nuclear power plant

Abstract

A nuclear power plant has a nuclear reactor including a reactor pressure vessel containing a plurality of fuel rods containing fissile material. The nuclear power plant also has means for submerging the reactor pressure vessel in water to thereby water cool the reactor pressure vessel in the event of an emergency requiring cooling of the nuclear reactor. The nuclear power plant also has a primary core catcher located outside the reactor pressure vessel, the primary core catcher being made of a material adapted to retain molten core material in the event that the core material escapes the reactor pressure vessel. The nuclear power plant also has a secondary core catcher outside the primary core catcher, the secondary core catcher being lined with a tank that is filled with water in normal use of the nuclear power plant to submerge the primary core catcher and thereby water cool the primary core catcher. The secondary core catcher is also made of a material adapted to retain molten material in the event that the molten material escapes the first core catcher.

Description

Technical Field

[0001] This disclosure relates to a nuclear power plant. Background Technology

[0002] Nuclear power plants convert the heat energy generated by the nuclear decay of fissile material in fuel assemblies into electrical energy. A pressurized water reactor (PWR) nuclear power plant has a primary coolant loop, which typically connects the following pressurized components: a reactor pressure vessel (RPV) containing the fuel assemblies; one or more steam generators; and one or more pressurizers. Coolant pumps in the primary loop circulate pressurized water through a piping system between these components. The RPV houses the nuclear reactor that heats the water in the primary loop. The steam generator acts as a heat exchanger between the primary and secondary loops, generating pressurized steam in the secondary loop to drive turbines. A pressurizer typically maintains the pressure in the primary loop at around 155 bar.

[0003] After passing through the turbine, the pressurized steam in the secondary loop is cooled and condensed in one or more condensers before returning to the steam generator. The condensers transfer heat from the condensed steam to the tertiary loop, which circulates water between the tertiary radiator (i.e., ocean, lake, or river) and the condenser, which is the final destination for waste heat from the nuclear power plant.

[0004] Nuclear power plant safety systems are designed to prevent a range of failures. The successful implementation of these safety measures ensures that nuclear power plant conditions remain within safe limits. Failure of these safety measures can lead to core damage, known as a "serious accident." Serious accident safety systems can be included in the design of nuclear power plants to protect people and the environment from the harmful effects of ionizing radiation by confining radioactive materials within the plant's containment structure.

[0005] Specifically, engineering structures can be included in nuclear power plants to confine the molten core, water can be supplied to cool these structures and maintain their structural integrity, and separate radiators can be provided to remove heat from the containment system. Summary of the Invention

[0006] In summary, this disclosure provides a nuclear power plant with enhanced safety through having multiple molten core emergency safety shells.

[0007] In a first aspect, this disclosure provides a nuclear power plant having:

[0008] A nuclear reactor, including a reactor pressure vessel that houses multiple fuel rods containing fissile material;

[0009] A device for cooling the exterior of a water-cooled reactor pressure vessel in emergency situations where cooling of the nuclear reactor is required;

[0010] A primary core trap outside the reactor pressure vessel, the primary core trap being formed of a material suitable for retaining the molten core material in the event of core molten material escaping from the reactor pressure vessel; and

[0011] A secondary core catcher is located outside the primary core catcher. The secondary core catcher is lined in a tank that is filled with water during normal operation of the nuclear power plant to submerge and thereby water-cool the primary core catcher. The secondary core catcher is formed of a material suitable for retaining the molten core material in the event that the molten core material escapes from the primary core catcher.

[0012] For the molten core material escaping from the reactor core melt at a nuclear power plant, at least three containment layers must have failed: the external water-cooled reactor pressure vessel, the water-cooled primary core catcher, and the secondary core catcher. Therefore, the probability of a complete containment failure is significantly reduced. This reduced probability is further enhanced by the substantial independence of the three containment layers. Furthermore, as the melt passes through the innermost to the outermost containment layers, its temperature and volume increase. The temperature increases due to decay heat, and the volume increases because the molten core first mixes with the molten material in the reactor pressure vessel and then with the molten material in the primary core catcher. This temperature increase leads to a greater temperature difference with the surrounding environment and the molten core material outside the containment, ultimately resulting in a greater heat flux and increasing the likelihood of solidification. The delay caused by the need to pass through these containment layers also reduces the decay heat level. Additionally, the increased volume reduces the bulk density of the decay heat, again increasing the likelihood of solidification.

