
The reactor core is normally housed in a pressure vessel, which is a part of the primary circuit. In addition to the pressure vessel, the primary circuit is, typically, made up of pipes, heat exchangers and pumps which are needed for the circulation of the coolant (water) and for the transfer of heat from the core to the heat exchangers.
The entire primary circuit (and some auxiliary circuits) are housed in a containment building. This is capable of containing, the primary coolant and radioactive products which, in case of an accident, might escape from the primary circuit.
The containment building comprises the last barrier to the release of radioactive products to the environment in case of an accident.
During normal reactor operation, radioactive products build up in the fuel and are retained in the fuel matrix inside the metal can. During an accident, there is a temporary mismatch between the heat produced in the fuel and the heat removed by the coolant (the first being larger than the second) and the fuel starts to overheat. Even if the reactor is shut down, with the insertion of control rods or coolant poisoning (boration), decay heat (a few percent of the nominal thermal power) continues to be generated inside the fuel. This is because of the accumulated radioactive products. If an emergency core cooling system (ECCS), is actuated, the temporary mismatch is overcome and the reactor is put back into a safe condition. If, on the other hand, the ECCS, for some reason is not actuated, the mismatch between heat produced and heat removed continues. This causes fuel overheating. If the ECCS function is not recovered in time, the fuel melts. The melting core falls to the bottom of the pressure vessel. It is then possible to have a meltthrough of the pressure vessel. In fact, this can occur even if the emergency cooling function is recovered, due to the low surface-to-volume ratio in the melt which accumulates on the bottom of the pressure vessel. The molten core which is mixed with the molten structural material is normally referred to as "Corium".
Even if the reactors are equipped with (1) redundant ECCS systems, (2) can be fed by onsite power supplies in case of loss of offsite power, (3) their design takes into account all the foreseeable accident loads, and (4) they are regularly tested and inspected, in the framework of the so-called "defense-in-depth" concept which permeates the approach to the safety of nuclear power plants, in recent times serious attention is being given to the degraded situation in which the ECCS is not operating. To protect the public and the environment from these improbable circumstances specific design features must be introduced.
Once a molten core has breached the pressure vessel bottom and has fallen into the cavity below, it is important to preserve the integrity of the containment building and the leaktightness to limit the release of radioactivity to the environment. Such a severe accident poses three main challenges to the containment integrity:
Existing solutions can be grouped into two broad categories:
The solution proposed by Golden (U.S. Pat. No. 4,036,688) involves installing under the reactor pressure vessel a crucible of refractory material which should retain any molten material from the pressure vessel, so as to prevent its interaction with the basemat.
This solution has at least two drawbacks.
Firstly, heat transfer from the melt-down is minimal because of the refractory crucible and upwards is minimal because of the low heat transfer coefficient of air, with the result that the temperature of the molten material increases well above the melting point and that there is internal heat generation. The consequence is that in the long term the refractory crucible itself is eroded and fails to retain the molten core.
Secondly, a large fraction of the radioactive products is released by evaporation from the molten material into the reactor containment building, with a consequent increase in the release of radioactivity into the environment.
Another proposed solution by West (U.S. Pat. No. 3,607,630) relates to spreading the molten core into a flat thin layer over the basemat and removing the heat by means of cooling pipes embedded in the base wall.
However, this solution requires a cavity having a very large cross-section. If the spreading area in not large enough an excessive temperature gradient arises through the base wall with the danger of its fracturing and the possibility of the molten material attacking the pipes. If this happens, the fission products can escape into the external environment.
This invention is a solution to this last problem (without worsening the other two). Other solutions have been proposed in the past to this problem but they have been based on an entirely different approach.
The present solution, which could be defined as a "core quencher", envisages the subdivision of the molten corium into a series of vertical layers and the cooling of it by direct transfer of heat to the cooling water into which the subdividing structure is immersed.