WO1995024719A1

WO1995024719A1 - A depressurisation system for plants operating with pressurised steam

Info

Publication number: WO1995024719A1
Application number: PCT/EP1995/000842
Authority: WO
Inventors: Giovanni Mini
Original assignee: Finmeccanica S.P.A. Azienda Ansaldo
Priority date: 1994-03-09
Filing date: 1995-03-07
Publication date: 1995-09-14
Also published as: IT1276043B1; ITMI940430A1; ITMI940430A0

Abstract

A depressurisation system (1) for plants operating with pressurised steam which operates by gravity injection of cold water from a tank (2) arranged in a higher position than the pressurised plant and having a delivery line (24) for delivery to the pressurised plant, and including at least one valve structure (30) operable to put the pressurised plant into hydraulic communication with the external environment and intended to open to achieve this communication at a predetermined pressure in the pressurised plant which corresponds to the substantial emptying of the tank (2) and to remain open until the pressurised plant is totally depressurised.

Description

"A depressurisation system for plants operating with pressurised steam" DESCRIPTION

The present invention relates to a system for depressurising plants operating with pressurised steam. The system is of the type in which cold water is injected by gravity from a tank positioned at a higher level than the pressurised system.

A typical application is in the field of water-cooled nuclear reactors where a sudden incident, for instance a loss of electrical power to the circulation pumps of the preliminary cooling circuit or the lack of availability of the heat sink, can lead to a rapid and dangerous increase in the internal pressure of the plant.

In systems of the type specified, depressurisation is effected by the injection into the pressurised plant of a great quantity of cold water which condenses the steam in the system and lowers the temperature, thereby causing the internal pressure to fall.

Various mechanisms are known for achieving the aforesaid cold-water injection. In the copending Italian Patent Application N. MI93A002070 by the same Applicant, a depressurisation system is described in which the raised tank is connected to the steam plant by a delivery line which forms a siphon and is connected to the narrow portion of an ejector through which steam is drawn by a condenser downstream of the ejector. The depressurisation caused by the passage of the steam draws the cold water from the tank through the siphon which is thus overcome. However, the depressurisation systems described above are not able to depressurise the pressurised plant completely as, especially in the case of nuclear plants, they are not able to use a sufficient quantity of water as this would require a raised tank of such large dimensions that it would be extremely difficult to build.

In addition, in the case of nuclear reactors, heat would continue to be generated inside the plant even after the water from the raised tank had been used up and hence the internal pressure would build up once again unless active intervention were possible. The technical problem at the basis of the present invention involves providing a depressurisation system able to overcome the problems cited with reference to the prior art.

These problems are overcome, according to the invention, by a depressurisation system of the type specified, characterised in that it includes at least one valve structure operable to put the pressurised plant into communication with the external environment, and intended to open, thereby achieving this communication, at a predetermined pressure in the pressurised plant corresponding to the substantial emptying of the aforesaid tank.

The main advantage of the depressurisation system of the invention lies in the fact that it ensures, permanently, that a pressurised-steam plant may be totally depressurised to the pressure of the external environment.

A further advantage is provided by the fact that the system of the invention can be assembled from components of simple design. An additional advantage of the present invention lies in the fact that it is adaptable to a vast range of pressurised plants, including any type of nuclear reactor, using either pressurised or boiling water.

Further characteristics and advantages of the invention will become clearer from the description of one embodiment of the depressurisation system for pressurised plants applied to a nuclear reactor using pressurised light water, provided hereafter with reference to the appended drawings, supplied purely by way of non-limitative example.

In these drawings: * Figure 1 is a diagram of one embodiment of the depressurisation system for pressurised plants according to the invention;

* Figure 2 is a cross section of a valve structure of the depressurisation system according to the invention in its closed position; and

* Figure 3 is a cross section of the valve structure of Figure 2 in its open position.

In the drawings, a depressurisation system for pressurised plants is generally indicated 1. The pressurised plant is constituted, in this example, by a nuclear reactor which is not shown overall, the only components illustrated being those which interact with the depressurisation system 1.

The system 1 includes a tank 2 filled to a level 3 with cold water. This level 3 divides the tank 2 into an upper portion 4 and a lower portion 5. The depressurisation system 1 also includes an ejector 6, having an inlet section 8, an outlet section 9 and a narrow section 10, and a condenser 7 having an inlet 11 and an outlet 12.

