ZA200602629B

ZA200602629B - Method and device for venting the primary circuit of a nuclear reactor

Info

Publication number: ZA200602629B
Application number: ZA200602629A
Authority: ZA
Inventors: Gilbert Renu
Original assignee: Areva Np
Priority date: 2003-10-23
Filing date: 2006-03-30
Publication date: 2008-04-30
Also published as: ATE445902T1; FR2861493B1; CN1871667A; DE602004023632D1; FR2861493A1; EP1676279B1; WO2005040434A3; EP1676279A2; WO2005040434A2; KR20060090247A

Description

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A METHOD AND APPARATUS FOR VENTING THE PRIMARY CIRCUIT OF

® A NUCLEAR REACTOR

The invention relates to a method and apparatus for venting the primary circuit of a nuclear reactor cooled by pressurized water.

Pressurized water nuclear reactors include a primary circuit in which there circulates water for cooling the nuclear reactor core, which is itself placed inside a reactor vessel. The cooling water under high pressure and at high temperature is used for transferring the heat taken from the nuclear reactor core to feed water which is heated and vaporized within steam generators. The primary circuit of the nuclear reactor comprises the nuclear reactor core which is filled with water under pressure throughout operation of the nuclear reactor, and at least two (and generally three or four) loops, each having disposed therein a steam generator and a primary pump for putting the primary water into circulation between the vessel of the reactor and the steam generator. Each of the loops of the primary circuit includes primary pipes of large diameter, one primary pipe connecting a primary portion of the steam generator to the vessel, a second pipe connecting a delivery portion of the primary pump to the vessel, and a third pipe connecting the primary portion of the steam generator to a suction portion of the primary pump. By means of the first pipe, referred to as the hot branch, the heated cooling water coming from the vessel reaches an inlet compartment of the primary portion of the steam generator, and via the second pipe, referred to as the cold branch, the cooling water that has passed through the primary portion of the steam generator is returned to the vessel from an outlet compartment of the primary portion of the steam generator, passing via a third pipe referred to as an intermediate branch connecting the primary portion of the steam generator to a suction portion of the primary pump.

The primary portion of the steam generator comprises o a bundle of tubes having two substantially vertical branches of great height that are connected to the inlet compartment and to the outlet compartment of the primary portion of the steam generator. The tubes of the bundle are generally U-shaped, each having a top portion (referred to as the curved portion if the tube is U- shaped) that constitutes one of the high points of the primary circuit of the nuclear reactor.

The nuclear reactor vessel is generally cylindrical in shape and presents a vertical axis with an access opening at its top end, which opening is closed by a generally domed closure head while the nuclear reactor is in operation. The top portion of the vessel under the closure head constitutes a second high point of the primary circuit.

The primary circuit of a pressurized water nuclear reactor also includes a pressurizer that is connected via an expansion line to the hot branch of one of the loops of the primary circuit so as to ensure that the primary water is maintained in pressure and temperature. The pressurizer comprises a casing having the general shape of a vertical axis cylinder of great height having in a bottom portion thereof electrical heater means for heating the primary water contained in the pressurizer, and having in an upper portion thereof a line for spraying primary water and a discharge line including protection valves.

The top portion of the pressurizer including the spray line constitutes the highest point of the primary circuit of the nuclear reactor. The top portion of the vessel closure head is connected to a venting duct for evacuating gas from the primary circuit and present under the closure head of the vessel. The top portion of the vessel constituting a high point of the primary circuit is at a level that is considerably lower than the level of the top portion of the pressurizer. The top portions

Lc! [4 . of the tubes in the bundles of the steam generators are

C at a level that is intermediate between the top level of the pressurizer and the top level of the vessel.

When the primary circuit of the nuclear reactor is filled with water, e.g. when initially starting the nuclear reactor or on restarting it after a period in which it has been shut down and during which the primary circuit has been emptied, it is necessary to exhaust all of the gas that is present in the primary circuit prior to restarting the nuclear reactor. If gas remains present in the primary circuit and is entrained by the cooling water while the nuclear reactor is in operation, then heat exchange between the cooling water and firstly the fuel elements of the core and secondly the tubes of the steam generators becomes much less effective, which leads to degraded operation in terms of thermal efficiency and safety associated with the effectiveness of core cooling. In addition, the flows of cooling water in the primary circuit run the risk of becoming unstable in the event of the primary cooling water containing gas when the nuclear reactor is restarted, and at high temperature the oxygen contained in the gas can lead to the primary circuit being corroded.

