GB2059667A - System for mitigating consequences of loss of coolant accident at nuclear power station - Google Patents

Abstract

The system according to the invention comprises a first room (1) which accommodates a reactor plant (3) and an active-type sprinkler means (10). As pressure rises in the first room (1) due to a release of steam from the lost coolant, most of the air contained in this first room (1) is driven out through holes (12) provided in walls (13) of the first room (1) in immediate proximity to a floor (18) of the first room (1), (14) wherefrom it proceeds to a second room (2) through channels (16) and a basin-type condenser (11) accommodated in the second room (2). The length of the channels (16) is selected so as to form a water seal in these channels (16) to prevent the back-flow of air from the second room (2) to the first room (1) and thus produce rarefaction in the first room (1). <IMAGE>

Description

SPECIFICATION System for mitigating consequences of loss of coolant accident at nuclear power station The present invention relates to nuclear power engineering and, more particularly, to a system for mitigating the consequences of a loss of coolant accident at a nuclear power station.

The system according to the invention is aimed at preventing radioactive contamination of the environment which may be the result of result of an accident at a nuclear power station where the coolant is water confined within a sealed coolant circuit and maintained at a high pressure and temperature.

The most serious accident that may occur at a nuclear power station is a loss of coolant accident which means an instantaneous rupture of a pipeline of a maximum diameter and free escape of the coolant at both ends of that pipeline. The coolant brings radioactive fission products to the reactor room. The steam released by the ebullient coolant raises the pressure in the reactor room so that there is a danger of a discharge of radioactive products into the atmosphere.

The present invention essentially consists in providing a system for mitigating the consequences of a loss of coolant accident at a nuclear power station, comprising a first room which accommodates a reactor plant and an active-type sprinkler means to condense steam released by the ebullient coolant after an accident, in which room pressure mounts as a result of an accident so that most of the air contained therein is driven out, no matter at which point of the reactor plant the loss of coolant occurs, through holes in the walls of the first room, which are arranged in immediate proximity to the floor of that room and spaced at a maximum possible distance from one another over the perimeter of the walls, to an intermediate chamber and through channels to a basin-type condenser intended to condense steam produced as a result of an accident and arranged in a second room, the length of the channels being selected so as to form a water seal to prevent the backflow of air from the second room to the first and thus produce rarefaction in the first room.

It is expedient that the system should be provided with a passive-type sprinkler means arranged above the first room, communicating with the first room and actuated right after an accident so as to condense steam released by the ebullient coolant before power supply is resumed and before the active-type sprinkler means is brought into action, and thus ensure fast rarefaction in the first room.

The passive-type sprinkler means may be of the type that comprises a vessel partially filled with water, a main pipe line connected with its first end to the vessel so that a water seal Is formed in the main pipeline, and a water sprinkling device connected to the second end of the main pipeline and intended to sprinkle water in the first room.

It is expedient that the passive-type sprinkler means should be provided with a means to speed up the actuation of the sprinkler means in case of an accident.

This means may be of the type that comprises at least one pipeline with a choke, through which the vessel communicates with the first room so that a water seal is formed in the pipeline.

It is advisable that one end of the pipeline should be immersed in water in the vessel and include an elbow rising above the level of water in the vessel for forming a water seal.

It is highly desirable that one end of the pipeline should be above the level of water in the vessel, and that a sleeve member should be arranged above this end of the pipeline with walls of the sleeve member being partially immersed in the water to form a water seal.

It is expedient that a water sprinkling means should be installed at that end of the pipeline which is found in the first room so as to sprinkle water in the first room.

The means to speed up the actuation of the passive-type sprinkler means may be a heat exchanger arranged in the first room and communicating by means of pipelines with that part of the vessel which is filled with water.

It is desirable that the means to speed up the actuation of the passive-type sprinkler means should include a pipeline to communicate the first room with that part of the vessel which is filled with air, and a choke installed in the main pipeline.

The means to speed up the actuation of the passive-type sprinkler means may also include a tank arranged below the vessel partially filled with water and formed by an expanded portion of the main pipeline.

It is expedient that the means to speed up the actuation of the passive-type sprinkler means should be provided with a water ejector communicating by means of a pipeline with the air-filled space of the main pipeline's elbow or sleeve member, or with the air-filled space of the tank.

