Patent Number: 043127037
Section: summary

FIELD OF THE INVENTION The invention relates to a nuclear reactor installation comprising a housing holding a reactor core and provided with means for dissipating the heat generated in the reactor core during operation, which means comprise a primary cooling fluid circuit consisting of at least one loop passing through the reactor core and housing in which is included a pump, as well as a secondary cooling fluid circuit arranged ouside the housing of the nuclear reactor and consisting of at least one closed loop in which is included a pump, the loop of the primary circuit being coupled by means of a heat exchanger to the loop of the secondary circuit, while the installation is furthermore provided with means for dissipating the decay heat which is produced in the core of the nuclear reactor after the reactor has been switched off, which dissipating means are at least partly formed or included within the primary cooling fluid circuit. BACKGROUND OF THE INVENTION Like nuclear reactor installations are well-known and comprise both installations of the so-called "loop" type and installations of the so-called "pool" type. As cooling fluid for dissipating heat generated in the reactor core for example, liquid sodium may be used as, for example, in a nuclear reactor of the fast breeder type. The liquid sodium transfers the heat from the primary circuit in the heat exchangers between the loops of the primary circuit and the respective loops of the secondary circuit heat to the fluid in the secondary circuit, which fluid is, for example, also liquid sodium. The heat thus transferred to the secondary circuit is subsequently used, for example, for the conversion of water to steam in one or more steam generators coupled to the secondary circuit for heat transfer. When the nuclear reactor is suddenly stopped or cut off, the heat production in the core quickly diminishes until it is only a few percent of the nominal power. This last value decreases only gradually in a period which may vary from a few hours to some days. The heat which is still generated in the reactor when it is cut off is called decay heat. Although the decay heat is only a few percent (for example, 1-3%) of the design power of the reactor, the decay heat may be considerable, particularly in recently proposed reactors having a very great design power. If the decay heat is not dissipated from the core, the reactor core will become superheated, with highly undesirable consequences: the stainless steel pins wherein the nuclear fuel is present in the reactor core could, for example, melt locally, resulting in reactions of the nuclear fuel with the sodium of the cooling fluid and the entire primary cooling fluid circuit could be strongly contaminated. For the dissipation of the decay heat the normal cooling system of the reactor which usually is also employed for the dissipation of the heat generated during operation could serve. Since this manner of dissipation of the decay heat alone is not considered safe enough, extra provisions can be made. In a fast breeder type reactor as now under construction, for example, the decay heat is transferred via the primary circuit to the secondary circuit and thence to the water-steam system. When the reactor is cut off, the pumps of this entire system continue to work slowly for this purpose. As an addition to this system there are also provided immersion coolers which are switched on when the reactor is cut off. A drawback of the utilization of the entire system of primary circuit, secondary circuit and water-steam system for the dissipation of the decay heat is that the secondary circuit and the water-steam system constitute part of the protective system of the nuclear reactor, as a result of which, beside the usually already set reliability requirements, extremely high safety requirements are set by the authorities. As a result of this, a scaling-up for future nuclear reactors is very difficult from a technical point of view. Now, the object of invention is to provide a nuclear reactor installation wherein an alternative provision is made for the dissipation of the decay heat, so that the entire system of primary circuit, secondary circuit and water-steam system is not needed for the decay heat dissipation, while nevertheless there is ensured an installation satisfying the highest safety and reliability requirements. THE INVENTION According to the invention, the outlined object is achieved with a nuclear reactor installation wherein the means for the dissipation of the decay heat at least consist of another third cooling fluid circuit arranged outside the housing of the nuclear reactor and comprising at least one closed circulating fluid loop in which is included a pump, coupled to an external loop of the primary circuit by means of the same heat exchanger which couples the primary circuit to the secondary circuit. Therefore, in the installation according to the invention dissipation of decay heat take place via the primary circuit, but the heat is transferred to a separate cooling circuit, to which may be connected one or more air coolers. In addition, there may be provided conventional immersion coolers as a supplementation to the decay heat dissipation via the primary circuit. Essential to the installation according to the invention is that the decay heat cooler (i.e. the heat exchanger between the primary circuit and the third circuit) is completely integrated in the intermediate heat exchanger (i.e. the heat exchanger between the primary circuit and the secondary circuit). The use according to the invention of a decay heat cooler integrated in the intermediate heat exchanger has a number of advantages. A first advantage is that the component in question is very compact. The integral combination of intermediate heat exchanger and decay heat cooler occupies less space than two separate components. Furthermore there is question of a saving of piping, because