Patent Number: 047939648
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a small natural circulation pressurized water nuclear reactor of the calogenic or electrogenic type intended for local use. Such a reactor can be installed on an ocean drilling platform, or on a river or sheet of water, in an isolated region not having the electric power necessary for the operation of an industrial installation. In view of these uses of a very particular type, such a reactor must be completely autonomous and transportable in a safe manner between a loading site, where its core is installed and its place of use. This reactor must also fulfil long term intrinsic safety conditions, preferably including the hypothesis of capsizing, because it has to be installed and transported at sea or on a river. Finally, the very local use of such a reactor makes it necessary for it to be constructed in a particularly simple manner, so as to reduce costs, increase reliability and simplify use thereof. In the particular case of the nuclear propulsion of ships, small pressurized water nuclear reactors are already used. These reactors are generally derived by homothetic transformation from large pressurized water reactors. In particular, they still have pumps for circulating water from the primary circuit, as well as auxiliary circuits ensuring the extraction of the residual power on shut down and in the case of the ship capsizing. Thus, these reactors are too complex and costly to be used for the local production of electricity or heat according to the invention. The present invention specifically relates to a pressurized water nuclear reactor of an original design and which is particularly simple, fulfilling all the imposed safety conditions, particularly in the case of capsizing, without having recourse to any auxiliary standby cooling circuit, whereby said reactor is also autopressurized and operates on a natural circulation basis, which makes it possible to eliminate the heating members of the pressurizer and the primary pumps indispensable in existing reactors. SUMMARY OF THE INVENTION The present invention therefore specifically relates to a pressurized nuclear reactor with circulation by natural convection, comprising a main vessel filled with water surmounted by a pressurized steam layer, said vessel containing in the lower part the reactor core and in the upper part a steam generator, internal structures channelling the circulation of water between the core and the steam generator, a confinement enclosure externally duplicating the main vessel and defining with the latter an intermediate space, wherein the main vessel is not thermally insulated, the intermediate space having an upper zone filled with pressurized neutral gas, an intermediate zone filled with water and communicating with the upper zone and defined between the enclosure and a thin ferrule sealingly connecting the confinement enclosure to the vessel, above the reactor core and a lower zone filled with water and defined between the thin ferrule, the vessel and the enclosure, the confinement enclosure being immersed in an external cooling liquid such as water and internally equipped with thermal insulation in the lower zone of the intermediate space, except in a lower part of the confinement enclosure located at a level below the reactor core. This special design of the main vessel, confinement enclosure and intermediate space defined between these two components makes it possible to limit thermal leaks to a minimum value during the normal operation of the reactor and to bring about a short term evacuation of the residual power dissipated in the reactor core, no matter what the slope of the latter, when power extraction by the secondary circuit of the steam generator is stopped. Thus, a nuclear reactor is obtained without any auxiliary circuit, requiring no continuous monitoring, able to function without intervention during the use period of the core and usable in an intrinsically safe manner in all cases where it is possible to ensure that the confinement enclosure remains immersed in the external cooling liquid. In a preferred embodiment of the invention, the upper zone of the intermediate space is formed in a spherical upper part of the confinement enclosure. This particular shape aids the condensation of the steam formed by the boiling of the water in the intermediate zone during low power operation of the reactor, the extraction of power by the secondary circuit being stopped. In this preferred embodiment, outside the spherical upper part of the confinement enclosure, the main vessel and the confinement enclosure have a cylindrical configuration centered on a common vertical axis, the thin ferrule also having a cylindrical configuration centred on said axis and being fixed by its upper end to the confinement enclosure, at the bottom of the spherical upper part and by its lower end to the main vessel. Preferably, pressure balancing means are provided between the lower zone and the upper and intermediate zones of the intermediate space. These means can be constituted by a swanneck tube projecting upwards into the intermediate zone from the thin ferrule. According to another interesting aspect of the invention, the main vessel also contains an annular reflector surrounding the reactor core, said reflector being formed from several separate sectors, normally positioned level with the core, each sector being able to move upwards with the aid of elastic means during an inclination of the reactor exceeding a given angle, e.g. approximately 60.degree.. If the ship capsizes, this feature makes it possible to ensure the smothering of the power dissipated by the reactor by introducing antireactivity into the latter. Thus, it is possible to compensate the fact that under these conditions that it is not possible for absorbing elements to drop or dropping cannot be completed. According to another feature of the invention, the main vessel contains at least one system of absorbing elements able to move in guide tubes provided in the reactor core during the actuation of the control means outside the vessel, said control means creating a rotary movement transmitted to a threaded rod located in the vessel and on which is mounted a nut carrying said system, via a mechanism comprising a magnetic coupler ensuring the transmission of the rotary movement through the vessel. These control means can also be outside the confinement enclosure. In this case, a second magnetic coupler is disposed on the confinement enclosure to ensure the transmission of the rotary movement through the latter. Means within the vessel are provided in this case to automatically disconnect said system of absorbing elements from the nut when the pressure in the vessel exceeds a given pressure and when the level of the water in the vessel drops below a given level. It is also possible to consider a manual dropping of the system of absorbing elements by means of a tube connecting the upper part of the vessel to the outside of the enclosure and permitting the pressure in the vessel to rise by injecting gases. This tube can also be used for injecting boron or any other soluble nuclear poison. This tube, equipped with a burster disk, is normally closed by sealing means. As the reactor core has an active part of given height, the guides preferably project beyond said active parts downwards by half said height and upwards by the equivalent of said height. The absorbing elements then have a length equal to one and a half times the height of the active part of the core, one half of the elements being absorbing over their entire length and the other half being absorbing over the upper two thirds of their length.