EP0070775B1 - Water-sodium steam generator - Google Patents

Description

The present invention relates to an improvement to sodium-water type steam generators used in particular in nuclear power plants.

In power plants, a boiler provides heat to the fluid in a closed loop circuit, this heated fluid then circulates in a steam generator in order to transfer its heat to water which is transformed into steam, this steam being then sent to the power station turbines.

In certain nuclear power plants, in particular of the fast neutron type, it is known to use liquid sodium as a fluid for transporting calories from the boiler to the steam generator. In this case, the steam generator consists of a heat exchanger, the primary circuit of which contains liquid sodium and the secondary circuit of which contains water transformed into vapor.

In such steam generators, special precautions are taken to avoid contact between the liquid sodium in the primary circuit and the water in the secondary circuit. Indeed, it is known that the mixture of sodium and water at high temperature causes very violent chemical reactions, with release of gas and sudden increase in the pressure prevailing in liquid sodium. These brutal, accidental chemical reactions which can resemble an explosion inside the steam generator, can cause the partial deterioration of certain parts of the steam generator, but also, due to the propagation of the pressure wave in the primary circuit pipes caused by the accidental explosion in the steam generator, can cause damage to the reactor core or intermediate exchangers, or damage to the circulation pumps or any other installation located on this primary circuit.

It is known to have various devices in different parts of the sodium circuit making it possible to limit the effects of the sodium-water reaction. It is known, for example, to connect a connection to the sodium outlet of the steam generator in a connection to a storage tank closed by a rupture membrane, this membrane being designed to rupture quickly as soon as the sodium pressure exceeds a predetermined value. , in order to establish a bypass of the overpressure sodium to a storage tank. It is also known to connect a bypass to the piping of the primary circuit reaching the circulation pumps and the intermediate exchangers, an expansion tank making it possible to greatly attenuate the intensity of the pressure wave, before it propagates. in these pumps and these intermediate exchangers.

It is also known to arrange the steam generator so that in its upper zone encloses a pocket of inert gas, which creates a free level which very strongly attenuates the pressure waves emitted in the pipes. arrival of sodium.

When a primary circuit containing liquid sodium includes a steam generator, the upper part of which encloses a gas pocket and also includes an expansion tank connected to the circuit by large-diameter piping, located at a certain distance of the steam generator, a mass oscillation phenomenon has been observed between the two gas pockets during accidental sodium-water reactions or during the simultaneous reactions which are carried out to test the installation.

This rocking phenomenon is similar to that which would exist in a system of communicating vessels closed at their upper part and into which a very high gas flow is suddenly injected into one of the two vessels. Likewise, in a conventional installation comprising a steam generator provided with a free level and an expansion tank connected to the circuit, the more the inertia of the mass of liquid sodium which is located between the two free levels important, the larger the amplitudes of the oscillations. While the expansion tank is intended to limit the effects of the pressure wave, it can be seen that it cannot eliminate significant overpressures in the circuit, harmful for the entire installation and in particular for the intermediate exchangers which are frequently installed to isolate a first liquid sodium circuit passing through the reactor core from the liquid sodium circuit passing through the steam generators.

Also known from document US-A-3 924 675 is a steam generator comprising an internal expansion tank constituted by a hollow internal cylinder provided with a non-return valve.

The present invention aims mainly to overcome the various drawbacks of known steam generators.

• - un circuit primaire constitué d'une enceinte allongée remplie de sodium liquide,
• - une zone d'entrée de l'enceinte dans laquelle est introduit le sodium liquide,
• - une zone de sortie de l'enceinte allongée dans laquelle est évacué le sodium liquide,
• - un réservoir rempli partiellement d'un gaz inerte et communiquant à sa partie inférieure avec l'enceinte allongée pour définir un niveau libre du sodium liquide,
• - et un circuit secondaire constitué d'une pluralité de tubes de circulation d'eau s'étendant à l'intérieur de l'enceinte allongée.

The present invention therefore relates to a sodium-water type steam generator, in particular for nuclear power plants, comprising:

• - a primary circuit consisting of an elongated enclosure filled with liquid sodium,
• - an enclosure entry zone into which the liquid sodium is introduced,
• - an exit zone from the elongated enclosure into which the liquid sodium is discharged,
• - a tank partially filled with an inert gas and communicating at its lower part with the elongated enclosure to define a free level of liquid sodium,
• - And a secondary circuit consisting of a plurality of water circulation tubes extending inside the elongated enclosure.