The nuclear reactor includes a pressuriser 13 filled with hot water 14 up to a level 15 with a steam head 16 above the hot water 14.

The tank 2 is at a level above the entire pressuriser 13 and the ejector 6, this in turn being above the level 15 of the hot water 14 inside the pressuriser 13 while the condenser 7 is below this level 15.

The nuclear reactor has a primary circuit of which the hot leg is indicated 17 in Figure 1.

A depressurisation circuit 25 branches from the hot leg 17 and is connected to the inlet end 31 of a valve generally indicated 30. The valve 30 also includes an outlet end 32 open to the external environment of the pressurised plant and is therefore able to put the pressurised plant into communication with this external environment.

The steam head 16 communicates with the upper portion 4 of the tank 2 through a first line 18.

As a result of this, the tank 2 and the pressuriser 13 are at the same pressure. A second line 20 opens from the first line 18 and is connected to the inlet section 8 of the ejector 6.

The outlet section 9 of the ejector 6 communicates with the inlet 11 of the condenser 7 through a third line 21 while the condenser outlet 12 communicates with the hot leg 17 through a first injection line 22 which opens thereto through a spray nozzle 23. The hot water 14 in the pressuriser 13 is in communication, through a fourth line 19, with the hot leg 17 of the main circuit.

As the condenser 7 is below the level 15 of the hot water 14 in the pressuriser 13 and as the circuit formed by the third line 21 , the condenser 7, the first injection line 22, the hot leg 17 and the fourth line 19 is open, the condenser 7 is completely flooded with water from the pressuriser 13 which fills the entire aforesaid circuit up to a point in the third line 21 which is at the same height as the level 15.

The lower portion 5 of the tank 2 is connected to the narrow section 10 of the ejector 6 through a delivery line 24 which constitutes a siphon circuit 24a.

The upper portion of the siphon circuit 24a is above the tank 2 so that cold water from this tank 2 fills the delivery line 24 up to a point in the ascending portion of the siphon circuit 24a which is at the same height as the level 3.

The aforesaid valve 30 will now be described in detail with reference to Figures 2 and 3.

This valve has a valve body 40 which includes the inlet end 31 and the outlet end 32 and a piloting mechanism 80. A tube-like inlet manifold 41 extends from the inlet end 31 and terminates with a stop surface 42 inside the valve chamber 43 from which an outlet manifold 44 extends to the outlet end 32.

A valve shutter member 45 is arranged in the valve chamber 43 and has a disc-shaped end face 46 formed to seal against the stop surface 42 so as to prevent hydraulic communication between the inlet manifold 41 and the outlet manifold 44 when the valve 30 is closed.

A stem 47 extends from the opposite side of the shutter member 45 from the disc-face 46 and is housed in a bore 48 which extends from the valve chamber so as to ensure a watertight seal. The stem 47 has at its opposite end a first piston head 49 arranged inside a first cylinder chamber 50. A watertight seal is also ensured between the first piston head 49 and the first cylinder chamber 50.

The first cylinder chamber 50 has a bottom surface 51 concentric with the stem 47 which passes through it and an opposing surface 52. When the valve 30 is closed, the first piston head 49 bears against the bottom surface 51.

A spring 53 is arranged between the first piston head 49 and the opposing surface 52 and is unstressed in the aforesaid situation, serving only as a damper should the valve 30 open suddenly.

The area of that surface of the first piston head 49 which faces the opposing surface 52 is greater than the area of the disc-face 46 of the shutter member 45 which faces the inlet end 31.

The stem 47, the first piston head 49 and the first cylinder chamber 50 constitute actuator means for opening and closing the valve 30.

The first cylinder chamber 50 communicates with the outside through a loading duct 54 which extends from the chamber and ends in a loading valve 55.

The piloting mechanism 80 includes a second cylinder chamber 81 containing a second piston head 82 which divides this second chamber 81 into a lower zone 83 and an upper zone 84 and also provides a watertight seal between these two zones 83, 84.

A first, preloaded, biasing spring 85 is housed in the upper zone 84 and acts on the second piston head 82, the function of which will be explained later.