[0013] In a second aspect, this disclosure provides a method for operating a nuclear power plant according to the first aspect, the method comprising:

[0014] In a normally operating nuclear power plant, the outer surface of the reactor pressure vessel is surrounded by air during normal operation; and

[0015] In emergency situations requiring cooling of the nuclear reactor, or during safety tests of a nuclear power plant, the exterior of the reactor pressure vessel is water-cooled.

[0016] The optional features of a nuclear power plant will now be described. These can be applied individually or in combination with any aspect of this disclosure.

[0017] The apparatus for a water-cooled reactor pressure vessel may include means for immersing the reactor pressure vessel in water.

[0018] A device for immersing a reactor pressure vessel in water may include a water-retaining jacket located outside the reactor pressure vessel, spaced apart from the reactor pressure vessel, such that in an emergency, the cavity between the jacket and the reactor pressure vessel can be filled with water to immerse the reactor pressure vessel, thereby water-cooling the reactor pressure vessel. Such a water-retaining jacket may be a primary core trap, but more preferably, the water-retaining jacket is a separate component, with the primary core trap located both outside the reactor pressure vessel and outside the water-retaining jacket, for example, with an air gap between the jacket and the primary core trap.

[0019] Conveniently, the aforementioned water-retaining jacket can act as a heat shield during normal operation of the nuclear reactor to retain heat in the reactor. The cavity between the jacket and the reactor pressure vessel is a gas cavity during such normal operation.

[0020] The apparatus for immersing a reactor pressure vessel may also include a supply system for supplying water to immerse the reactor pressure vessel in an emergency, such as for supplying water to the cavity between the water jacket and the reactor. For example, the supply system may include one or more tanks that can be gravity-fed into the cavity. Such gravity supply reduces reliance on pumps and other power sources.

[0021] In an alternative embodiment, means for the exterior of a water-cooled reactor pressure vessel may include spraying water onto the exterior or immersing the exterior in water.

[0022] Nuclear power plants may also have one or more heat exchangers arranged to condense steam formed by the boiling of water submerged in the reactor pressure vessel. For example, a nuclear power plant may be arranged, for instance, by shaping the containment structure so that the condensed steam returns to the cavity between the jacket and the reactor pressure vessel. The cold side of the heat exchanger may be one or more localized radiators, such as additional water tanks. One or more heat exchangers may also be arranged to condense steam formed by the boiling of water in the filling tank.

[0023] Primary core catchers are typically metal core catchers, such as steel core catchers. However, primary core catchers can also be ceramic core catchers.

[0024] Secondary core catchers are typically ceramic core catchers. As previously mentioned, if the molten core reaches the secondary core catcher, it will melt through the reactor pressure vessel and the primary core catcher, both of which typically have melting points of around 1500°C (e.g., if formed of steel). Therefore, by forming a secondary core catcher of ceramic material, its melting point can be increased to, for example, greater than 2000°C, which allows the molten core to solidify on it without causing cracking. The secondary core catcher can be externally air-cooled. In some embodiments, the secondary core catcher may include a metallic core catcher. The secondary core catcher may take the form of a tank liner.

[0025] This invention may include or comprise as part of a nuclear reactor power plant (referred to herein as a nuclear reactor). In particular, this invention may relate to a pressurized water reactor. A nuclear reactor power plant may have a power output between 250 megawatts (MW) and 600 megawatts (MW), or between 300 MW and 550 MW.

[0026] A nuclear reactor power plant can be a modular reactor. A modular reactor can be considered as a reactor composed of multiple modules that are manufactured off-site (e.g., in a factory) and then assembled on-site into a nuclear reactor power plant by connecting these modules together. Any of the primary, secondary, and / or tertiary loops can be formed using modular construction.