According to the invention, the reservoir permanently forms a total passage in the elongated enclosure. slightly free without any restriction in both directions favoring the propagation of an acoustic wave towards the free level of liquid-gas sodium where it is reflected with a significant loss of energy, in order to avoid in the whole of the steam generator the propagation of this acoustic wave resulting from a brutal accidental chemical reaction between sodium and water, a tube bringing in the upper part of the tank the inert gas and a tube extending vertically inside the tank, its end lower being located relatively low in said tank, so that the level of the free surface of the liquid sodium does not fall below the lowest point of the tube at the time of introduction of the neutral gas through the tube into the tank.

• - la figure 1 représente schématiquement un circuit de refroidissement pour centrales nucléaires conforme à l'invention,
• - la figure 2 représente schématiquement en coupe longitudinale un générateur de vapeur conforme à la présente invention.

A preferred embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended drawings in which:

• FIG. 1 diagrammatically represents a cooling circuit for nuclear power plants according to the invention,
• - Figure 2 shows schematically in longitudinal section a steam generator according to the present invention.

Referring to FIG. 1, a secondary cooling circuit for nuclear power plants can be distinguished, the general diagram of which is entirely conventional. There are, in known manner, a set of pipes 1, 2, 3 which transport liquid sodium in a closed circuit. The liquid sodium circulates so as to take the calories in the intermediate exchanger 5 to give them to the steam generator 6 by causing, in this steam generator, the transformation of water into steam in the secondary circuit, this steam being used to conventional way to turn the power station turbines. After passing through the steam generator, the liquid sodium returns via line 3 to the circulation pump 4, before starting its cycle again.

The steam generator 6 is therefore a heat exchanger, the primary circuit of which contains liquid sodium and the secondary circuit of which contains water. If a rupture occurs in this steam generator causing a mixture of a certain quantity of sodium with water, it follows a brutal chemical reaction which has the effects of an explosion inside the steam generator, and which causes a sudden rise in pressure in the sodium circuit. On a conventional installation, the pressure wave which results from this explosion can propagate through line 3 then through line 1, arrive in the intermediate exchanger 5 and cause it to break. To protect this intermediate exchanger 5 which constitutes a barrier between the radioactive liquid sodium passing through the reactor core and the uncontaminated liquid sodium passing through the steam generator, it has already been proposed to have a bypass on the pipe bringing the sodium to the intermediate exchanger 5 an expansion tank of large size. capacity, connected to the pipeline by a large diameter tubing. To illustrate this known solution, we have arranged in dotted lines in FIG. 1 such a branch connection of an expansion tank 7. On the other hand, we have represented a steam generator 6 including the introduction chamber 8 sodium is conventionally placed at the top of the steam generator and contains an inert gas pocket 9 which determines a free level of sodium. We have noticed that when we thus had in a known manner the whole of this steam generator 6 connected by pipes 1, 2 and 3 to this expansion tank 7, during the simulations of the sodium-water reactions which were carried out to test the installation, the sodium masses contained in the pipes are driven in oscillations of large amplitudes associated with significant variations of pressure in the circuit which can be of the same order of magnitude as the pressure waves which would exist if there was no expansion tank. We therefore note that this expansion tank 7 is not very effective.

The present invention aims to overcome these drawbacks by designing a steam generator 6 comprising certain new provisions which make it possible to avoid the connection of the expansion tank 7 to the circuit by means of a large diameter pipe 24 intended to limit the propagation of a pressure wave.

The steam generator specific to our invention, making it possible to avoid such a connection, is shown in FIG. 2 in more detail. This steam generator comprises a cylindrical enclosure 1 of elongated shape and arranged vertically, filled with circulating liquid sodium constituting the primary circuit. Liquid sodium arrives at the steam generator 6 through the pipes 2, is introduced into the inlet chamber 8 located at the top of the steam generator, flows from top to bottom inside the cylindrical enclosure 1, reaches to the outlet zone 9 located at the bottom of the steam generator from where it is discharged via the pipe 11. The steam generator shown in this exemplary embodiment comprises a secondary circuit consisting of a multitude of tubes arranged helically 12 and in which circulates water introduced by the lower part 13 of the tubes and emerging from the upper part 14 of the tubes in the form of vapor. As the tubes 12 cannot be wound helically with a radius that is too small, the central part of the steam generator is not occupied by the tubes 12 and it is therefore planned to install a cylinder 15 of elongated shape therein. This cylindrical casing 15 extending longitudinally in the center of the steam generator is not completely useless since it advantageously contributes to the rigidity of the assembly and / or to keeping the tubes 12 in place. But in the steam generator 6 in accordance with the present invention this cylindrical envelope 15 com also carries various arrangements which allow it to perform other functions which we will describe below and which constitute the essential feature of this invention.