Filling the primary circuit completely with cooling water without leaving any residual gas is an operation that is difficult, insofar as the primary circuit includes the high points as mentioned above. In particular, the gas present in the top portions of the tubes in the bundles of the steam generators can be difficult to exhaust while filling the primary circuit.

While the primary circuit is being filled, complete venting of the primary circuit, i.e. exhausting all of the gas present in the primary circuit, requires lengthy and complex operations to be performed, thereby lengthening the time required for putting a reactor back into operation after a period in which it has been stopped. In addition, these complex operations present risks that can be mastered only by adopting procedures @® that are extremely severe and that require the installation to be subjected to transients.

Until now, two methods of venting the primary circuit of a nuclear reactor have been proposed and those methods are described below for a reactor having a primary circuit with three loops.

A first method consists in filling the entire circuit with cooling water and then in performing venting via venting lines located at the high points constituted by the top portion of the pressurizer and by the closure head of the vessel. After performing this initial crude venting that is not suitable in particular for exhausting the gas contained in the top portions of the bundles of the steam generators, the pressure in the primary circuit is raised to a level that allows the primary pump to be started, where this level is generally 25 bars. One primary pump is then started for a short period of time.

The air trapped in the top portions of the tubes of the heat exchanger bundles is expelled and accumulates under the closure head of the vessel. Thereafter pressure is reduced to 3 bars so that operators can act in complete safety at low pressure to vent using the venting lines of the pressurizer and of the vessel.

This complete cycle comprising raising the pressure in the primary circuit to 25 bars by injecting water into the circuit, starting one of the primary pumps, and then returning the primary circuit to a pressure of 3 bars by taking water from the primary circuit, and finally venting the high points, needs to be performed with each of the primary pumps in all three loops of the primary circuit, and then using all three primary pumps simultaneously in all three loops. When pressure is lowered from 25 bars to 3 bars by taking water from the primary circuit, it is necessary to wait before venting so as to allow the emulsion of primary water and air to separate so as to release the air and gas that is to be @ disposed of by venting.

The volumes of water that need to be injected into the primary circuit in order to raise its pressure from 3 5 bars to 25 bars, or that need to be extracted in order to lower its pressure from 25 bars to 3 bars depend on the volume of residual air in the primary circuit, but under all circumstances, repeating numerous cycles in which pressure is caused to vary between 25 bars and 3 bars, requires large volumes of water to be used. The water drawn off to lower the pressure must be considered as effluent and needs to be subjected to special treatment prior to being reused or disposed of. The known method that is generally used for venting the primary circuit is lengthy and expensive.

A second method has therefore been proposed for venting the primary circuit while seeking to avoid the need for numerous pressurization cycles. In that second method, the level of water in the pipework of the loops in the primary circuit is determined during filling so ~ + that it occupies the midplane of the primary pipework, and a vacuum suction pump is connected to the venting duct of the pressurizer. After establishing a sufficient vacuum in the primary circuit, the circuit is filled completely with water. A pressure of about 25 bars is established in the primary circuit, and all three primary pumps are set into operation for a short period of time.

The pressure in the primary circuit is then lowered to 3 bars and final venting is performed from the top portion of the vessel and from the pressurizer.

That method has the advantage of being shorter and easier to implement than the usual method as described above, but it presents drawbacks and risks that put considerable restrictions on the possibilities of using it. During evacuation, the residual heat removal system (RRA) for cooling the nuclear reactor when not in operation needs to operate with the level of primary water occupying the midplane of the pipes in the loops of @® the primary circuit. This configuration runs potential risks, and certain safety authorities do not accept this procedure. In addition, evacuating the primary circuit can lead to subsequent faulty operation of sensors in the primary circuit, such as level sensors in the pressurizer.