It is desirable that a pipeline should communicate the tank with the intermediate chamber, and that one end of this pipeline should be arranged inside the tank and at some distance from the tank's bottom.

The means to speed up the actuation of the passive-type sprinkler means may be provided with a gas ejector installed between the first room and intermediate chamber and having the tank's pipeline communicating with its suction chamber.

Ths system according to the invention produces rapid rarefaction in the first room fol lowing a less of coolant accident and fully prevents contamination of the environment with radioactive products released in the first room.

A better understanding of the present invention will be had from a consideration of the following detailed description of preferred embodiments thereof, taken in conjunction with the accompany drawings, wherein: Figure 1 is an elevation view of a system for mitigating the consequences of a loss of coolant accident at a nuclear power station, in accordance with the invention; Figure 2 is a section taken on line ll-il of Fig. 1 (for reasons of simplicity, Fig. 1 shows a hole in the wall of the first room matched with the plane of the section taken on line ll-ll); Figure 3 is an elevation view of the passivetype sprinkler means, a part of the first room and a part of the intermediate chamber of the system in accordance with the invention;; Figure 4 is an elevation view of a passivetype sprinkler means similar to that of Fig. 3, but featuring two water sprinklers; Figure 5 is an elevation view of an alternative embodiment of the sprinkler mean, which also shows a part of the first room and a part of the intermediate chamber of the system in accordance with the invention; Figure 6 is an elevation view of the passivetype sprinkler means of Fig. 3 with a different means to speed up the actuation of the passive-type sprinkler means; Figure 7 is an elevation view of the passivetype sprinkler means of Fig. 3, featuring yet another version of the means to speed up the actuation of the passive-type sprinkler means; Figure 8 is an elevation view of an alternative embodiment of the sprinkler means, similar to that of Fig. 7; Figure 9 is an elevation fiew of the sprinkler means of Fig. 7, featuring a water ejector;; Figure 10 is an elevation view of another version of the passive-type sprinkler means of the system in accordance with the invention; Figure 11 is an elevation view of a alternative embodiment of the sprinkler means, similar to that of Fig. 8; Figure 12 is an elevation view of another alternative embodiment of the sprinkler means, similar to that of Fig. 8; Figure 13 is an elevation view of yet another alternative embodiment of the sprinkler means, similar to that of Fig. 8; Figure 14 is a graph showing the change of pressure after an accident in the first room, second room and air filled space of the passive-type sprinkler means of the system in accordance with the invention.

Referring to the accompanying drawings, the system for mitigating the consequences of a loss of coolant accident at a nuclear power station comprises a first room 1 (Fig. 1) and a second room 2.

In the embodiment shown in Fig. 1 the second room 2 is arranged below the first room 1. However, another mutual arrangement of the first and second rooms is possible.

The first room 1 is confined within a hermetically sealed envelope conventionally represented by the thick line, and accommodates a reactor plant 3 which comprises a reactor 4 and a coolant circuit 5 including, in turn, steam generators 6, pumps 7 and gate valves 8 which are connected to the reactor 4 by means of a pipeline 9. The present specification only lists that equipment of the reactor plant 3 which is shown in the drawings. It must be borne in mind, however, that the list is much longer and includes, for example, ventilation equipment which is not shown in the drawings because it is irrevelant to the present invention.

The first room 1 also accommodates an active-type sprinkler means 10 to condense steam produced by the boiling coolant following an accident.

The second room 2 is confined within a sealed envelope which is also conventionally shown by the thick line; the second room 2 accommodates a basin-type condenser 11 intended to condense steam released as a result of an accident. A release of steam in the room 1 raises pressure in the room 1 so that air contained therein is driven out through holes 12 provided in walls 1 3 of the room 1 to an intermediate chamber 14 and then through inlets 1 5 of channels 1 6 and their outlets 17 and the basin-type condenser 11 to the second room 2.

The holes 1 2 in the walls 13 of the first room 1 are found on the boundary between the walls 13 and a floor 18 of the first room 1, i.e. in immediate proximity to the floor 18.

