Patent Number: 051026163
Section: summary

The present invention relates to integral water cooled nuclear reactors with pressurisers, and is particularly applicable to water cooled nuclear reactors of the integral pressurised water reactor (PWR) type and the integral indirect cycle boiling water reactor (BWR) type with integral pressurisers. However the invention is also applicable to integral water cooled nuclear reactors with separate pressurisers and to dispersed or loop type pressurised water reactors (PWR's) with separate pressurisers. A problem with water cooled nuclear reactors is that under some severe accident conditions effective cooling of the nuclear reactor core can be lost very quickly. Emergency core cooling systems are provided in the prior art, but under some severe circumstances these are not sufficiently fast acting to restore cooling before some damage to the nuclear reactor core occurs. The design philosophy of these emergency core cooling systems is to restore cooling to the nuclear reactor core before the core damage results in an uncoolable nuclear reactor core geometry rather than to prevent damage to the nuclear reactor core under all circumstances. A further problem with water cooled nuclear reactors is the long term removal of residual heat from the nuclear reactor core in the event that heat removal by the normal means is lost. Residual heat removal systems are provided in the prior art for such emergencies. Emergency core cooling and residual heat removal systems of the prior art are controlled and operated by active components which may fail to function when required. Residual heat removal systems of the prior art also have active pumping components. Such active components require external electrical or other energy sources which may fail to operate during emergency conditions. To mitigate such possibilities the emergency core cooling and residual heat removal systems of the prior art and their support systems are replicated leading to complication and high cost. Such prior art emergency core cooling and residual heat removal systems of the prior art make it difficult to produce cost effective water cooled nuclear reactor power plants of low and moderate power rating. The present invention seeks to provide an emergency core cooling and residual heat removal system which maintains nuclear reactor core cooling at all times during severe accident conditions by passive safety systems, which are continuously available when the nuclear reactor is operating normally, to prevent nuclear reactor core damage. The present invention also seeks to provide a low cost water cooled nuclear reactor power plant in low and moderate power ratings by simplification of safety systems and by obviating the need for replication. According to the present invention a water cooled nuclear reactor comprises a pressure vessel, a reactor core, a primary water coolant circuit arranged to cool the reactor core, the reactor core and at least a portion of the primary water coolant circuit being located in the pressure vessel, a pressuriser having a water space and a steam space, at least one full pressure reactor core cooler means, a first pipe means to interconnect an upper portion of the primary water coolant circuit with each full pressure reactor core cooler means, a second pipe means to interconnect a lower portion of the primary water coolant circuit with each full pressure reactor core cooler means, each first pipe means having a first inverted U-bend, each first inverted U-bend of the first pipe means passes through the water space and steam space of the pressuriser to form a vapour lock within each first inverted U-bend, whereby each vapour lock in normal operation substantially prevents a natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure reactor core cooler means and the second pipe means to the primary water coolant circuit, each vapour lock sensing abnormal operation of any of the reactor core, the primary water coolant circuit, the pressuriser or loss of primary water coolant and thereby being displaced from the first inverted U-bend to allow a natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure reactor core cooler means and the second pipe means to allow relatively cool primary water coolant from the full pressure reactor core cooler means flow into or through the primary water coolant circuit. Preferably at least one of the full pressure reactor core cooler means may comprise a full pressure emergency core coolant tank having a reserve supply of primary water coolant, the first pipe means interconnects an upper portion of the primary water coolant circuit with an upper portion of the full pressure emergency core coolant tank, the second pipe means interconnects a lower portion of the primary water coolant circuit with a lower portion of the full pressure emergency core coolant tank, at least a portion of the full pressure emergency core coolant tank being positioned above the reactor core, the first pipe means having a first inverted U bend, the first inverted U-bend of the first pipe means passes through the water space and steam space of the pressuriser to form a vapour lock within the first inverted U-bend, whereby the vapour lock in normal operation substantially prevents a natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure emergency core coolant tank and the second pipe means to the primary water coolant circuit, the vapour lock sensing abnormal operation of the reactor core, the primary water coolant circuit, the pressuriser or loss of primary water coolant and thereby being displaced from the first inverted U-bend to allow a natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure emergency core coolant tank and the second pipe means to the primary water coolant circuit to allow relatively cool primary water coolant in the full pressure emergency core coolant tank to flow through the reactor core, or to allow primary water coolant vapour to be vented from the primary water coolant circuit through the first pipe means into the full pressure emergency core coolant tank to facilitate a gravity feed of primary water coolant from the full pressure emergency core coolant tank into the primary water coolant circuit. At least one of the full pressure reactor core cooler means may comprise a full pressure residual heat removal heat exchanger, the first pipe means interconnects an upper portion of the primary water coolant circuit with an upper portion of the full pressure residual heat removal heat exchanger, the second pipe means interconnects a lower portion of the primary water coolant circuit with a lower portion of the full pressure residual heat removal heat exchanger, at least a portion of the full pressure residual heat removal heat exchanger