[0027] The nuclear reactor disclosed herein may include a primary loop, which includes a reactor pressure vessel; one or more steam generators; and one or more pressurizers. The primary loop circulates a medium (e.g., water) through the reactor pressure vessel to extract heat generated by nuclear fission in the core, then delivers the heat to the steam generators and transfers it to the secondary loop. The primary loop may include one to six steam generators; or between two and four steam generators; or may include three steam generators; or a range of any of the foregoing values. The primary loop may include one, two, or more pressurizers. The primary loop may include a loop extending from the reactor pressure vessel to each of the steam generators, the loop carrying a hot medium from the reactor pressure vessel to the steam generator and a cooling medium from the steam generator back to the reactor pressure vessel. The medium may be circulated by one or more pumps. In some embodiments, each steam generator in the primary loop may include one or two pumps.

[0028] In some embodiments, the medium circulating in the primary loop may include water. In some embodiments, the medium may include a neutron-absorbing material (e.g., boron, gadolinium) added to the medium. In some embodiments, the pressure in the primary loop during full-power operation may be at least 50 bar, 80 bar, 100 bar, or 150 bar, and the pressure may reach 80 bar, 100 bar, 150 bar, or 180 bar during full-power operation. In some embodiments, when water is the medium in the primary loop, the temperature of the heated water leaving the reactor pressure vessel during full-power operation may be between 540 and 670 Kelvin (K), or between 560 and 650 Kelvin, or between 580 and 630 Kelvin. In some embodiments, when water is the medium in the primary loop, the temperature of the cooled water returning to the reactor pressure vessel during full-power operation may be between 510 and 600 Kelvin, or between 530 and 580 Kelvin.

[0029] The nuclear reactor disclosed herein may include a secondary loop, which includes a water circulation loop that extracts heat from the primary loop in a steam generator to convert water into steam to drive a turbine. In embodiments, the secondary loop may include one or two high-pressure turbines and one or two low-pressure turbines.

[0030] The secondary loop may include a heat exchanger to condense the steam into water as it returns to the steam generator. The heat exchanger may be connected to the tertiary loop, which may include a large volume of water to act as a radiator.

[0031] The reactor vessel may include a steel pressure vessel, which may be 5 to 15 meters high, or 9.5 to 11.5 meters high, and have a diameter of 2 to 7 meters, or 3 to 6 meters, or 4 to 5 meters. The pressure vessel may include a reactor body and a reactor top cover located vertically above the reactor body. The reactor top cover is connected to the reactor body by a series of studs passing through flanges on the reactor top cover and corresponding flanges on the reactor body.

[0032] The reactor top cover may include an integrated top cover assembly, in which multiple elements of the reactor structure can be combined into a single element. The combined element includes a pressure vessel top cover, a cooling shroud, a control rod drive mechanism, a missile shield, lifting equipment, a crane assembly, and a cable tray assembly.

[0033] A reactor core can consist of numerous fuel assemblies, each containing fuel rods. Fuel rods can be formed from pellets of fissile material. Fuel assemblies can also include spaces for control rods. For example, a fuel assembly can provide a shell for 17×17 grid rods, totaling 289 spaces. Of these 289 spaces, 24 can be reserved for the reactor's control rods, each control rod potentially formed by 24 control bars attached to the main arm, and one space can be reserved for instrumentation tubes. Control rods can be moved into and out of the core to control the fission process experienced by the fuel by absorbing neutrons released during nuclear fission. A reactor core can include 100 to 300 fuel assemblies. Complete insertion of control rods can typically result in a subcritical state leading to reactor shutdown. Up to 100% of the fuel assemblies in a reactor core can contain control rods.

[0034] The control rods can be moved via a control rod drive mechanism. This mechanism commands and powers the actuators to lower and raise the control rods into and out of the fuel assemblies, and to maintain their position relative to the reactor core. The rods of the control rod drive mechanism can be rapidly inserted into the control rods to enable rapid reactor shutdown (i.e., emergency shutdown).