no separate loop need be passed from the primary system to the decay heat cooler. A further advantage of the installation according to the invention is that there is no required increase of the pumping capacity to circulate the cooling fluid in the primary circuit. In general, in a decay heat cooler all the fluid should run through the cooler, but the cooling surface should be relatively small in order to keep the influence of the decay heat cooler during normal operation as small as possible. For that reason one will perhaps space the tubes widely apart in the cooler. When the rate of the fluid in the primary circuit becomes slight, then, however, the heat transfer situation may become bad. Since this is undesirable a fair pressure drop in the cooler even at low through-flow rates is needed. However, this requires a much higher pressure drop at full load. In the installation according to the invention, thanks to the integrated construction, the tubes of the intermediate heat exchanger provide sufficient mixing, even of slowly flowing fluid from the primary circuit around the relatively widely apart spaced tubes of the decay heat cooler so that a uniform cooling is provided for. In a highly suitable embodiment of the installation according to the invention, the integrated heat exchanger comprises a vessel-shaped housing having arranged therein a bundle of a plurality of straight tubes almost vertically extending in the housing and arranged around a central tube which projects above and below the housing and terminates in a first collecting chamber, which collecting chamber at the top side is closed by a tube plate wherein terminate the lower ends of part of the tubes of the bundle. The upper ends of the tubes of this part of the bundle terminate in a tube plate which constitutes the lower wall of a second collecting chamber which is provided with a discharge line. The remaining tubes of the tube bundle, which comprise the more outwardly situated tubes of the bundle, at the bottom side terminate into a torus-shaped collecting chamber which is arranged around the first collecting chamber and/or the lower end of the therein terminating tubes, and at the top end terminates into an other torus-shaped collecting chamber which is arranged around the top end of the tubes terminating into the second chamber. The lower torus-shaped collecting chamber is connected a supply line leading to outside the housing and the top torus-shaped collecting chamber is connected to a discharge line also leading to outside the housing. The housing is also provided with a supply line and a discharge line for supplying and discharging, respectively, of primary cooling medium to the space in the housing which is not occupied by the tube bundle, the collecting chambers and the lines and tubes leading thereto or therefrom. Preferably, in this heat exchanger annular flow baffles are arranged in the space destined for primary cooling medium around the tubes of the tube bundle. The annular flow baffles are in substantially parallel planes which are almost perpendicular to the axis of the central tube, so such that the primary cooling medium moving along the tubes in operation alternately flows along the tubes between the first and second collecting chamber and the tubes between the two torus-shaped collecting chambers. Preferably, the baffles consist of first annular bodies having a diameter at the inside of the ring almost equal to the outer diameter of the central tube and an outer diameter almost equal to the diameter of the envelope of the tubes of the part of the tube bundle between the first and second collecting chamber, and second annular bodies having an inner diameter in the order of twice the outer diameter of the central tube and an outer diameter almost equal to the diameter of the envelope of the entire tube bundle, which first and second annular bodies are arranged alternately and spaced apart from each other. In such an integrated intermediate heat exchanger and decay heat cooler, throughflow of the intermediate heat exchanger and the decay heat cooler takes place alternately, so that transients during shut down from normal operation to decay heat dissipation are restricted. An other embodiment of the integrated intermediate heat exchanger and decay heat cooler installation according to the invention comprises a vessel-shaped housing having arranged therein a bundle of a plurality of straight tubes extending almost vertically, which tubes at the bottom and top side terminate into tube plates which close a first collecting chamber situated substantially above the tube bundle and a second collecting chamber situated substantially below the tube bundle. The housing is provided with a supply opening for feeding the primary cooling fluid into the first collecting chamber and with a discharge opening for discharging it from the second collecting chamber. It is supplied thereto from the first collecting chamber via the tubes. The housing is also provided with means for supplying secondary cooling fluid in to the space around the tubes at their lower end and discharging same from the space at the top end. In each tube of the tube bundle there is arranged a concentric second tube which projects outside the tube and is passed through the first and second collecting chamber and at the bottom side terminates into a central supply tube for cooling fluid for decay heat dissipation and at the top side terminates into a tube plate which constitutes a closure for a third collecting chamber. This chamber is provided with a discharge opening for discharging the cooling fluid for dissipating the decay. In such a design it is necessary to arrange in each tube of the intermediate heat exchanger, a concentric tube for the cooling fluid for decay heat dissipation, since otherwise during decay heat dissipation there may occur great differences in temperature between the tubes among themselves. This would lead to undesirable thermal