The cylindrical casing 15 is arranged longitudinally in the center of the steam generator, vertically, is closed at its upper part 16 and has openings 17 at its lower part. On the other hand, the interior of this envelope 15 is filled with a certain amount of an inert gas 18 which forms a pocket at the upper part of the envelope 15 and which defines a free surface 19 of the liquid sodium. A tube 20 brings the necessary inert gas into the upper part of the envelope 15, and a tube 21 extends vertically inside the enclosure 15, its lower end being situated relatively low in the envelope 15, so that, when the inert gas is introduced into the chamber 18 through the tube 20, the level of the free surface 19 of the liquid sodium does not drop below the lowest point of the tube 21. If in the generator steam 6 as described above, an explosion due to a violent sodium-water reaction occurs, the resulting pressure wave is considerably damped, inside the steam generator, due to the elasticity of the pocket gas 18, thus preventing this pressure wave from propagating in significant proportions through the sodium outlet pipe 11, towards the other devices located in this circuit, that is to say mainly the circulation pumps and especially the intermediate exchangers. In other words, the volume occupied by the cylindrical envelope 15 which sometimes exists in certain known steam generators is arranged according to the present invention so as to constitute an expansion tank inside the steam generator, produced economically since it is made up largely of elements that exist anyway, which takes no additional space, and which strictly limits the steam generator mechanical stress caused by an accidental sodium-water reaction therein.

Of course, the heat transfer circuit comprising a steam generator in accordance with the present invention, as shown in FIG. 1, can advantageously be supplemented by a deviation, close to the outlet of liquid sodium from the steam generator, this deviation being closed in regime normal by a rupture membrane intended to rupture when the liquid sodium pressure at the outlet of the steam generator exceeds a predetermined value, in order to put this outlet of the steam generator into communication with the storage tank 30. It may be noted that this arrangement of rupture membrane and storage tank is known per se and plays here a role entirely identical to that which it plays in installations comprising a conventional steam generator. It can also be noted that it is known that such a rupture membrane 31 only partially attenuates the amplitude of the pressure wave during a possible sodium-water reaction in the steam generator.

Furthermore, it is advantageous to have, in the installation specific to the invention (FIG. 1), at the level of the circulation pump 4, an expansion vessel 22 connected to the pipe 3 by a small diameter tube 23, this expansion vessel 22 only making it possible to compensate for the variations in sodium expansion. Of course its connection by a small tube to the pipe 3 is not at all suitable for the function of limiting the propagation of the pressure wave.

One would think that, in the steam generator 6 specific to our invention, the simultaneous existence of two free surfaces 10 and 19 surmounted by a pocket of inert gas 9 and 18 is likely to also cause a system of oscillations of the level of these two free surfaces. In fact, the phenomenon of oscillations can only take on a significant extent when the distances which separate the two free surfaces are large enough, implying a fairly large mass of displaced liquid sodium. However, in the steam generator 6, the distance separating the two free surfaces 10 and 19 is very small since these two surfaces are located in the same device, and consequently the mass of liquid sodium existing between these two free surfaces is substantially the mass of liquid sodium contained in the cylindrical enclosure 1 of the steam generator, which is too small to produce oscillations of significant magnitude.

It is possible to apply the provisions specific to the present invention to a steam generator whose tubes of the secondary circuit would be arranged other than helically, or whose inputs and outputs of liquid sodium would be arranged otherwise.

Claims (1)

1. Steam generator of the sodium/water type especially for nuclear installations comprising:

   - a primary circuit comprising an elongated container (1) filled with liquid sodium,

   - an inlet manifold (8) for the elongated container into which the liquid sodium is introduced,

   - an outlet manifold (9) for the elongated container (1) into which the liquid sodium is evacuated,

   - a reservoir (15) partially filled with inert gas and communicating at its lower end with the elongated container (1) to define a free level (19) of liquid sodium, and

   - a secondary circuit comprising a plurality of water circulation tubes (12) extending in the interior of the elongated container,

   
   characterized by the fact that the reservoir (15) permanently forms in the elongated container (1) a totally free passage without any restriction in the two senses favouring propagation of an acoustic wave towards the free level (19) of liquid sodium/gas at which it is reflected with a considerable loss of energy, whereby to avoid in the steam generator assembly, propagation of said acoustic wave resulting from a violent accidental chemical reaction between sodium and water, a tube (20) leading the inert gas into the upper part of the reservoir (15) and a tube (21) extending vertically in the interior of the reservoir (15), its lower end being located relatively low in said reservoir whereby the level of the free surface (19) of liquid sodium does not descend below the lowest point of tube (21) at the moment of introduction of the inert gas through the tube (20) into the reservoir (15).

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