[0035] The primary loop can be housed within the containment structure to maintain steam from the primary loop in the event of an accident. The containment diameter can be between 15 and 60 meters, or between 30 and 50 meters. The containment structure can be formed of steel or concrete, or steel-lined concrete. The containment can house one or more lifting devices (e.g., a ring crane). The lifting device can be mounted on top of the containment above the reactor pressure vessel. The containment can be contained within or supported outside a water tank used for emergency cooling of the reactor. The containment can contain equipment and facilities that allow for reactor refueling, fuel assembly storage, and transport of fuel assemblies inside and outside the containment.

[0036] A nuclear power plant may include one or more civil structures to protect reactor components from external hazards (such as missile attacks) and natural hazards (such as tsunamis). The civil structures may be made of steel, concrete, or a combination of both. Attached Figure Description

[0037] The embodiments will now be described by way of example only with reference to the following figures, wherein:

[0038] Figure 1 This is a schematic diagram of the various parts of the PWR nuclear power plant; and

[0039] Figure 2 This is a schematic diagram of the emergency safety shell of a nuclear power plant's molten core. Detailed Implementation

[0040] The RPV 12, containing the fuel assembly, is located in the center of the nuclear power plant 10. Surrounding the RPV are three steam generators 14, which are connected to the RPV via pressurized water and primary coolant loop piping 16. Coolant pumps 18 circulate pressurized water around the primary coolant loop, delivering hot water from the RPV to the steam generators and cooling water from the steam generators to the RPV.

[0041] The voltage regulator 20 maintains the water pressure in the primary coolant circuit at approximately 155 bar.

[0042] In steam generator 14, a heat exchanger transfers heat from pressurized water to feed water circulating in pipe 22 of the secondary coolant circuit, thereby generating steam to drive a turbine, which in turn drives a generator. The steam then condenses in one or more condensers (not shown) before returning to the steam generator. The condensers transfer heat from the condensed steam to a tertiary coolant circuit (not shown), which circulates water between a tertiary radiator (i.e., an ocean, lake, or river) and the condenser.

[0043] In addition to the primary, secondary, and tertiary circuits, nuclear power plant 10 also has an emergency safety shell for the molten core, such as... Figure 2 As shown. In particular, nuclear power plants implement a three-layer melt retention strategy: 1) IVR (In-Vessel Retention), 2) EVCC (External-Vessel Core Cooling), and 3) ACCC (Air-Cooled Ceramic Core Catch). This provides three opportunities to retain the melt instead of one. Moving the list down from 1) to 3) reduces the equipment required for the appropriate performance of each layer, thus reducing system dependency constraints and the conditional probability of failure. The layer order also advantageously reduces the diffusion area of ​​the core melt. As the list moves down from 1) to 3), the melt temperature and melt volume increase, thus increasing the temperature difference with the surrounding environment and outside the vessel, prompting the melt to generate a greater heat flux, thereby increasing the likelihood of solidification. The delay caused by having to pass through these layers also reduces the decay heat level, and the increased mass reduces the decay heat volume density, again increasing the likelihood of solidification.

[0044] These three layers are also different, which helps to improve their reliability and robustness.

[0045] More specifically, the IVR layer contains a cavity submerged between the RPV 12 and the heat shield 24 to solidify the core melt within the lower top cover of the RPV. During normal operation, this cavity is filled with air, and the heat shield is used to retain heat within the reactor. However, in emergency situations requiring reactor cooling, the heat shield transforms into a water-holding jacket, allowing the RPV to be submerged in cooling water. This water is supplied by a system such as a storage tank 26 located above the reactor pressure vessel, allowing water to be gravity-fed into the cavity.

[0046] Water entering the chamber vaporizes into steam upon contact with RPV 12, but the supply system can be configured to continuously replenish this lost water and maintain the water level in the chamber at a given level. Any excess water entering the chamber can be directed to the filling tank 36. The power plant may also have one or more heat exchangers 28 arranged to condense the steam. Conveniently, these heat exchangers 28 can be mounted on the walls of the power plant's containment structure, and the condensed steam can then be directed back into the chamber or filling tank 36. The cold side of the heat exchangers is a suitable radiator, such as one or more additional cold water tanks 30.