The object of the invention is thus to propose a method of venting the primary circuit of a nuclear reactor cooled by pressurized water, in order to enable the primary circuit to be filled completely with water, said primary circuit comprising a vessel containing the reactor core that is generally cylindrical in shape having a vertical axis with a top end closed by a closure head, and at least two loops, each comprising first and second primary pipes in communication with the inside of the vessel, a steam generator having a primary portion connected to the vessel by the first pipe, and including a bundle of tubes having substantially vertical branches, and a primary pump having a delivery portion connected to the vessel via the second primary pipe and a suction portion connected to the primary portion of the steam generator via a third primary pipe, together with a pressurizer connected to the first primary pipe of only one of the loops of the primary circuit via a pressurization duct, the highest points of the primary circuit being constituted in order of decreasing height: by the top portion of the pressurizer including a first gas exhaust duct; by the top portions of the branches of the bundles of tubes in the steam generators; and by the top portion of the vessel closed by a closure head, having a second gas exhaust duct, the method being suitable for being implemented under very good safety conditions by operations that are very simple and less numerous than in the methods of the prior art.

To this end, the method of the invention provides the following steps: \

; + filling the primary circuit with water and ( establishing the pressure of the water in the primary circuit at a level that enables the primary pumps to operate; + setting into operation a first primary pump of a loop that is not connected to the pressurizer via a pressurization duct; + continuously evacuating the gas contained in the primary circuit and entrained by the water set into circulation by the primary pump from the top portion of the vessel, at least while the first primary pump is in operation; + stopping the first primary pump; - putting into operation, in succession, each of the primary pumps, and then putting all of the primary pumps of the primary circuit into operation simultaneously, while simultaneously continuously exhausting gas from the top portion of the vessel; and - performing final venting of the primary circuit from the high points of the pressurizer and of the vessel via the first and second exhaust ducts after stopping the primary pumps and lowering the pressure of the water in the primary circuit down to a level lower than the pressure that enables the primary pumps to operate.

In order to make the invention well understood, there follows a description by way of example and with reference to the accompanying figures of an implementation of the invention in a three-loop nuclear reactor.

Figure 1 is a diagrammatic elevation and section view of one loop of the primary circuit of a pressurized water nuclear reactor.

Figure 2 is a diagrammatic view of the primary circuit of a three-loop nuclear reactor including means for implementing the venting method of the invention.

Figure 3 is a fragmentary view in elevation and in section of a nuclear reactor loop including means for \

implementing the method of the invention in a first ( variant implementation.

Figure 4 is a section and elevation view of a primary circuit of a nuclear reactor including means for implementing the method of the invention in a second variant implementation.

Figure 5 is a fragmentary view in elevation and in section of the primary circuit of a nuclear reactor including means for implementing the method of the invention in a third variant.

Figure 6 is a fragmentary view in elevation and in section of a primary circuit loop of a nuclear reactor with a pressurization line from the pressurizer, and including means for implementing the method of the invention in a fourth variant.

Figure 1 shows a loop of a primary circuit of a pressurized water nuclear reactor, the primary circuit being given overall reference 1.

The primary circuit that is to receive pressurized water for cooling the nuclear reactor comprises in particular a vessel 12 and a plurality of loops 3, each comprising a steam generator 4 and a primary pump 5 for circulating the primary cooling water under pressure within the loop of the primary circuit.

Figure 1 shows water 20 filling a portion of the vessel 12 and of the loop 3 of the primary circuit, during a filling stage, after the primary circuit 1 has been emptied and before the nuclear reactor is restarted.

The loop 3 of the primary circuit as shown comprises a first main pipe 6a referred to as the hot branch, connecting the vessel 12 to an inlet compartment of the primary portion of the steam generator 4, a second primary pipe 6b, or cold branch, connecting a delivery portion from the primary pump 5 to the vessel 12, and a third pipe 6c connecting an outlet compartment of the primary portion of the steam generator 4 to a suction portion of the primary pump 5.

The pressurizer 17 of the primary circuit 1 is @® connected via an expansion duct 18 of large diameter to the first primary pipe 6a or hot branch.