stresses. A further embodiment of the integrated heat exchanger to be used in the installation according to the invention comprises a vessel-shaped tube having a tube vertically extending along the axis which projects above the housing and extends into the housing adjacent to the lower end thereof where the tube terminates into a first collecting chamber, as well as having a second vertical tube having a greater inner diameter than the outer diameter of the first-mentioned tube, which second tube is arranged concentrically around the first tube, at the top side projects above the housing and extends into the housing adjacent to the lower end thereof, where the tube terminates into a second collecting chamber. The surrounds the first collecting chamber, which first tube is sealingly passed above the housing through the wall of the second tube and the two tubes are closed at the top side in a suitable manner. Both tubes are provided with a supply opening, which can be connected to respective supply lines, while from the first collecting chamber project a plurality of tube-shaped lines and extend through the second collecting chamber and are passed sealingly through the wall thereof. From the second collecting chamber also project a plurality of tube-shaped lines, which pass from the first chamber and from the second chamber adjacent the chambers through a first supporting plate and are combined to form one single bundle which subsequently extends helically around the central tubes towards the top side of the tube. There the tubes originating from the first collecting chamber terminate via a second supporting plate into a third collecting chamber and the tubes originating from the second collecting chamber terminate via the supporting plate into a fourth collecting chamber, while the supporting plates which are also arranged around the central tubes define a pair of the housing wherein the primary cooling fluid is introduced via a supply opening in the wall of the housing and from which via a discharge opening in the wall the primary cooling fluid can be discharged again, while the third and the fourth collecting chamber are provided with openings for connecting thereto the respective discharge lines. An advantage of such an integrated heat exchanger having a helically wound tube bundle is the great surface area that is available for heat transfer (from the primary cooling medium via the tube walls to the secondary cooling media), and the proper mixing which can be achieved. That is to say, that the primary cooling medium is distributed homogeneously over the entire space wherein the helically wound bundle is present, so that no great differences in temperature occur across a horizontal cross-section through the heat exchanger. The tubes of the helically wound bundle may extend in superposed layers, wherein in each layer there are present both the tubes of the intermediate heat exchanger and of the decay heat cooler. In this connection the ratio between the number of tubes of the intermediate heat exchanger and the number of tubes of the decay heat cooler depends on the amount of decay heat to be dissipated after the nuclear reactor has been cut off. There has to be taken into account the maximum allowable differences in temperature between the tubes themselves and between the various layers of tubes in the helically wound bundle. An expert in the field of heat exchangers will be able to determine the most favourable numbers. For the decay heat dissipation system in the installation with a helically wound bundle there are several possibilities. For example, the system can be used for generating electrical energy and for that purpose be connected to a steam generator, just like the secondary system of the intermediate heat exchanger. In that case, the cooling medium in the decay heat cooler can be kept at almost the same temperature as the cooling medium in the intermediate heat exchanger. Differences in temperature between the various tubes can thereby be avoided to a considerable extent. An other possibility is that the cooling medium in the decay heat cooler is not circulated during normal use of the nuclear reactor. Then the temperature profile of this cooling medium over the bundle will be almost equal to that of the primary cooling medium. The differences in temperature occurring with the tubes of the intermediate heat exchanger are acceptable. In a less favourable alternative, the cooling medium of the decay heat dissipation system is circulated without the heat being transferred. Then on certain positions in the integrated heat exchanger relatively high temperature gradients may exist, so that this alternative is not preferred. Finally, it is possible to circulate the cooling medium in the decay heat dissipation system at a low rate during normal operation of the reactor, and to take heat from the medium by means of an external cooler. By doing so, undesirable temperature gradients are avoided, but the heat dissipation in the external cooler decreases the efficiency of the total reactor installation. DETAILED DESCRIPTION OF THE INVENTION In an integrated heat exchanger with helically wound bundle intended for use in a reactor having an output of 5000 MWh, at an output of the intermediate heat exchanger of 625 MWh, and if there is started from an output of the decay heat dissipation corresponding to 5% of the reactor output, that is to say an output of 62.5 MWh for each decay heat cooler, it is possible to attain a proper temperature distribution over the helically wound bundle if it is built up of 26 layers with total of 2197 tubes, 1950 tubes of which constitute part of the intermediate heat exchanger and 247 tubes of the decay heat cooler. The total bundle length in this installation is more than 6 meters. If there is a decay heat dissipation of 2% of the reactor output, there are required 2025 tubes, 1920 of which are in the intermediate heat exchanger and 105 in the decay heat cooler.