Safety injection device for pressurised-water nuclear reactor

Abstract

The invention relates to a device (1) for injecting a liquid coolant (2) into a primary circuit (CP) of a nuclear reactor, the device comprising a first liquid-coolant accumulator (10) configured to inject the liquid coolant into the primary circuit by gravity, the device being characterised in that it comprises: - a second accumulator (20) connected to the first accumulator (10), the second accumulator comprising the liquid coolant in a manner held under pressure by a gas (21); - a connecting pipe (23) extending between the first accumulator and the second accumulator; - a valve (24) extending between the first accumulator and the second accumulator such that, when the valve opens, the liquid coolant flows via the connecting pipe (23) from the second accumulator to the first accumulator, so as to fill the first accumulator.

Description

[0001] Description

[0002] Title: Safety Injection Device for Pressurized Water Nuclear Reactor

[0003] TECHNICAL FIELD

[0004] The technical field of the invention relates to the injection of refrigerant into a primary circuit of a nuclear reactor in the event of a primary coolant loss accident.

[0005] EARLIER ART

[0006] Figure 1 schematically illustrates the main components of the primary circuit of a pressurized water reactor (PWR). In this example, the reactor consists of a vessel (V) connected to four cooling loops, two of which are shown, forming the primary circuit (CP) through which a heat transfer fluid circulates. In this example, the heat transfer fluid is pressurized water: the water is maintained under pressure (155 bar) so that it remains liquid up to an average temperature of 300 °C. Each cooling loop includes pipes extending between the vessel, a steam generator (SG), and a pump (P). Some components are not shown, such as the pressurizer.

[0007] A breach in the primary circuit is an accidental event, usually referred to as a Loss of Coolant Accident (LOCA). This results in a pressure drop and a loss of the primary circuit's water inventory. The consequences can include heating of the fuel rods, which must be kept to a minimum to maintain reactor core cooling and preserve the integrity of the fuel cladding. An auxiliary circuit, usually designated a Safety Injection System (RIS), is configured to inject borated water into the primary circuit or the reactor vessel. Several RIS safety injection systems can be installed, which activate based on the pressure in the primary circuit.This allows the fuel elements to remain immersed in the event of a loss of coolant and ensures the evacuation of residual power generated by the reactor core after its automatic shutdown.

[0008] Figures 2A and 2B represent two injection devices that could potentially be used to ensure water injection into the primary circuit.

[0009] Figure 2A shows a device, designated as a "bloated accumulator," consisting of a container filled with liquid water pressurized at the top by an inert gas, for example, nitrogen, which forms a nitrogen head (N2). In an accident situation, when the pressure in the primary circuit is lower than the pressure of the inert gas, the liquid water stored in the container is injected into the primary circuit via an injection line (LIS) through a valve (C). The valve opens when the pressure in the primary circuit falls below the pressure of the nitrogen head. This type of injector is currently used in pressurized water reactors operating in France. One drawback is that the injection flow rate is high and depends on the rate of depressurization of the primary circuit. This can result in wasted water injected through the breach.

[0010] JPH0483198 describes the use of a pressurized accumulator to inject water into a primary circuit of a boiling water reactor.

[0011] Figure 2B illustrates another type of injection device, a "gravity accumulator." This device consists of a container filled with liquid water. A pressure equalization line (LEP) connects the container to the primary circuit. In an accident situation, the LEP is connected to the primary circuit, or to the tank, so that the pressure inside the container reaches the pressure of the primary circuit. The liquid water can then flow into the primary circuit through an injection line (LIS), via a valve (C). Valve C opens when the pressure in the primary circuit becomes lower than the pressure in the accumulator. With this type of injector, the flow is gravity-fed due to the pressure equilibrium achieved. This results in a more controlled injection flow rate, preventing excessive water leakage through the breach. The use of such a device was described in Cadiou T.et al, “Multi-scale study of an innovative safety system for PWR”, Nuclear Engineering and Design, NED-21-000077R1, 2022.

[0012] However, a gravity-fed accumulator has a limited capacity because its size is restricted by constraints related to its footprint and height relative to the primary circuit, which are necessary for the water to flow by gravity. It would be possible to fill this type of accumulator, but this would require a pumping system powered by an electrical energy source. However, relying on an electrical energy source is a drawback from a reliability standpoint.