The primary portion of the steam generator 4 comprises a water box 9 subdivided by a partition into an inlet compartment 9a and an outlet compartment 9b for the nuclear reactor cooling water, and a tube bundle 10 in which each tube has one end communicating with the inlet compartment and its other end communicating with the outlet compartment of the water box 9. The tube bundle 10 is shown in conventional manner in the form of a single large-diameter tube, but in reality the tube bundle of a steam generator comprises a very large number of tubes that are long and of small diameter (e.g. 3000 tubes to 5000 tubes), each tube being folded into a U- shape and comprising two straight branches of very great length disposed vertically and connected at their bottom ends respectively to the inlet compartment and the outlet compartment of the water box 9 of the steam generator.

The curved portions of the U-shaped tubes of the steam generator are juxtaposed and constitute the top portion of the steam generator bundle.

The vessel 12 of the nuclear reactor is generally cylindrical in shape about a vertical axis having a top portion giving access to the inside of the vessel, which top portion is closed by a closure head 12a during operations of filling, venting, starting, and also while the nuclear reactor is in operation.

The water 20 introduced into the primary circuit 1 fills the vessel of the nuclear reactor up to a top level 20a beneath the closure head 12a of the vessel, fills the pressurizer 17 up to a top level 20b in the top portion of the pressurizer, and fills the primary portion of the steam generator up to a water level 20c in the top portions of the tubes making up the bundle 10.

Gas, constituted mainly by air fills the top portion of the vessel above the water level 20a and below the

. closure head 12a, fills the top portion of the ® pressurizer above the water level 20b, and fills the top portion of the steam generator bundle 10 above the filling level 20c.

The venting of the primary circuit to it being filled completely with water in order to restart the nuclear reactor consists in evacuating the gas contained in the high portions of the primary circuit, and in particular in the top portions of the vessel, of the pressurizer, and of the steam generator bundle, so as to ensure that the primary circuit 1 is completely filled with water.

The vessel 12 has a venting line 27 connected to the top portion of the vessel closure head 12a with valves 28 and 28' located therein suitable for being remotely controlled, thus enabling the air that accumulates under the vessel closure head 12a to be exhausted when the valves are opened.

The pressurizer 17 includes a venting line 2 connected to its top portion and having a valve 21 located therein. The venting line may be constituted by the connected pressurizer spray line on which the venting line 14 is disposed.

In general, the primary circuit of the nuclear reactor is vented by opening the venting lines of the pressurizer and of the vessel, once after the steam generator has been filled with water, and then after the primary pumps of the primary circuit loops of the reactor have been put successively into operation, so as to entrain the air and the gas that is trapped in the high : portions of the primary circuit, and in particular in the top portions of the bundles 10 of the steam generators 4.

As mentioned above, in the context of a venting method that is commonly used, it is necessary to perform numerous cycles during which the pressure in the primary circuit goes from a high pressure at which the pumps are put into operation (e.g. 25 bars) to a low pressure (e.g.

3 bars) in which gas can be exhausted without danger, and

C then back again from the low pressure to the high pressure.

In particular, it is necessary to pressurize the primary circuit to 25 bars prior to each operation of starting a pump, and it is necessary to reduce pressure down to 3 bars prior to exhausting the gas that has accumulated in the high portions of the primary circuit of the nuclear reactor.

In addition, prior to exhausting gas, it is necessary to wait for separation of the water and gas emulsion that has accumulated in this way in the high portions of the vessel and of the pressurizer that include the venting means.

These various cycles need to be repeated each time the primary pumps are put into operation separately and whenever all three pumps situated on the three loops of the primary circuit are put into operation.

Figure 2 shows in conventional manner a primary circuit 1 of a three-loop nuclear reactor including additional means for implementing a venting method of the invention.