As shown in Fig. 2, the holes 12 are spaced at a maximum distance from one another, wherefrom a maximum amount of air is forced from the first room 1 through the holes 1 2 (Fig. 1) and the basin-type condenser 11 to the second room 2, regardless at which point of the reactor plant 3 a leakage of coolant may occur.

The length of the channels 1 6 is selected so as to form a water seal to prevent the backflow of air from the second room 2 to the first room 1 and thus produce rarefaction in the room 1. The height in metres of the channels 16 should be not less than the tenfold ratio of the combined volumes of the first room 1 an intermediate chamber 14 to the volume of the second room 2.

According to the invention, the system is provided with a passive-type sprinkler means 19 arranged above the first room 1 and communicating therewith. The passive-type sprinkler means 1 9 is actuated right after an accident, before the active-type sprinkler means 10 is brought into action; this speeds up the condensation of steam released as a result of the accident and cuts down the time required to produce rarefaction in the first room 1.

The passive-type sprinkler means 1 9 comprises a vessel 20 partially filled with water 21, and a pipeline.22 whereof one end communicates with the water-filled part of the vessel 20 so that water seal is formed in the pipeline 22. For this purpose, the pipeline 22 is bent to form an elbow which rises above the level of the water 21. The means 1 9 further includes a water sprinkling device 23 connected to the second end of the pipeline 22 and intended to sprinkle water in the first room 1. In the embodiment under review, these functions are performed by a pipe 24 carrying water sprinklers 25. Of course, the sprinkler may be of any other type, such as a trough with a perforated wall.

As stated above, the first room 1 accommodates the active-type sprinkler means 10, while the second room 2 accommodates the basin-type condenser 11. The active-type sprinkler means 10 comprises a pipe 26 carrying sprinklers 27 and connected by a pipeline 28 to a pump 29 which supplies water 30 to the sprinklers 27 from a tank 31 or a sump 32 provided in the floor 1 8 of the first room 1; the water 30 is passed through a heat exchanger 33 and an ion-exchange filter 34.

From the tank 31 or sump 32, the water 30 is fed through valves 35 and 36, respectively.

The basin-type condenser 11, comprises a tank 37 filled with water 38 wherein there are immersed outlets 39 of pipelines 40 whose inlets 41 are connected to the outlets 1 7 of the channels 16.

Such is one of the embodiments of the system for mitigating the consequences of a loss of coolant accident at a nuclear power station, in accordance with the invention. The system makes it possible to bring down the pressure in the hermetically sealed envelope and produce rarefaction in the room accommodating the reactor plant, where most of the radioactive products are released as a result of an accident.

Other alternative embodiments of the system according to the invention are dealt with below; these make it possible to accelerate the condensation of steam in the room accommodating the reactor plant so that condensation siarts before the active-type sprinkler means is put into action, whereby rarefaction can be rapidly produced in the reactor plant room.

With this aim in view, the passive-type sprinkler means 1 9 (Fig. 3) is provided with a means 42 intended to speed up the actuation of the means 1 9 in case of an accident.

The means 42 comprises a pipeline 43 with a choke 44. The pipeline 43 serves to connect the vessel 20 to the first room 1 so that a water seal is formed in this pipeline 43. For this purpose, one end of the pipeline 43 is immersed in the water 21 in the vessel 20 and bent to form an elbow which rises above the level of the water 21. According to the embodiment under review, the means 42, intended to rapidly bring the passive-type sprinkler means 1 9 into action, comprises a single pipeline 43 with the choke 44. The means 42 may comprise a plurality of pipelines with their respective chokes, which increases the outflow of water from the vessel, accelerates the condensation of steam and thus cuts down the time it takes to bring the passive-type sprinkler means into action.

According to Fig. 4, the end of the pipeline 43, which is found in the first room 1, carries the sprinkling device 23 to sprinkle the water 21 in the room 1. The sprinkling device 23 is a pipe 45 with water sprinklers 46 mounted thereon. An increase in the area of contact between water and the steam-and-air mixture in the room 1 accounts for a faster condensation of steam and, consequently, a faster operation of the sprinkler means 1 9.

Fig. 5 presents another embodiment of the passive-type sprinkler means 1 9.

