NUCLEAR POWER PLANT

Abstract

A nuclear power plant has a nuclear reactor including a reactor pressure vessel which houses plural fuel rods containing fissile material. The nuclear power plant further has means for submerging the reactor pressure vessel in water and thereby water-cooling the reactor pressure vessel in the event of an emergency requiring cooling of the nuclear reactor. The nuclear power plant further has a primary core catcher outwardly of the reactor pressure vessel, the primary core catcher being formed of a material suitable for retaining molten corium in the event corium escapes the reactor pressure vessel The nuclear power plant further has secondary core catcher outwardly of the primary core catcher, the secondary core catcher lining a tank which is water-filled in normal use of the plant to submerge and thereby water-cool the primary core catcher. The secondary core catcher is also is formed of a material suitable for retaining molten corium in the event corium escapes the primary core catcher.

Claims

1. A nuclear power plant having: a nuclear reactor including a reactor pressure vessel which houses plural fuel rods containing fissile material; means for water-cooling the exterior of the reactor pressure vessel in the event of an emergency requiring cooling of the nuclear reactor; a primary core catcher outwardly of the reactor pressure vessel, the primary core catcher being formed of a material suitable for retaining molten corium in the event corium escapes the reactor pressure vessel; and a secondary core catcher outwardly of the primary core catcher, the secondary core catcher lining a tank which is water-filled in normal use of the plant to submerge and thereby water-cool the primary core catcher, the secondary core catcher being formed of a material suitable for retaining molten corium in the event corium escapes the primary core catcher.

2. The nuclear power plant according to claim 1, wherein the means for water-cooling the exterior of the reactor pressure vessel comprises means for cooling the reactor pressure vessel by submerging the reactor pressure vessel in water.

3. The nuclear power plant according to claim 2, wherein the means for submerging the reactor pressure vessel in water comprises a water retention jacket outside the reactor pressure vessel, the jacket being spaced from the reactor pressure vessel such that a cavity between the jacket and the reactor pressure vessel is fillable with water to submerge and thereby water-cool the reactor pressure vessel in the event of the emergency.

4. The nuclear power plant according to claim 3, wherein the water retention jacket functions as a thermal insulation shield in normal operation of the nuclear reactor to retain heat in the reactor, the cavity between the jacket and the reactor pressure vessel being an air cavity in such normal operation.

5. The nuclear power plant according to claim 1, wherein the means for water-cooling the exterior of the reactor pressure vessel comprises a supply system for supplying the water for submerging the reactor pressure vessel in the event of the emergency.

6. The nuclear power plant according to claim 1, further having one or more heat exchangers arranged to condense steam formed by the boiling of the water submerging the reactor pressure vessel.

7. The nuclear power plant according to claim 1, wherein the primary core catcher is a metal core catcher.

8. The nuclear power plant according to claim 1, wherein the secondary core catcher is a ceramic core catcher.

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

10. A method of operating the nuclear power plant according to claim 1, the method including: normally operating the power plant, the outer surface of the reactor pressure vessel being surrounded by air during the normal operation; and water-cooling the exterior of the reactor pressure vessel in the event of an emergency requiring cooling of the nuclear reactor, or in the event of a safety test of the power plant.

11. The method according to claim 10, wherein water-cooling the exterior of the reactor pressure vessel comprises submerging the reactor pressure vessel in water.

Description

[0037] Embodiments will now be described by way of example only, with reference to the following drawings in which:

[0038] FIG. 1 is a schematic diagram of parts of a PWR nuclear power plant; and

[0039] FIG. 2 is a schematic diagram of molten core emergency containment levels of the plant.

[0040] An RPV 12 containing fuel assemblies is centrally located in the plant 10. Clustered around the RPV are three steam generators 14 connected to the RPV by pipework 16 of the pressurised water, primary coolant circuit. Coolant pumps 18 circulate pressurised water around the primary coolant circuit, taking heated water from the RPV to the steam generators, and cooled water from the steam generators to the RPV.