The primary circuit 1 comprises a vessel 12 having a closure head 12a closing its top portion, and three loops 13, 23, and 33, each having a steam generator and a primary pump located therein. The first loop 13 has a steam generator 14 and a pump 15. The second loop 23 has a steam generator 24 and a primary pump 25. Finally, the third loop 33 has a steam generator 34 and a primary pump 35. Each of the loops 13, 23, and 33 of the primary circuit 1 has a first primary pipe or hot branch connecting the vessel 12 to the primary portion of the steam generator of the loop, a second primary pipe or cold branch connecting a delivery portion of the pump 15 to the vessel 12, and an intermediate branch connecting the primary portion of the steam generator to the primary pump of the loop.

References 16a, 26a, and 36a designate the hot

C branches, references 16b, 26b, and 36b designate the cold branches, and references 16c, 26c, and 36c designate the intermediate branches of the respective loops 13, 23, and 33.

The pressurizer 17 of the primary circuit 1 is connected via an expansion duct 18 of larger diameter to the hot branch 16a of the first loop 13 of the primary circuit.

Two cold branches of the loops of the primary circuit (e.g. the loops 13 and 33) are connected by ducts 19a and 19b to a spray line 19 opening out into the top portion of the pressurizer. Branching from the spray line 19, there is disposed a venting line 2 including a controlled valve 21.

The top portion of the pressurizer 17 is also connected to a discharge line 22 having controlled valves 29 and 29' located therein.

A venting line 27 is secured to the top portion of the vessel 12 and opens out under the closure head 12a, the venting line including venting valves 28 and 28°'.

As explained below, the method of the invention can be implemented by connecting exhaust means and/or fluid recovery means to the end 30 of the venting line 27 of the vessel 12.

In a first embodiment shown in Figure 3, the fluid exhaust means connected to the end 30 of the venting line 27 for the vessel are constituted by a single pipe 32 enabling the end 30 of the venting duct 27 of the vessel to be connected to the end 31 of the discharge line 22 of the pressurizer 17. This line 32 also has two purge lines 22' and 27' and two isolating valves 28" and 29%.

In Figure 3, there can be seen the first loop 13 of the primary circuit to which the pressurizer 17 is connected via the expansion line 18 connected to the hot branch 16a.

Implementation of the venting method of the ® invention is described below in a first implementation with reference to Figures 2 and 3.

Initially, the primary circuit is filled with water and the pressure of the water in the primary circuit is raised to a level that allows the primary pumps disposed in the loops 13, 23, and 33 to be set into operation.

This pressure level is generally about 25 bars.

The valves 28 and 28' of the venting line 27 of the vessel are opened as are the valves 29 and 29' of the isolating lines connected to the top portion of the pressurizer 17.

A first primary pump in one of the loops 23 and 33 is set into operation, i.e. in one of the two loops whose hot branches 26a and 36a are not connected to the pressurizer via an expansion line. By way of example, the pump 35 of the loop 33 is initially set into operation so as to cause the water contained in the primary circuit and the air contained in the high portions of the primary circuit to circulate.

In the loop 33 whose primary pump 35 has been put into operation, the pump 35 delivers the water contained in the loop 33 tec the vessel via the cold branch 36b (continuous-line arrow 37 representing water flow).

Simultaneously, water containing air contained in the high portions of the bundle is sucked in by the pump 35 from the steam generator 34 (dashed line arrow 38 representing a flow of air or a water-air emulsion).

In the loops 23 and 13 in which the primary pumps are not in operation, fluid circulation occurs in the opposite direction to normal circulation when the primary pump is in operation.

Water coming from the vessel is caused to flow into the cold branches 26b and 16b of the loops 23 and 13 in the direction going from the vessel towards the primary pump, as shown by continuous-line arrows 39 and 41.