The piloting mechanism 80 also includes a first pilot valve 86 and a second pilot valve 87 having respective first and second actuator stems 88, 89 and respective second and third biasing springs 102, 103 which keep the pilot valves 86, 87 closed. A control rod 90 extends upwardly from the second piston head 82 and projects from the second cylinder chamber 81 through a watertight hole 91. Outside the second cylinder chamber 81 , a projection 92 projects perpendicularly from the control rod 90 so as to act either on the first actuator stem 88 or on the second actuator stem 89 of the pilot valves 86, 87 respectively, in opposition to the biasing springs 102, 103, according to the position of the second piston 82 inside the second cylinder chamber 81.

At its other end, the control rod 90 is connected to a pneumatic actuator 100 attached to the piloting mechanism by a first frame element 101.

Finally, a first duct 93 opens from the inlet manifold 41 and divides into first and second branches 94, 95 which open into the lower zone 83 of the second cylinder chamber 81 and into the first pilot valve 86 respectively.

A second duct 96 opens from the first cylinder chamber 50 and divides into third and fourth branches 97, 98 which open into the first pilot valve 86 and the second pilot valve 87 respectively, the latter in turn communicating with the exterior through a third duct 99. During normal operation of the nuclear reactor, the reactor's internal pressure acts on the disc-face 46 of the shutter member 45. This pressure is also transmitted through the first duct 93 and the first branch 94 to the second piston 82 which, overcoming the resilient force of the first biasing spring 85, remains raised whereby the projection 92 acts on the first actuator stem 88 of the first pilot valve 86 to maintain it open, against the resilient force of the second biasing spring 102.

The opening of the first pilot valve 86 puts the inlet manifold 41 into hydraulic communication with the first cylinder chamber 50 through the first duct 93, the second branch 95, the first pilot valve 86, the third branch 97 and the second duct 96. The Pressure in the first cylinder chamber 50 is thereby equalised with that in the pressurised plant.

As the surface area of the first piston 49 facing the opposing surface 52 is greater than the surface area of the disc-face 46 of the shutter member 45 facing the inlet end 31 and as the pressure is equal on the two faces, a resultant force arises which holds the disc-face 46 pressed against the stop surface 42, preventing hydraulic communication between the pressurised plant and the outside environment.

The operation of the system 1 according to the invention for depressurising plants under pressure will now be described.

With reference in particular to Figure 1 , it can be noted that, when the level 15 inside the pressuriser 13 falls as a result of an incident which causes a loss of water from the primary circuit, the level of the water in the line 21 also falls until the condenser 7 is empty. When this happens, the condenser 7 starts to condense the steam which begins to fill it.

This phenomenon draws steam from the pressuriser 13 through the first line 18, the second line 20, the ejector 6 and the third line 21. The passage of steam through the ejector 6 reduces the pressure in the narrow section 10.

This low pressure sucks steam from the delivery line 24 and thereby also causes the water in it to rise until it passes the summit of the siphon circuit 24a. When this is reached, the water in the tank 2 is injected, simply by gravity, through the delivery line 24, the ejector 6, the third line 21 , the condenser 7 and the first injector line 22 into the hot leg 17 of the primary circuit, being sprayed thereinto through the spray nozzle 23.

As a result, the steam present therein condenses rapidly causing the pressure in the nuclear reactor to fall to a predetermined level.

This level corresponds to a particular calibration of the first biasing spring 85 of the valve 30 which, at this point, urges the second piston 82 downwards.

With reference in particular to Figure 3, the projection 92 of the control rod 90 is also moved until it acts on the second actuator stem 89 of the second pilot valve 87, causing this to open against the action of the third biasing spring 103.

At the same time, the second biasing spring 102 is no longer opposed by the respective first actuator stem 88, which is freed from contact with the projection 92, and closes the first pilot valve 86.

CONFIRMAΗON COPY The opening of the second pilot valve 87 puts the outside environment and the first cylinder chamber 50 into hydraulic communication through the third duct 99, the second pilot valve 87, the fourth branch 98 and the second duct 96. The pressure in the first cylinder chamber 50 is thus equalised with that of the environment outside the pressurised plant, which, presumably, is much lower than the aforesaid predetermined pressure.

At this point, the force applied to the disc-face 46 of the shutter member 45 is much greater than the opposing force on the other end of the stem 47 as this is due to the pressure of the external environment. There is thus a resultant force which lifts the shutter member 45, opening the valve

30.

Hydraulic communication is thus achieved between the external environment and the pressurised plant, with the result that the plant is totally depressurised. In a preferred embodiment of the invention, the shutter member 45 is attached to an elastic element 56 connected to the valve body 40 by means of a second frame element 57.

