Patent Number: 053496159
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a device for the retention of core melt-through-in the event of a core melt-through accident in light-water reactors and also to a reactor plant having such a device. Because of the afterheat of the reactor core, which is produced because of the radioactive fission products, the core melts if the cooling water empties from the reactor pressure vessel because of an accident; the molten mass of the core penetrates the wall of the pressure vessel. If there is no retention device to collect the molten mass, then the foundation of the building is also breached by the molten mass, with the concrete being decomposed because of the high melting temperature (over 2500.degree. C.) with the formation of hydrogen, carbon monoxide, carbon dioxide and water vapour. The molten mass of the core is essentially composed of uranium oxide, zirconium and steel in the ratio of roughly 6:1:3. Various safety concepts have been developed to prevent the hypothetical consequences of serious accidents in light-water reactors, in particular of core melt-through accidents. Thus for example a core melt-through retention device ("core catcher") has been proposed, in which a shaft-like collecting basin for the molten mass is provided beneath the reactor core (R. Hammond, J. Dooley, 1982 "Retrofitting Core Catcher to Nuclear Plants", NUREG/CR-2941; or U.S. Pat. No. 4,036,688). This known device consists of an upper section, which is cylindrical (diameter: 3.5-5 mm, height: 10-15 m) and a lower section, which tapers conically downwards (height: 20-25 m). The upper section is lined with highly refractory material and the lower section is constructed as a water-cooled, double-walled steel crucible. This crucible is filled with a siliceous or oxidic ceramic bulk material. With the cooling system provided there is the danger of the inner crucible wall being perforated by the molten mass, as a result of which radioactive fission products of the molten mass can be released into the atmosphere. SUMMARY OF THE INVENTION The object of the invention is to create inside the containment a collecting basin for the molten mass of the core, which is constructed as a crucible for the safe and long-term retention of the molten mass at high temperatures. The molten mass of the core collected in the crucible of the device according to the invention is cooled in the cooling basin by sump water, which comes partly from the broken primary water circuit. The water vaporised on cooling condenses on the walls of the containment and flows back into the containment sump again. The crucible lining should be as inert as possible with respect to the molten mass of the core. When the molten mass solidifies a solid layer--a "secondary crucible"--is constructed in the edge zones of the crucible, which produces a protective action for the wall of the retention device. Until the formation of this secondary crucible, the material of the lining can only react with the molten mass to a limited extent. It is therefore advantageous if a non-oxidising ceramic is chosen for the crucible lining. So that the protective secondary crucible is quickly formed, the heat has to be conducted away easily by the lining. High-temperature isostatic pressed boron nitride--"HIP-BN"--meets these requirements (thermal conductivity: 49 W/m.multidot.K at 20.degree. C.; 28 W/m.multidot.K at 800.degree. C.) and offers various other advantages. Thanks to the boron a recriticality of the molten mass of the core can be prevented. Boron nitride (BN) has a poor wettability with respect to the molten mass of the core. The melting point of BN is high; it is roughly 3000.degree. C. BN is characterised by a good thermal fatigue resistance and a good compressive strength. BN is resistant in air up to a temperature of 1000.degree. C. With regard to construction the simplest method is to construct the protuberances on the crucible vat as circular cylindrical cooling tubes. Low-alloy steel can be used for the external crucible wall. On the bottom of the vat between the cooling tubes is advantageously provided a thick guard plate, with which the lining can be protected against the impact of the molten mass of the core. This guard plate is melted by the molten mass of the core if the requirements are met (i.e. in the event of a core melt-through accident). The crucible lid, which forms a water-tight seal for the crucible, can be composed of steel plates, for example by horizontal plates being welded to vertical plates disposed in a honeycomb, which protrude into the interior of the crucible vat. The honeycomb lid reinforcement can be placed on the guard plate of the base of the vat. With the retention of the molten mass of the core if the requirements are met, first of all the kinetic energy of the falling molten mass of the core is converted into deformation energy; then the crucible lid is melted through. The volume of the crucible has such dimensions that the crucible is filled by the molten mass to roughly 10 cm above the vat base, for example. The cooling basin may be provided as the deepest part of the containment sump; it can be constructed as a cavity in the foundation plate of the containment. The crucible lid guarantees that no water penetrates into the crucible, even if it is flooded by incoming primary water. The cooling basin is always filled with water. A distinction can be made between two ways in which the reactor pressure vessel may malfunction, which are referred to by the designations "low pressure path" and "high pressure path" respectively. In the case of the low pressure path the molten mass of the core flows through a melted-open perforation in the reactor vessel. In the case of the high pressure path the lower spherical section of the reactor pressure vessel is centrifuged away with the molten mass of the core located therein. In order to control the high pressure path, a collecting structure has to be provided between the reactor pressure vessel and the retaining device, with which the retaining device can be screened from the wall piece blown off the pressure vessel. This collecting structure is advantageously constructed in a funnel shape so that the molten mass can be directed into the central region of the retaining device. The steam produced during the cooling of the retention device filled with molten mass can freely spread in the interior of the containment. Thanks to the condensation of the steam on the walls an equilibrium pressure can be balanced out; this depends on the heat dissipation of the containment to the atmosphere. It is possible for the pressure to rise to an unacceptable level. In order to redress such a pressure build-up, the containment has to be connected to a filtered pressure relief device (device for the filtered relief of pressure).