Water coming from the vessel reaches the steam generators

Claims

® 1. A method of venting the primary circuit (1) of a nuclear reactor cooled by pressurized water, so as to enable the primary circuit (1) to be filled completely with water, the primary circuit comprising a vessel (12) containing the reactor core that is generally cylindrical in shape, having a vertical axis and a top end closed by a closure head (12a), and at least two loops (13, 23, 33) each having first and second primary pipes (16a, 26a, 36a; 16b, 26b, 36b) in communication with the inside of the vessel (12), a steam generator (14, 24, 34) having a primary portion containing a bundle (10) of tubes having branches that are substantially vertical, the primary portion being connected to the vessel (12) by the first pipe (16a, 26a, 36a), and a primary pump (15, 25, 35) having a delivery portion connected to the vessel (12) via the second primary pipe (16b, 26b, 36b) and a suction portion connected to the primary portion of the steam generator (14, 24, 34) via a third primary pipe (1l6c, 26c, 36c¢), and a pressurizer (17) connected to the first primary pipe (16a) of one of the loops (13) of the primary circuit (1) via a pressurizing duct (18), the highest points of the primary circuit (1) being constituted in order of decreasing height: by the top portion of the pressurizer (17) having a first venting duct (2) placed on the spray duct (19) connected to the pressurizer (17); by the top portions of the branches of the bundles (10) of tubes in the steam generators (14, 24, 34); and by the top portion of the vessel (12) closed by a closure head (12a) and including a second venting duct (27); the method being characterized by the following steps: - filling the primary circuit (1) with water and establishing the pressure of the water in the primary circuit (1) at a level that enables the primary pumps (15, 25, 35) to operate;

ES BN + setting into operation a first primary pump (35) Sv ® of a loop (33) that is not connected to the pressurizer (17) via a pressurizing duct (18); - continuously evacuating the gas contained in the primary circuit (1) and entrained by the water set into circulation by the primary pump (35) from the top portion (12a) of the vessel (12), at least while the first primary pump (35) is in operation; - stopping the first primary pump (35); + putting into operation, in succession, each of the primary pumps (35, 25, 15), and then putting all of the primary pumps (15, 25, 35) of the primary circuit (1) into operation simultaneously, while simultaneously continuously exhausting gas from the top portion of the vessel (12); and

. performing final venting of the primary circuit from the high points of the pressurizer (17) and of the vessel (12) via the first and second venting ducts (27, 22, 19, and 2) after stopping the primary pumps and lowering the pressure of the water in the primary circuit (1) down to a level lower than the pressure that enables the primary pumps (15, 25, 35) to operate.
2. A method according to claim 1, characterized by the fact that gas is exhausted from the top portion of the vessel (12) via the first venting duct (27) towards the top portion of the pressurizer (17) which is itself vented by the second venting duct (2) via the spray duct (19) .
3. A method according to claim 1, characterized by the fact that gas is exhausted from the top portion of the vessel (12) into a tank (47) having a bottom portion that is connected to the first venting duct (27) .
4. A method according to claim 3, characterized by the fact that the tank (47) is vented at a pressure level that enables the primary pump (15, 25, 35) in the primary ® circuit to operate.
5. A method according to any one of claims 2, 3, and 4, characterized by the fact that the fluid in the first venting duct (27) is set into forced flow by means of a pump (50, 68).
6. A method according to any one of claims 1 to 5, for a primary circuit of a nuclear reactor having three loops (13, 23, 33): a first loop (13) including a hot branch (16a) connected via a pressurization duct (18) to the pressurizer (17); and second and third loops (23 and 33) having respective hot branches (26a, 36a) that are not connected to the pressurizer (17) via a pressurization duct, the method being characterized by the facts that the first primary pump (35, 25) to be set into operation is the primary pump (35, 25) of one of the third and second loops (33, 23) of the primary circuit (1), that after stopping the first primary pump (35, 25), the primary pump (25, 35) of the second or third loops (23, 33) respectively is put into operation, and then after stopping the primary pump of the second or third loop (23, 33), the primary pump of the first loop (15) of the primary circuit (1) is put into operation, the flow of water in each loop of the primary circuit in which a pump is put into operation taking place in the direction of water circulation during normal operation of the nuclear reactor, and taking place in the opposite direction in each of the loops of the primary circuit in which the primary pump is not put into operation.
7. Apparatus for venting the primary circuit (1) of a nuclear reactor cooled by pressurized water, to enable the primary circuit (1) to be filled completely with water, the primary circuit comprising a vessel (12) containing the reactor core that is generally cylindrical in shape having a vertical axis and a top end closed by a @® closure head (12a), and at least two loops (13, 23, 33) each comprising first and second primary pipes (16a, 26a, 36a; 16b, 26b, 36b) in communication with the inside of the vessel (12), a steam generator (14, 24, 34) having a primary portion comprising bundles (10) of tubes having branches that are substantially vertical and connected to the vessel (12) via the first pipe (16a, 26a, 36a), and a primary pump (15, 25, 35) having a delivery portion connected to the vessel (12) via the second primary pipe (16b, 26b, 36b), and a suction portion connected to the primary portion of the steam generator (14, 24, 24) via a third primary pipe (1l6c, 26c, 36c), together with a pressurizer (17) connected to the first primary pipe (16a) of one of the loops (13) of the primary circuit (1) via a pressurization duct (18), the highest points of the primary circuit (1) being constituted, in order of decreasing height: by the top portion of the pressurizer (17) that includes a first venting duct (2) placed on the spray line (19) connected to the pressurizexr (17); by the top portions of the branches of the bundles (10) of tubes in the steam generators (14, 24, 34); and by the top portion of the vessel (12) closed by a closure head (12a) including a second venting duct (27), the apparatus being characterized by the fact that it includes means (32, 47) connected to the second venting duct (27) of the vessel (12) of the nuclear reactor for continuously evacuating gas reaching the top portion of the vessel (12) of the nuclear reactor while at least one of the primary pumps (15, 25, 35) of the primary circuit (1) of the nuclear reactor is in operation.
8. Apparatus according to claim 7, characterized by the fact that the means (32) for continuously exhausting gas reaching the top portion (12a) of the vessel (12) comprise a connection duct (32) for connecting one end (31) of the first venting duct (27) of the vessel (12) to

