Method and system for bringing a nuclear power plant into a safe state after extreme effect

Abstract

The method and system for bringing a nuclear power plant to a safe state after extreme effect reduce the temperature of the coolant after extreme effect. The system includes inlet and outlet pipelines, a steam generator, a storage tank and a heat exchanger, a separation tank above the steam generator and connected by two pipelines to a storage tank, a pump, a control unit. The method involves filling the system with coolant, feeding the coolant from the steam generator through the inlet pipeline and the storage tank to the heat exchanger, and feeding the coolant through the outlet pipeline back to the steam generator, wherein the pump is turned on for feeding the coolant and subsequent operation of the system. The first air valve is used to maintain pressure in the system, ensuring the absence of boiling of the coolant.

Claims

1. A system for bringing a nuclear power plant into a safe state after extreme effect, the system comprising: a steam generator; a separation tank; a storage tank; a pump; a heat exchanger; a control unit configured to control the system; a first water valve; a second water valve; a first air valve; an inlet pipeline from the steam generator to the separation tank; two pipelines comprising a first pipeline and a second pipeline, the first pipeline configured to provide a path for water in the separation tank to be transferred to the storage tank, the second pipeline configured to provide a path for gas in the separation tank to be transferred to the storage tank; a discharge pipeline from the storage tank to the pump; and an outlet pipeline from the storage tank to the pump and from the pump to the steam generator; the separation tank located above the steam generator and connected by the two pipelines to the storage tank; and wherein the heat exchanger is installed in the outlet pipeline downstream of the pump, the first water valve is installed in the inlet pipeline, and the separation tank is connected with the storage tank by the first pipeline with the second water valve installed therein and the second pipeline with the first air valve installed therein.

2. The system according to claim 1, wherein a deaerator configured to remove steam from the system is used as the storage tank.

3. The system according to claim 1, wherein the system comprises a vertical steam discharge pipeline with a second air valve; and the steam generator is equipped with the vertical steam discharge pipeline with the second air valve.

4. The system according to claim 1, wherein the system comprises a plurality of steam generators connected in parallel to each other to the inlet and outlet pipelines.

5. The system according to claim 1, wherein at least a part of the inlet pipeline is configured to have an upward slope towards the separation tank.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURE

(1) FIG. 1 shows a system for bringing a nuclear power plant into a safe state after extreme effect.

(2) The system for bringing a nuclear power plant into a safe state after an extreme effect consists of steam generator 1, second air valve 2 connected to it through a steam discharge pipeline, inlet pipeline 3 with first water valve 5 installed in it, connecting steam generator 1 to separation tank 4, which is connected to storage tank 8 through the two pipelines with second water valve 6 and first air valve 7 installed in them, the storage tank is connected to steam generator 1 through outlet pipeline 9, wherein pump 10, heat exchanger 11 and third water valve 12 are installed. In a preferred embodiment, the storage tank is connected by a pipeline to the feedwater make-up tank (not shown in the FIGURE).

PREFERRED EMBODIMENT OF THE INVENTION

(3) A system for bringing a nuclear power plant into a safe state after extreme effect in the preferred embodiment works as follows. After using passive heat removal systems, for example, SG SPHR, or other heat removal systems, the coolant temperature of the NPP will be reduced to 130° C., the system control unit opens the valve between storage tank 8 and the feedwater make-up pipeline, thereby letting feedwater make-up having a temperature of about 25° C. into storage tank 8 to a certain level; it opens third water valve 12 and closes first water valve 5, turns on pump 10, maintains a certain water level in steam generator 1 (about 3.7 in), performs heating of inlet pipeline 3 and outlet pipeline 8, wherein a pressure in the system is maintained at about 0.27 MPa through second air valve 2. Then, when the temperature of the wall of separation tank 4 is reached the value of 125° C., the control unit opens first water valve 5 and sets it in the mode of maintaining a constant liquid flow rate (about 7.5 kg/s per steam generator 1 when using four steam generators 1 in the system). Thereafter, the first air valve 7 is opened, which, similarly to second air valve 2, starts to work in the mode of maintaining the pressure at a level of about 0.27 MPa, and when separation tank 4 reaches a certain level, second water valve 6 starts to work in the mode of maintaining the liquid level. Maintaining the specified steam pressure in the system is required in order to avoid boiling of saturated water in the steam generator when the pressure decreases. Then, after the flooding of steam generator 1 and pipelines, third water valve 12 can be switched to the mode of maintaining increased liquid flow (up to 12.5 kg/s, up to 50 kg/s for a total of four steam generators). Then, the reactor is cooled down to a temperature of 70° C., which can take several days. Upon reaching a temperature of 70° C., a passive heat removal system ensures the removal of residual heat during all the time necessary for this, which can be up to 60 days. In this case, in the preferred embodiment, when the pressure in the system is lower than 98 kPa, first 7 and second 2 air valves are opened to their full section area and turned off from the pressure maintenance mode in the system, wherein there is no longer any danger of the coolant boiling up in steam generator 1 at that moment and there is no need in pressure regulation, and atmospheric pressure is sufficient for the most efficient heat exchange process. All of the above processes are controlled by a control unit (not shown in the FIGURE).