The calibration of the elastic element 56 is such that, when the pressure in the plant approaches that of the external environment and therefore the force which maintains the interceptor element 45 raised from the stop surface 42 tends to zero, the resilient force of the elastic element 56 at least ensures that the valve 30 is kept open.

It remains to describe how the nuclear reactor can be pressurised in the presence of the valve 30. During this step, the pneumatic actuator 100 is put into operation so as to hold the projection 92 of the control rod 90 in an intermediate position between the actuator stems 88, 89 whereby both of the pilot valves 86, 87 are closed. Next, the loading valve 55 is opened and compressed air is injected into the first cylinder chamber 50 through the loading duct 54. By adjusting the pressure of the compressed air it is possible to overcome the force of the elastic element 56 and of the mounting pressure in the nuclear reactor during its pressurisation. Once the pressure in the nuclear reactor equals that in the chamber

50, the loading valve 55 is closed and the pneumatic actuator is put into operation so as to move the projection 92 of the control rod 90 until it interferes with the first actuator stem 88 of the first pilot valve 86, opposing the resilient force provided by the second biasing spring 102. As explained earlier, the pressure in the first chamber 50 is thus equalised with that in the pressurised plant.

At the end of this step, the pneumatic actuator 100 is deactivated so that the valve 30 will remain closed so as not to interfere with the pressurisation which has been achieved in the plant. In addition to the aforesaid advantages, the depressurisation system

1 of the invention operates entirely passively, relying only on the laws of physics, thanks to its structural characteristics.

It is however possible to imagine numerous variants of the depressurisation system 1 described above.

11

CONFIRMAΗON COPY In particular, the depressurisation line 25 may open not only from the hot leg 17 of the pressurised plant but also from other components thereof, such as the pressuriser 13 or the fourth connecting line 19.

Many other variations and alterations may be made to the depressurisation system 1 of the invention without departing from the protective scope of the invention, as claimed in the following Claims.

Claims

1. A depressurisation system (1) for plants operating with pressurised steam which operates by gravity injection of cold water from a tank (2) arranged in a higher position than the pressurised plant and having a delivery line (24) leading to the pressurised plant, characterised in that it includes at least one valve structure (30) operable to put the pressurised plant into communication with the external environment and intended to open, thereby achieving this communication, at a predetermined pressure in the pressurised plant which corresponds to the substantial emptying of the tank (2).

2. A depressurisation system (1) according to Claim 1 , wherein the valve structure (30) includes a valve body (40) having at least one inlet end (31) for delivery from the pressurised plant, this inlet end (31) communicating with at least one outlet end (32) opening to the external environment, and a valve shutter member (45) interposed between the inlet end (31) and the outlet end (32), this shutter member (45) having associated actuator means (47, 49, 50) for opening the valve structure (30) controlled by a piloting mechanism (80) of the actuator means (47, 49, 50) sensitive to the attainment of the aforesaid predetermined pressure in the pressurised plant. 3. A depressurisation system (1) according to Claim 2, wherein:

- the actuator means include a stem (47) extending from the shutter member (45), this stem (47) having a first piston head (49) at its end housed in a first cylinder chamber (50);

- the piloting mechanism (80) having:

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CONFIRMAΗON COPY - a second cylinder chamber (81) housing a second piston head (82) which divides it into a lower zone (83) and an upper zone (84), the lower zone (83) being in hydraulic communication with the pressurised plant and the upper zone (84) housing a first biasing spring (85);

- first and second piloting valves (86, 87) including respective first and second actuator stems (88, 89);

- a control rod (90) extending upwardly from the second piston head (82) and from which projects a projection (92) positioned so as to act upon the first and second actuator stems (88, 89) according to the position of the second piston (82) in the second cylinder chamber (81);

- the first cylinder chamber (50) being in hydraulic communication with the pressurised plant through the first pilot valve (86) and with the external environment through the second pilot valve (87).

4. A depressurisation system (1) according to Claim 3, wherein:

- the first cylinder chamber (50) is in hydraulic communication with the external environment through at least one loading duct (54) which has a loading valve (55); - the control rod (90) is connected to a pneumatic actuator (100).

5. A depressurisation system (1) according to Claim 2, wherein the shutter member (45) is connected to the valve body (40) by at least one resilient element (56).