EN one end (31) of a discharge duct (22) connected to the ® top portion of the pressurizer (17) that is also connected to the first venting duct (2) via a spray duct (19).
9. Apparatus according to claim 8, characterized by the fact that a pump (50) is inserted in the connection duct (32).
10. Apparatus according to claim 7, characterized by the fact that the means for continuously exhausting gas reaching the top portion (12a) of the vessel (12) of the nuclear reactor are constituted by a tank (47) having a bottom portion connected via a duct (46) to one end (30) of the second venting duct (27) of the vessel (12) of the reactor and to means (48, 68) for driving circulation of the water entrained into the tank (47) by the gas reaching the top portion of the vessel (12), with a top portion thereof being connected to an effluent exhaust tank (62).
11. Apparatus according to claim 10, characterized by the fact that the means (48) for driving circulation of water from the bottom portion of the tank (47) are constituted by a duct (48) connected to a point at low pressure of at least one loop of the primary circuit in which at least one valve (56, 57) is disposed.
12. Apparatus according to claim 10, characterized by the fact that the means for driving a flow of water from the bottom portion of the water in a tank (47) are constituted by a duct (48) connected to at least one of the branches of a loop of the primary circuit (1) of the nuclear reactor having a circulation pump (68) and at least one stop valve (56, 57) disposed therein.
13. Apparatus according to claim 12, characterized by the ® fact that a motor-drive valve (63') and an expander device (71) with a calibrated orifice are disposed in the effluent exhaust duct (61) connected to the top portion of the tank (47), and by the fact that the motor-driven valves (56, 60) are disposed respectively in the duct (48) connecting the bottom portion of the tank (47) to the primary circuit, and in the duct (46) connecting the bottom portion of the tank (47) to the end (30) of the second venting duct (27) of the vessel (12).
14. Apparatus according to claim 10, characterized by the fact that the means for driving water circulation in the bottom portion of a tank (47) are constituted by a duct (48) connecting the bottom portion of the tank (47) to one end (31) of the discharge duct (22) of the pressurizer of the nuclear reactor, having disposed therein a pump (68) and at least one top valve (29, 29', 56) .
15. Apparatus according to claim 14, characterized by the fact that at least a first motor-driven valve (56) and at least a second motor-driven valve (60) are disposed respectively in the duct (48) connecting the bottom portion of the tank (47) to the end (31) of the discharge duct (22) of the pressurizer, and in the duct (46) connecting the bottom portion of the tank (47) to the end (30) of the second venting duct (27) of the vessel (12).