(4) In a preferred embodiment of the group of inventions, a deaerator is used as storage tank 8, and the piping system of the secondary system of the NPP with WWER already used in normal operation of the NPP as inlet 3 and outlet 9 pipelines, wherein the deaerator is located below the steam generator, and inlet pipeline 3 in the systems currently used in NPPs with WWER is located with a decrease from steam generator 1 towards the deaerator, which is rational for the normal operation of the secondary system of NPPs with WWER, since it allows the collection of moisture after the passage of steam through this section at its lower point and to remove it to the drainage system so as to avoid its feed to the NPP turbine. This solution allows to use the systems already existing in the secondary system of the NPP to bring the NPP to a safe state, however, in the emergency operation mode of the inventive system, not steam passes through steam pipeline 3, but the steam-water mixture and therefore the reduction of inlet pipeline 3 creates the conditions for the occurrence of steam blockages in pipeline 3, as a result, hydraulic impacts. That is why separation tank 4, located above steam generator 1, is added to the system, and at least a part of inlet pipeline 3 is placed with a slope upward towards separation tank 4. This solution avoids the accumulation of steam blockages. In addition, in order to remove excess steam from steam generator 1, a vertical steam discharge pipeline with second air valve 2, configured to relieve steam pressure when the pressure exceeds 0.27 MPa, is additionally introduced into the system in the preferable embodiment, since lower pressure can lead to boiling water and therefore poses a threat to the integrity of the piping of the system. The steam discharge pipeline can be made wide enough, up to 3 meters in diameter, in order to avoid turbulent effects during steam removal.

(5) The use of a deaerator as storage tank 8 also makes it possible to use its blowdown system to remove steam from the system. In addition, it is also rational to use other standard secondary systems of WWER NPPs. In particular, in a preferred embodiment of the claimed group of inventions, the standard feedwater make-up system of NPPs is used as an external source of feedwater make-up, the standard secondary system pump of an NPP with WWER is used as a pump, and the standard cooling system of non-critical consumers of NPPs is used as heat exchanger 11.

(6) When supplying feed water to the system, its flow rate can be chosen so that the process of filling steam generator 1 and pipelines 3 and 9 occurs with saturated water. Due to this, it is possible to avoid condensation hydraulic impact arising from the meeting of steam with a cold liquid. One of the main conditions for the occurrence of condensation hydraulic impact is the underheating of water relative to steam, the critical values of which are 15° C. and above. The most likely section wherein condensation hydraulic impact can occur during filling is the steam collecting header of steam generator 1. As calculations showed, when filling the system, it is rational to maintain water at a temperature close to the saturation temperature and reduce it only after pipelines 3 and 9 are completely filled.

(7) Calculations carried out using software have shown that when using four steam generators connected to the inventive system in parallel, using water from the feed system and the cooling system for non-critical consumers, it is possible to cool the NPP from 130° C. to 70° C. without causing hydraulic impact for 60 hours. In addition, the calculations showed that even in the event of failure of one of the steam generators, cooling the system using three steam generators according to the proposed method and system is quite safe and allows to bring the NPP to a safe state at a temperature of 70° C.

INDUSTRIAL APPLICABILITY

(8) The method and system for bringing a nuclear power plant into a safe state after an extreme effect can be applied in nuclear power plants with water-water energetic reactor to bring them to a safe state after an extreme effect.