[0013] The combined use of both types of accumulators has been considered for VVER-type pressurized water reactors, as described in the publication Redondo-Valero E, "Safety margins improvement by means of passive second stage hydroaccumulators in a VVER-1000 / V320 reactor," Nuclear Engineering and Design, Vol. 414, December 2023. The accumulators are used sequentially: the swollen accumulator is used in the short term, when the pressure in the primary circuit is high, while the gravity accumulator is used in the longer term, at low pressure. However, the water flow resulting from the swollen accumulator remains difficult to control, with the risk of significant water loss through the breach created in the primary circuit.

[0014] The inventors propose an injection device that can be used for a long time after the occurrence of an accident such as a loss of primary refrigerant.

[0015] DESCRIPTION OF THE INVENTION

[0016] The object of the invention is a device for injecting a refrigerant liquid into a primary circuit of a nuclear reactor, extending around a reactor vessel, the device comprising a first refrigerant liquid accumulator, configured to inject, by gravity, the refrigerant liquid into the primary circuit, the device being characterized in that it comprises: - a second accumulator, connected to the first accumulator, the second accumulator comprising the refrigerant liquid maintained under pressure by a gas;

[0017] - a connecting pipe, extending between the first accumulator and the second accumulator;

[0018] - a valve, extending along the connecting pipe, so that under the effect of an opening of the valve, the refrigerant flows, through the connecting pipe, from the second accumulator to the first accumulator, so as to fill the first accumulator.

[0019] The valve can be configured to be open when the refrigerant liquid, in the first accumulator, reaches a low point during gravity injection.

[0020] The device may include a pressure balancing line extending between the first accumulator and the primary circuit or reactor vessel, the pressure balancing line being configured to bring steam from the primary circuit or vessel to the level of an upper end of the first accumulator, the device being such that the valve is configured to open under the effect of steam flowing from the pressure balancing line.

[0021] The valve can be pneumatic, the valve being configured to move from a closed position to an open position under the effect of a movement of a moving part.

[0022] The device may include: - a reservoir, connected to the valve, containing an auxiliary fluid, the auxiliary fluid having a vaporization temperature, at atmospheric pressure, of less than 100 °C;

[0023] - a branch, extending along the first accumulator, and connected to the latter between an upper junction and a lower junction, the branch being such that

[0024] • the lower and upper junctions open on either side of the lower threshold of the first accumulator;

[0025] • the bypass extends, in a sealed manner, through the tank, the portion of the bypass extending through the tank forming an exchanger, conducive to heat exchange between the bypass and the tank;

[0026] - so that under the effect of the steam in the exchanger, the auxiliary fluid vaporizes, inducing an increase in pressure in the tank, causing the moving part to move.

[0027] Preferably, in the exchanger, the bypass forms undulations in the tank so as to maximize heat exchange.

[0028] The invention will be better understood by reading the explanation of the examples of embodiment presented, in the continuation of the description, in connection with the figures listed below.

[0029] FIGURES

[0030] Figure 1 schematically illustrates the main components of a primary circuit.

[0031] Figure 2A represents a swollen-type accumulator.

[0032] Figure 2B represents a gravity accumulator.

[0033] Figures 3A and 3B show an embodiment of a safety injection device according to the invention.

[0034] PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0035] The invention is based on an injection device combining a swollen accumulator as described in Figure 2A, and a gravity accumulator, as described in connection with Figure 2B. An important aspect of the invention is that these two accumulators are used in series, as described below.

[0036] In the description, the refrigerant is assumed to be water, or to be composed mostly of water.

[0037] Figures 3A and 3B describe an embodiment of a device 1 according to the invention. Figure 3A shows the device during normal reactor operation. Figure 3B shows the device used during a loss-of-coolant accident to inject water into the primary circuit of a nuclear reactor. The device 1 shown in Figures 3A and 3B comprises a first accumulator 10, configured to inject water into the primary circuit by gravity. The volume of the first accumulator 10 is preferably greater than 10 m³ 3 It can be between 10m 3 and 20 m 3 .

[0038] The accumulator 10 extends between an upper end 11 and a lower end 12. It is intended to store water 2. A water injection line 32 extends from the lower end 12 to the primary circuit or to the reactor vessel.

[0039] A pipe 30, forming a pressure equalization line, connects the upper end 11 to the primary circuit or the reactor vessel via a valve 31. During normal reactor operation, the pipe 30 is closed by a valve 33. Air or an inert gas 3 is introduced at the upper end 11, above the water 2, at ambient pressure. In the event of an accident (see Figure 3B), the pipe 30 is opened, allowing the injection of steam 31 at the upper end 11. This equalizes the pressure applied to the water in the accumulator 10 and in the primary circuit. The water 2 from the accumulator 10 can thus flow by gravity into the primary circuit, following the opening of a valve 34, through a safety injection line 32, without the need for pumping, which contributes to the safety of the cooling of the reactor core.

