Patent Number: 053496175
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general terms to pressurized water nuclear reactors and more specifically relates to the problem of the need of removing the residual power or after-power from the core in the case of a programmed or accidental reactor shutdown. 2. Brief Description of Related Art Firstly the term residual power will be defined. On shutting down a reactor by introducing a high antireactivity into the core, the number of fissions in the latter becomes very rapidly negligible after a few seconds. However, the radioactivity of the fission products developed in the core during the normal reactor operating period continues to release a significant power, which can represent approximately 7% of the operating power at the time of reactor shutdown. Therefore, no matter why the shutdown has taken place and in particular when it occurs as a result of a depressurization incident with respect to the primary circuit, it is necessary to have means for removing said residual power or after-power from the core without the heating leading to catastrophic conditions and which could even bring about core meltdown. Conventionally three means have been used up to now for removing the residual power from pressurized water reactors. They are constituted by the steam generator, the system for cooling the reactor on shutdown and the safety injection device for accidental situations. The steam generator, whose normal function is to absorb heat, can obviously continue to serve a heat exchange function with the primary water following reactor shutdown. This process, which can last several hours, becomes inoperative when the pressure and temperature respectively drop to approximately 30 bars and 180.degree. C. Thus, the steam generators and secondary circuit are not designed for removing heat at low temperature and low pressure. As from this time it is the system for cooling the reactor on shutdown which comes into action by injecting cold water into the primary circuit. Thus, within about 15 hours it is possible to bring the core to a temperature below about 100.degree. C. The safety injection circuit ensures the emergency cooling of the core and the rapid insertion of antireactivity into it in all cases where there is an accidental depressurization of the primary circuit and which can even lead to a complete break in said circuit. It fulfils its function by as rapidly as possible injecting boric acid cooling solution into the reactor core. These various means, whose operation is satisfactory unfortunately suffer from a number of deficiencies, which will be given hereinafter. The distance between the cold air source and the core can lead to an inadequate operation of these means. Thus, the more equipment existing between the core and the cold source, the greater the failure risk (pipe breaks, poor operation of a valve, motor, etc.). The design of the steam generator only enable it to operate at high pressures and temperatures. At low pressures and temperatures, the shutdown reactor cooling system is used for removing the residual power. Generally, the operational overlap range of the two systems is narrow and requires a special procedure. During intervention on the steam generator, the water level in the primary circuit is at mid-height in the hot and cold pipes and the shutdown reactor cooling system openings are just below this level. Special precautions relative to the operation of the shutdown reactor cooling system have to be taken, so as to avoid any air entrainment risk and the formation of vortexes leading to the disappearance of the residual power removal function. Following a primary coolant loss incident, the steam generators and shutdown reactor cooling system can become completely unavailable, even on a long term basis. The only way to remove the residual power is the safety injection device, which is an active system. However, in this hypothesis, a possible disappearance of electric sources leads to a stoppage to the removal of the residual power. As has been shown, existing systems may be defective and this may lead to serious consequences for the reactor and its environment. Various solutions have already been proposed for improving the safety of the nuclear reactor residual power removal apparatus. Virtually all the solutions proposed consist of introducing an auxiliary heat exchanger into the reactor vessel. Reference can be made in this connection to the CEA FR-A-8,103,632, which recommends the introduction of an exchanger into the reactor vessel for extracting the heat from the heat transfer fluid without using loops. However, in order for such a system to be effective, it is necessary for the heat transfer fluid to be able to flow between the core and the exchanger. This arrangement within the actual vessel is not described and the vessel design proposed is completely different from that of presently used vessels. Other documents, such as the article "A water level initiated decay energy cooling system" by Charles W. Forsberg, Oak Ridge National Laboratory, pp. 229 ff, Nuclear Technology, Vol. 96, November 1991, also describe water reactors with integrated exchangers for removing residual power. These are astute "heat switch" systems controlling the heat exchange between the primary circuit and the exchanger. However, these systems are cumbersome, are not compatible with existing pressurized water reactors and are really intended for other reactor types. SUMMARY OF THE INVENTION The present invention relates to an apparatus for removing the residual power from the core of a reactor making it possible, by using as in the prior art apparatus an auxiliary exchanger within the vessel, to solve the aforementioned problems in all cases where the vessel remains filled with primary water. This apparatus for removing the residual power from the core of a pressurized water nuclear reactor, having a primary water circulation in accordance with a hairpin path in the reactor vessel and for this purpose having two concentric ferrules defining an external annular compartment, in which the cold primary water describes a downward path and a central cylindrical compartment containing the actual core, in which the primary water flows from bottom to top, accompanied by heating, through the core, is characterized in that it comprises a third ferrule defining a complementary annular space between the two preceding compartments, said annular space being linked in its lower part by a first orifice issuing into the external annular compartment with the cold water of the primary circuit and in its upper part and by a second orifice issuing into the central compartment with the hot water of the primary circuit and an auxiliary heat exchanger located in said complementary annular space, said auxiliary exchanger being supplied autonomously by a second heat transfer fluid, which is independent of the primary cooling water of the reactor core. The presence of a third ferrule and a complementary annular space between the core and the periphery of the vessel consequently makes it possible to create an area in which there is a flow of primary water, either by the vacuum or pressure drop effect when the primary circuit is still operating, or by a thermosiphon effect if the latter flow is interrupted. The auxiliary heat exchanger located in the complementary annular space formed in this way is supplied independently of said primary circuit by a second heat transfer fluid able to issue to the outside of the reactor on any cold source such as a condenser, air cooler, etc., so that the apparatus according to the invention in all circumstances ensures a good removal of the residual core power, even following reactor shutdown. According to an important feature of the present invention, following the closure of the first orifice, the removal apparatus comprises means for increasing the vacuum effect in the vicinity of the bottom of the complementary annular space. It is sometimes necessary to use these means when the pressure drop of the primary liquid through the core becomes excessive and would compromise the primary fluid flow in the complementary annular space housing the auxiliary heat exchanger. Two particularly interesting embodiments are envisaged within the scope of the present invention for obtaining these vacuum effect increasing means. In a first embodiment, said means for increasing the vacuum effect incorporate means for closing the first orifice and a series of radial, cylindrical pipes extending, in the vicinity of the base of the core, in the external annular compartment, said cylindrical, radial pipes issuing onto openings provided for this purpose every so often on the periphery of the lower portion of the intermediate ferrule in the complementary annular space and being provided on their wall with longitudinal slots for linking with the external annular compartment. In this first embodiment, it is the flow of the primary water around the longitudinal slots of the cylindrical pipes, which creates a greater pressure reducing or vacuum effect than could be obtained with the first orifice of the overall apparatus. In this connection, the best results are obtained when there are two such longitudinal slots for communicating with the external annular compartment on each pipe and when their azimuth position on the surface of the cylindrical pipes forms an angle .phi. close to 80.degree. in the main downward flow direction in the external annular compartment. In the second embodiment of the means for increasing the vacuum effect, the apparatus has in the external annular compartment below the auxiliary heat exchanger, an annular chamber linked by a series of openings with the complementary annular space and by an annular slot with the external annular compartment, said annular chamber having an extension in the radial direction of the vessel much that it creates in the external annular compartment, a constriction or narrowing which brings about an increase, at the location of the preceding slot, of the flow rate of the downward primary fluid in said external annullar compartment of the auxiliary heat exchanger. In this embodiment, the physical principle applied is similar to the previous one to the extent that the increase of the vacuum effect is simultaneously obtained by the positioning of the slot in an area where the flow of fluid brings about a vacuum and by a restriction of the channel offered in the external annular compartment to the primary water flow.