[0047] The EVCC layer is provided by a metallic (typically steel) primary core catcher 32, located outside the heat shield 24 and typically separated from it by an air gap. This core catcher can be immersed in water 34 in a permanently filled additional tank 36 (discussed below), making its outer surface water-cooled and enhancing its ability to absorb heat from the IVR layer, thereby safely retaining any molten core escaping from the IVR layer. The EVCC layer has no moving mechanical parts, and the thickness of the primary core catcher is sufficient to withstand the mass transfer of molten core from the RPV.

[0048] Nevertheless, given the decreasing likelihood of failure of the primary core catcher 32 (e.g., through jet ablation), the ACCC layer includes a water-filled tank 36, internally lined with a ceramic secondary core catcher 38, and mounted on a robust containment base 40. Typically, the ceramic liner and the tank wall behind the containment base are made of concrete. The outer surface of the tank is air-cooled.

[0049] The ACCC layer has no moving parts and no water replenishment requirement. The ceramic secondary core trap 38 is configured such that it does not melt upon contact with the molten core material, or melts slowly enough to allow the molten core material to refreeze before melting.

[0050] It should be understood that the present invention is not limited to the embodiments described above, and various modifications and improvements can be made without departing from the concept described herein. Except in cases of mutual exclusion, any of the features may be used alone or in combination with any other feature, and this disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A nuclear power plant (10) having: Nuclear reactors, including: A reactor pressure vessel (12) that contains multiple fuel rods containing fissile material; A means for water cooling the exterior of the reactor pressure vessel (12) in an emergency where cooling of the nuclear reactor is required, the means for water cooling the exterior of the reactor pressure vessel includes means for immersing the reactor pressure vessel in water. A primary core trap (32) outside the reactor pressure vessel (12), the primary core trap being formed of a material suitable for retaining the molten core material in the event of core melt escaping from the reactor pressure vessel; and A secondary core catcher (38) is located outside the primary core catcher. The secondary core catcher is lined in a tank (36) which is filled with water during normal operation of the nuclear power plant to submerge and thereby water-cool the primary core catcher. The secondary core catcher is formed of a material suitable for retaining the molten core material in the event that the molten core material escapes from the primary core catcher (32). The device for immersing the reactor pressure vessel (12) in water includes a water-retaining jacket (24) located outside the reactor pressure vessel. The jacket is spaced apart from the reactor pressure vessel so that in an emergency, the cavity between the jacket and the reactor pressure vessel can be filled with water to immerse the reactor pressure vessel, thereby water-cooling the reactor pressure vessel. The water-retaining jacket (24) acts as a heat shield during normal operation of the nuclear reactor to retain heat in the reactor. The cavity between the jacket and the reactor pressure vessel (12) is a gas cavity during such normal operation.

2. The nuclear power plant according to claim 1, wherein the means for water cooling the exterior of the reactor pressure vessel (12) includes a supply system for supplying water to submerge the reactor pressure vessel in an emergency.

3. The nuclear power plant according to claim 1, further comprising one or more heat exchangers (28) arranged to condense steam formed by the boiling of water immersing the reactor pressure vessel.

4. The nuclear power plant according to claim 1, wherein the primary core catcher (32) is a metal core catcher.

5. The nuclear power plant according to claim 1, wherein the secondary core catcher (38) is a ceramic core catcher.

6. The nuclear power plant according to claim 1, wherein the secondary core trap is externally air cooled.

7. A method of operating a nuclear power plant (10) according to any one of claims 1 to 6, the method comprising: During normal operation of the nuclear power plant, the outer surface of the reactor pressure vessel (12) is surrounded by air. as well as In emergency situations requiring cooling of the nuclear reactor or during safety tests of the nuclear power plant, the exterior of the reactor pressure vessel is water-cooled.

8. The method of claim 7, wherein water cooling of the exterior of the reactor pressure vessel comprises immersing the reactor pressure vessel in water.

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