[0040] Device 1 includes a second accumulator 20, connected to the first accumulator 10. The second accumulator is of the swollen accumulator type, as described in relation to Figure 2A, containing water 2 maintained under pressure by an inert gas 5, for example, nitrogen. The inert gas pressure is, for example, 20 bar. The volume of the second accumulator 20 is preferably equivalent to the volume of the first accumulator 10. The second accumulator 20 is not intended to inject water directly into the primary circuit, as described in the prior art, but to fill the first accumulator 10. Thus, the second accumulator 20 allows for indirect injection of water into the primary circuit, via the first accumulator 10.

[0041] A distinctive feature of the device is that it includes a connecting pipe 23, linking the second accumulator 20 to the first accumulator 10. Thus, the first accumulator 10 and the second accumulator 20 are arranged in series, connected to each other by the connecting pipe 23. The connecting pipe 23 is designed to allow the first accumulator 10 to be filled from the second accumulator 20 when the water level in the first accumulator 10 falls below a certain low threshold. A valve 24 is located on the connecting pipe. The valve 24 is configured to be open or closed depending on the water level in the first accumulator 10. During normal reactor operation, the valve 24 is closed. When an event occurs, involving the injection of water into the primary circuit, the valve 24 is configured to be opened after the water level in the first accumulator has reached a low threshold.

[0042] Preferably, valve 24 is a passive valve, requiring no electrical power supply. To this end, valve 24 is configured to move from the closed position, blocking the connecting pipe 23, to the open position, under the combined effects of the steam inlet 31 through the pressure balancing pipe 30 and the lower threshold being reached in the first accumulator 10. It is advantageous for the valve to open independently of any electrical supply. This allows device 1 to be considered a Category C device according to the classification established by the IAEA (International Atomic Energy Agency) IAEA-TECDOC-1624 "Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plants".

[0043] In the example shown in Figures 3A and 3B, valve 24 is connected to a reservoir 14 containing an auxiliary fluid 4. The auxiliary fluid is different from the refrigerant 2. In particular, the auxiliary fluid 4 has a lower evaporation temperature than the refrigerant 2, which is water. The auxiliary fluid can be a refrigerant with a low evaporation temperature, below 100°C, and preferably below 80°C or 70°C. The auxiliary fluid can be a fluid used in cooling devices, for example, NOVEC 649 (Manufacturer 3M, registered trademark) or 3M Novec HFE-7100.

[0044] The reservoir 14 is traversed by a branch 13 from the first accumulator 10. The branch extends, along the first accumulator 10, between an upper junction 13 s and a lower junction 13j. The upper junction 13 sopens, in the first accumulator 10, between the lower junction 13j and the upper end 11. The lower junction 13j opens, in the first accumulator, between the upper junction 13 s and the lower end 12. The lower junction 13i opens below the lower threshold of the accumulator 10, the crossing of which triggers the opening of the valve 24. The upper junction 13 sopens above the lower threshold. Thus, the lower threshold extends on both sides of the upper and lower junctions. The branch 13 extends through the reservoir 14, being sealed against it. The auxiliary fluid 4 in the reservoir is in thermal contact with the branch 13, without being able to enter the branch 13. The portion of the branch 13 extending into the reservoir 14 forms a heat exchanger 15, conducive to heat exchange between the fluid circulating inside the branch 13 and the auxiliary fluid 4 extending into the reservoir 14, around the heat exchanger 15. Preferably, the heat exchanger 15 forms corrugations, so as to increase the heat exchange with the auxiliary fluid 4.

[0045] The auxiliary liquid preferably has a lower saturation temperature compared to that of the water in the primary circuit, so that the pressure of the auxiliary liquid transformed into steam, at the heating temperature produced by the exchanger 15, allows the opening of the valve 24.

[0046] Figure 3B shows the system after a primary coolant loss accident. The pressure equalization line 30 is open, allowing steam 31 from the primary circuit or reactor vessel to enter at the upper end 11 of the first accumulator 10. This results in pressure equilibrium between the primary circuit and the water 2 inside the first accumulator 10. Consequently, the water 2 from the first accumulator 10 can flow by gravity into the primary circuit without overpressure, thus restoring the primary circuit's water inventory. The water 2 flowing from the first accumulator 10 is in liquid phase. Its temperature can be around 50°C.