According to Fig. 5, one end of a pipeline 47, through which the vessel 20 communicates with the first room 1, is above the level of the water 21. Arranged above this end of the pipeline 47 is a sleeve member 48 whose walls are partially immersed in the water 21 to form a water seal. One end of a pipeline 49 with the choke 44, through which the vessel 20 communicates with the first room 1, is also above the level of the water 21; a sleeve member 50 is arranged over this end of the pipeline 49. The walls of the sleeve member 50 are also partially immersed in the water 21 to form a water seal.

According to Fig. 6, the means 42 to speed up the actuation of the passive-type sprinkler means 1 9 of Fig. 3 comprises a heat exchanger 51 arranged in the first room 1 and communicating with the water-filled part of the vessel 20 by means of pipelines 52 and 53.

According to Fig. 7, the means 42 to speed up the actuation of the passive-type sprinkler means 1 9 of Fig. 3 comprises a pipeline 54 (Fig. 7) whereby the air-filled part of the vessel 20 communicates with the first room 1, and a choke 55 installed in the pipeline 22.

According to Fig. 8, the means 42 to speed up the actuation of the passive-type sprinkler means 1 9 of Fig. 7 additionally includes a tank 56 (Fig. 8) arranged below the vessel 20 and formed by an expanded portion of the pipeline 22. According to Fig. 9, the means 42 to speed up the actuation of the passivetype sprinkler means 1 9 of Fig. 7 is provided with a water ejector 57 (Fig. 9) communicating through a pipeline 58 with the air-filled space within the elbow of the pipeline 22.

The ejector 57 accelerates the filling of the pipeline 22 with water and thus considerably improves the effectiveness of the passive-type sprinkler means 1 9.

The passive-type sprinkler means 1 9 of Fig.

10 is such that the water seal in the pipeline 47 is formed as shown in Fig. 5, whereas the means to speed up the actuation of the sprinkler means 1 9 is like that of Fig. 9. The embodiment of Fig. 10 differs from the already discussed versions in that a water ejector 59 communicates through a pipeline 60 with the sleeve member 48 of the pipeline 47.

The sprinkler means 1 9 of Figs. 11, 1 2 and 1 3 is similar to that of Fig. 8.

The embodiment of Fig. 11 is characterized by that the means 42 to speed up the actuation of the passive-type sprinkler means 1 9 is provided with a water ejector 61 which communicates through a pipeline 62 with the airfilled space within the tank 56.

The embodiment of Fig. 1 2 is characterized in that the tank 56 is connected by a pipeline 63 to the intermediate chamber 14. one end of the pipeline 63 is inside the tank 56, at some distance from a bottom 64 of the tank 56.

The embodiments of Fig. 1 3 is characterized in that the means 42 to speed up the actuation of the passive-type sprinkler means 1 9 is provided with a gas ejector 65 disposed between the first room 1 and intermediate chamber 14. The suction chamber of the gas ejector 65 communicates with the pipeline 63 of the tank 56.

The operating principle of the system for mitigating the consequences of a loss of coolant accident at a nuclear power station in accordance with the present invention is as follows.