[0041] A pressurises 20 maintains the water pressure in the primary coolant circuit at about 155 bar.

[0042] In the steam generators 14, heat exchangers transfer heat from the pressurised water to feed water circulating in pipework 22 of a secondary coolant circuit, thereby producing steam which is used to drive turbines which in turn drive an electricity-generator. The steam is then condensed in one or more condensers (not shown) before returning to the steam generators. The condensers transfer heat from the condensed steam to a tertiary coolant circuit (not shown) which circulates water between a tertiary heatsink (i.e. the sea, a lake, or a river) and the condensers.

[0043] In addition to the primary, secondary and tertiary circuits, the power plant 10 has molten core emergency containment levels, shown schematically in FIG. 2. In particular, the plant implements a triple layer melt retention strategy, namely: 1) IVR (in vessel retention), 2) EVCC (ex-vessel corium cooling), 3) and ACCC (air cooled ceramic core catcher). This gives three opportunities for melt retention, rather than just one. Moving down the list from 1) to 3), the equipment demanded for proper performance of each layer reduces such that system dependency constraints are reduced, and the conditional probability of failure shrinks. The ordering of the levels also advantageously reduces the corium spread area. The melt temperature and melt volume increase on moving down the list from 1) to 3), and so the temperature difference with the surroundings and out-of-containment increases, promoting greater heat fluxes from the melt and thereby improving the likelihood of solidification. The delay produced in having to progress through the layers also reduces decay heat levels, and the increased mass reduces decay heat volumetric density, again improving the likelihood of solidification.

[0044] The three levels are also diverse, which helps to promote their reliability and robustness.

[0045] More particularly, the IVR level involves flooding of a cavity formed between the RPV 12 and a thermal insulation shield 24 to solidify core melt in the lower head of the RPV. In normal operation, this cavity is air-filled and the shield operates to retain heat in the reactor. However, in the event of an emergency requiring cooling of the nuclear reactor, the shield becomes a water retention jacket that allows the RPV to be submerged in cooling water. This water comes from a supply system, such as water storage tank 26 which can be located above the RPV so that its water may be gravity-fed into the cavity.

[0046] The water entering the cavity boils off as steam on contact with the RPV 12, but the supply system can be configured to constantly replenish this lost water and maintain the water in the cavity at a given level. Any excess water to the cavity may be channelled to the water filled tank 36. The plant may further have one or more heat exchangers 28 arranged to condense the steam. Conveniently these can be mounted on the wall of a containment structure of the plant, and the condensed steam can then be channelled back to the cavity or to the water filled tank 36. The cold side of the heat exchangers is a suitable heatsink, such as one or more further cold water tanks 30.

[0047] The EVCC level is provided by a metal (typically steel) primary core catcher 32 which is located outside the shield 24 and is typically spaced from the shield by an air gap. This core catcher may be submerged in the water 34 of a permanently filled further tank 36 (discussed below) so that its outer surface is water-cooled, enhancing its ability to extract heat from and thereby safely retain any corium which escapes the IVR level. The EVCC level has no moving mechanical parts and the primary core catcher is thick enough to withstand corium mass transfer from the RPV.

[0048] Nonetheless, in the increasingly unlikely event that the primary core catcher 32 fails (e.g. through jet ablation thereof), the ACCC level comprises the water-filled tank 36 which is internally lined by a ceramic secondary core catcher 38 and mounted on a substantial containment basement 40. Typically, the walls of the tank behind the ceramic liner and the containment basement are formed of concrete. The outer surface of the tank is air-cooled.

[0049] The ACCC level also has no moving parts and no water replenishment requirements. The ceramic secondary core catcher 38 is configured such that it does not melt on contact with molten corium, or melts slowly enough such that re-freeze of the corium is achieved before melt through.

[0050] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.