[0047] As the water level in the first accumulator 10 drops, the steam 31 progresses through the bypass 13 until it reaches the heat exchanger 15. The temperature of the steam 31 is, for example, between 100°C and 120°C. As the water level drops in the first accumulator 10, the water level also drops in the heat exchanger 15. This causes the steam 31 to progress through the heat exchanger 15. The temperature of the heat exchanger 15 increases, leading to vaporization of the auxiliary liquid 4 and a gradual increase in pressure within the reservoir 14, around the heat exchanger 15.

[0048] As the water level in the first accumulator 10 drops, the heat exchange surface exposed to steam in the heat exchanger 15 increases, and the pressure in the reservoir 14 rises. Once a pressure threshold, known as the trigger threshold, is reached, the valve 24 opens, injecting pressurized water from the second accumulator 20 into the first accumulator 10. This fills the first accumulator 10, thus extending the gravity flow of water in the primary circuit. The valve 24 can be a pneumatic valve with a piston, or another moving part, which moves in response to the increasing pressure in the reservoir 14. The pressure increase in the reservoir 14 causes the piston to move, triggering the opening of the valve 24.

[0049] The reservoir 14 is advantageously situated at an elevation extending on either side of the lower threshold of the first accumulator 10. The volume of the reservoir 14 and the configuration of the exchanger 15 are determined so as to trigger the opening of the valve 24 when the water crosses the lower threshold during the gravity flow of the water.

[0050] The use of a passive valve, opened under the effect of vaporization of the auxiliary liquid 4, is particularly interesting because it allows the first accumulator to be filled at a low vapor temperature, between approximately 100°C and 150°C.

[0051] It is not necessary for the valve to be reversible. On the contrary, it is preferable that after the valve has been opened, it remains open, by means of a mechanical locking device, so as to facilitate the flow of water contained in the secondary accumulator 20 rather than stopping the supply in the event of a rise in the level in the gravity accumulator.

[0052] Device 1 allows the use time of the first gravity-fed accumulator 10 to be extended by filling it from the second accumulator 20. This allows the water supply to the primary circuit to be extended by controlling the filling rate, since the water flow in the primary circuit is done by gravity.

Claims

9 DEMANDS 1. Device (1) for injecting a coolant (2) into a primary circuit (PC) of a nuclear reactor, extending around a reactor vessel (V), the device comprising a first coolant accumulator (10) configured to inject, by gravity, the coolant into the primary circuit, the device comprising - a second accumulator (20), connected to the first accumulator (10), the second accumulator containing the refrigerant liquid kept under pressure by a gas (5); - a connecting pipe (23), extending between the first accumulator and the second accumulator; - a valve (24), extending along the connecting pipe, so that under the effect of an opening of the valve, the refrigerant flows, through the connecting pipe (23), from the second accumulator to the first accumulator, so as to fill the first accumulator; the device being characterized in that it comprises a pressure balancing line (30), extending between the first accumulator (10) and the primary circuit or the reactor vessel, the pressure balancing line being configured to bring the steam from the primary circuit or the vessel to the level of an upper end (11) of the first accumulator (10); the device being such that the valve (24) is configured to open under the effect of the steam (31) flowing from the pressure balancing line.

2. Device according to claim 1, wherein the valve (24) is configured to be open when the refrigerant liquid, in the first accumulator, reaches a low point during gravity injection.

3. Device according to any one of the preceding claims, wherein the valve (24) is pneumatic, the valve being configured to move from a closed position to an open position under the effect of a displacement of a moving part.

4. Device according to claim 3, wherein the device comprises: - a reservoir (14), connected to the valve (24), containing an auxiliary fluid (4'), the auxiliary fluid having a vaporization temperature, at atmospheric pressure, of less than 100 °C; - a bypass, extending along the first accumulator, and connected to the latter between an upper junction (13 s ) and a lower junction ( 13j), the shunt being such that • the lower and upper junctions open on either side of the lower threshold of the first accumulator; "the bypass extends, in a sealed manner, through the reservoir (14), the portion of the bypass extending through the reservoir forming an exchanger (15), conducive to heat exchange between the bypass and the reservoir; - so that under the effect of the steam in the exchanger, the auxiliary fluid vaporizes, inducing an increase in pressure in the tank, causing the moving part to move.

5. Device according to claim 4, wherein in the exchanger, the bypass forms undulations in the reservoir so as to maximize heat exchange.

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