In case of rupture of the pipeline 9 (Fig. 1) of the reactor plant 3, the coolant escapes into the first roorn 1 to boil and release steam which fills the room 1. There is a rise of the pressure in the room 1, as shown by curve 66 of the graph of Fig. 14, where time r is plotted as abscissas and pressure P is plotted as ordinates. From the room 1 (Fig. 1), the steam-and-air mixture is force through the holes 1 2 (Figs. 1 and 2) in the walls 1 3 and through the intermediate chamber 14 and channels 1 6 (Fig. 1) to the basin-type condenser 11. The latter condenses the steam, whereas air bubbles through the water into the second room 2.Simultaneously, the steam-and-air mixture is driven from the first room 1 through the pipeline 22 to the waterfilled space inside the vessel 20 of the passive-type sprinkler means 1 9. The steam condenses, but the air accumulates in the airfilled space inside the vessel 20, raising the pressure therein. The pressure in the air-filled space of the vessel 20 increases in proportion to the increase of pressure in the room 1, the former being less than the latter by a value corresponding to the thickness of the layer of water above the end of the pipeline 22 im- mersed in the water. The duct, through which the steam-and-air mixture is forced from the first room 1 to the condenser 11, has a certain hydraulic resistance; it must also be remembered that the release of steam from the coolant is at its maximum at the onset of the accident.These factors make it clear that the pressure in the room 1 reaches a maximum several seconds after the onset of the accident, as shown by curve 66 of Fig. 14, and decreases as the ingress of steam to the room 1 becomes less intensive than the outflow of the steam-and-air mixture from the room 1 to the condenser 11. The decrease of the pressure in the room 1 is intensified by the condensation of steam on cool surfaces inside the room 1. As the pressure in the room 1 is reduced to a value at which the difference of pressure in the air-filled space of the vessel 20 (curve 67 of Fig. 14) and room 1 is greater than the height of the column of liquid in the water seal of the pipeline 22 (point "a" on curve 67 of Fig. 14), the water 21 is forced from the vessel 20 into the sprinkling device 23 and is sprinkled in the first room 1.The steam condensation in the room 1 is intensified until a rarefaction is produced, keeping in mind that much od the air initially filling the room 11 has been driven with the steam-and-air mixture to the condenser 11 and room 2.

As the pressure in the room 1 becomes lower than that in the room 2, as shown by curve 68 of Fig. 14, water is forced from the tank 37 of the condenser 11 to the channels 16 to form a water seal which prevents the backflow of air from the room 2 to the room 1 and intermediate chamber 14. Thus rarefaction in the room 1 can be maintained over a prolonged period of time. This object is also achieved by bringing the active-type sprinkler means 10 into the operation, which condenses steam produced in the room 1 by the afterheat of the reactor 4.

In the embodiment of Fig. 3, the flow of water from the vessel 20 to the sprinkling device 23 is intensified and the passive-type sprinkler means 19 is rapidly actuated by reducing the pressure in the room 1 with the aid of the means 42 which starts operating as soon as the difference of pressures in the airfilled space of the vessel 20 and between the choke 4 and water seal of the pipeline 43 of the means 42 is greater than the value corresponding to the height of the column of water in the water seal. From the vessel 20, water is driven into the pipeline 43 and sprinkled in the room 1, which helps to reduce the pressure in the room 1 and cuts down the time it takes to actuate the passive-type sprinkler means 19.

According to the embodiments of Figs. 4 and 5, the sprinkling of water supplied from the vessel 20 to the room 1 is made more effective by using the water sprinklers 46.

According to the embodiment of Fig. 6, the steam released in the room 1 is condensed on the surface of the heat exchanger 51, which reduces the pressure in the room 1 and the time it takes to bring the passive-type sprinkler means 1 9 into action.

According to the embodiment of Fig. 7, the choke 55 operates so that the pressure in the pipeline 22 rises more slowly than the pressure in the air-filled space of the vessel 20. As soon as the difference of pressures in the airfilled space of the vessel 20 and between the choke 55 and water seal of the pipeline 22 becomes higher than the value corresponding to the height of the column of water in the water seal, the pipeline 22 is filled with water to produce a siphon, and water is fed to the sprinkling device 23 and sprinkled in the room 1.

According to the embodiment of Fig. 8, the increase of pressure in the pipeline 22 is slowed down by the tank 56 formed by an expanded portion of the pipeline 22. This is another way to rapidly bring the sprinkler means 1 9 into action.

The embodiments of Figs. 9 and 10 operate as follows. As water is forced from the vessel 20 to the sprinkling device 23, the water ejector 57 is brought into play to suck the air from above the water seal in the pipeline 22, which improves the functioning of the siphon.

The same effect is attained in the embodiment of Fig. 11, where the ejector 61 sucks air from the tank 56.

The sprinkler means of Fig. 1 2 operates as follows. As soon as the steam-and-air mixture is forced through the holes 12 (Fig. 1) out of the room 1, a pressure difference is produced between the room 1 (Fig. 12) and intermediate chamber 14. A pressure difference is also produced between the air-filled space of the vessel 20 and the intermediate chamber 14.

As a result, water is force from the vessel 20 into the tank 56 and to the sprinkling device 23 which sprinkles it in the room 1.

According to the embodiment of Fig. 13, part of the steam-and-air mixture is forced from the room 1 to the intermediate chamber 1 4 through the gas ejector 65 which also sucks air from the tank 56 and thus facilitates the filling of the tank 56 with water coming from the vessel 20.

The system for mitigating the consequences of a loss of coolant accident at a nuclear power station according to the present invention operates so that it takes only 3 to 5 minutes to produce rarefaction in the reactor plant room which is the most dangerous place from the veiwpoint of radioactive contamination. It is significant that rarefaction is produced without resorting to such active-type means as check valves, which accounts for a high reliability of the system and makes it an effective tool to prevent radioactive contamination of the environment.

Claims (14)

1. A system for mitigating the consequences of a loss of coolant accident at a nuclear power station, comprising a first room accommodating a reactor plant and an activetype sprinkler means to condense steam released by the boiling coolant after an accident, in which room pressure is mounted as a result of an accident so that most of the air contained in the first room is driven out, no matter at which point of the reactor plant a leak of coolant may occur, through holes provided in the walls of the first room in immediate proximity to the floor of this room and spaced at a maximum distance from one another over the perimeter of the walls, to an intermediate chamber and through channels to a basin-type condenser intended to condense steam released as a result of an accident and arranged in a second room, the length of the channels being selected so as to form a water seal in these channels to prevent the backflow of air from the second room to the first and thus produce rarefaction in the first room.

2. A system as claimed in claim 1, provided with a passive-type sprinkler arranged above the first room, communicating with the latter and actuated immediately after an accident to condense steam released due to ebullition of the coolant and rapidly produce rarefaction in the first room.

3. A system as claimed in claim 2, wherein the passive-type sprinkler means comprises a vessel partially filled with water, a main pipeline connected with its first end to the vessel so that a water seal is formed in this main pipeline, and a water sprinkling device connected to the second end of the main pipeline and intended to sprinkle water over the first room.

4. A system as claimed in claim 3, wherein the passive-type sprinkler means is provided with a means to speed up the actuation of the passive-type sprinkler means in case of an accident.

5. A system as claimed in claim 4, wherein the means speed up the actuation, of the passive-type sprinkler means comprises at least one pipeline with a choke, which pipeline communicates the vessel with the first room so that a water seal is formed in this pipeline.

6. A system as claimed in claim 3 or 5, wherein one end of the pipeline is immersed in the water in the vessel and has an elbow rising above the water level to form a water seal.

7. A system as claimed in claim 3 or 5, wherein one end of the pipeline is above the water level and has a sleeve member arranged over it, whose walls are partially immersed in the water to form a water seal.

8. A system as claimed in claim 5, wherein one end of the pipeline, which extends into the first room, carries a water sprinkling device to sprinkle water over the first room.

9. A system as claimed in claim 5, wherein the means to speed up the actuation of the passive-type sprinkler means is a heat exchanger arranged in the first room and communicating through a pipeline with the water-filled space inside the vessel.

10. A system as claimed in claim 4, wherein the means to speed up the actuation of the passive-type sprinkler means includes a pipeline to communicate the air-filled space inside the vessel with the first room, and a choke installed in the main pipeline.

11. A system as claimed in claim 10, wherein the means to speed up the actuation of the passive-type sprinkler means additionally includes a tank arranged below the vessel and formed by an expanded portion of the main pipeline.

1 2. A system as claimed in claim 10 in combination with claims 6 or 7 or 11, wherein the means to speed up the actuation of the passive-type sprinkler means is provided with a water ejector communicating through a pipeline with the air-filled space inside the elbow of the main pipeline, or of the sleeve member of the main pipeline, or of the tank.

1 3. A system as claimed in claim 11, wherein the tank communicates through a pipeline with the intermediate chamber one end of the pipeline being inside the tank, at some distance from its bottom.

14. A system as claimed in claim 13, wherein the means to speed up the actuation of the passive-type sprinkler means is provided with a gas ejector disposed between the first room and intermediate chamber, the pipeline of the tank being connected to the suction chamber of the gas ejector.

1 5. A system for mitigating the consequences of a loss of coolant accident at a nuclear power station, substantially as hereinbefore described with reference to the accompanying drawings.