being positioned above the primary water coolant circuit, the first pipe means having a first inverted U-bend, the first inverted U-bend of the first pipe means passes through the water space and steam space of the pressuriser to form a vapour lock within the first inverted U-bend whereby the vapour lock in normal operation substantially prevents a natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure residual heat removal heat exchanger and the second pipe means to the primary water coolant circuit, the vapour lock upon abnormal operation of the reactor core, the primary water coolant circuit, the pressuriser or loss of primary water coolant is thereby displaced from the first inverted U-bend to allow a natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure residual heat removal heat exchanger and the second pipe means to the primary water coolant circuit to allow relatively cool primary water coolant to flow through the reactor core. The full pressure residual heat removal heat exchanger and the full pressure emergency core coolant tank may be integrated and fludily connected in flow series such that they share a common first pipe means, first inverted U-bend and second pipe means. The full pressure residual heat removal heat exchanger and the full pressure emergency core coolant tank may be separate and have their own respective first pipe means, first inverted U-bend and second pipe means. At least a portion of the full pressure emergency core coolant tank may be positioned above the primary water coolant circuit. The inverted U-bend in the first pipe means may have an electrical immersion heater to assist in the formation and maintenance of the vapour lock and to facilitate the removal of incondensible gases. Each first pipe means may have hydrostatic thermal seals which facilitate the circulation of warm water eddy currents within the first pipe means during normal operation of the reactor plant, but which prevent the warm water eddy currents from entering the full pressure reactor core cooler means in normal operation of the nuclear reactor and which allow the natural circulation of primary water coolant from the primary water coolant circuit through the first pipe means, the full pressure reactor core cooler means and the second pipe means if the vapour lock is displaced from the first inverted U-bend by abnormal operation of the reactor core, the primary water coolant circuit or the pressuriser. A second inverted U-bend in each first pipe means may form one hydrostatic thermal seal. A U-bend in the first pipe means may also form a hydrostatic thermal seal. Each second pipe means may have a hydrostatic thermal seal preventing thermal convection of warm water from the primary water coolant circuit to the full pressure reactor core cooler means during normal operation of the reactor. A U-bend in the second pipe means may form the hydrostatic thermal seal. At least one pair of inverted U-bend and normal U-bend connected in series in each second pipe means may form the hydrostatic thermal seal, the inverted U-bend is positioned in a relatively hot region and the normal U-bend is positioned in a relatively cool region to produce alternating stratified zones of lower and higher water density in the hydrostatic thermal seal. The pressuriser may have an auxiliary vessel, the auxiliary vessel having a water space and a steam space, at least the water space of the auxiliary vessel being interconnected with the water space of the pressuriser, the first inverted U-bend of the first pipe means passes through the water space and steam space of the auxiliary vessel. The pressuriser may have an auxiliary vessel, the auxiliary vessel having a water space and a steam space, at least the water space of the auxiliary vessel being interconnected with an upper portion of the primary water coolant circuit, the first inverted U-bend of the pipe means passes through the water space and steam space of the auxiliary vessel. The steam space of the auxiliary vessel may be interconnected with the steam space of the pressuriser. The auxiliary vessel may have an electrical immersion heater to maintain saturation conditions in the water space and steam space of the auxiliary vessel. The auxiliary vessel may define a portion of the first inverted U-bend and the steam space of the auxiliary vessel may form the vapour lock. A relatively small vent may interconnect the vapour lock and the steam space of the pressuriser to allow the flow of in-condensible gases from the vapour lock to the steam space of the pressuriser, to assist in the formation and maintenance of the vapour lock in normal operation and to provide the vapour lock with the required transient response. The full pressure emergency core cooling and residual heat removal system may have at least one residual heat removal means to remove heat from the primary water coolant in the full pressure emergency core cooling and residual heat removal system. Each full pressure emergency core coolant tank may be integrated with the full pressure residual heat removal heat exchanger having at least one residual heat removal circuit to remove heat from the primary water coolant in a combined full pressure emergency core cooling and residual heat removal system. The at least one combined full pressure emergency core cooling and residual heat removal tank may have an enclosed region, the first pipe means interconnects the primary water coolant circuit and the enclosed region, the enclosed region having one of the residual heat removal circuits to increase the heat transfer rate from the primary water coolant to the residual heat removal circuit. The residual heat removal circuit may comprise a first heat exchanger positioned in the full pressure emergency core coolant and residual heat removal tank, a second heat exchanger positioned outside of the full pressure emergency core coolant and residual heat removal tank, ducting interconnecting the first and second heat exchangers to convey working fluid therebetween. The reactor pressure vessel may be positioned in the full pressure emergency core coolant tank. The reactor pressure vessel and pressuriser may be positioned within the combined full pressure emergency core coolant and residual heat removal tank as an integral unit, the full pressure emergency coolant tank becoming the integrated pressure vessel and the reactor vessel becoming a thermal and flow control boundary between the primary circuit and the reserve volume of full pressure emergency coolant. A second low pressure emergency core cooling and residual heat removal system may comprise a tank having a further reserve supply of primary water coolant at low pressure, at least a portion of the low pressure emergency core cooling and residual heat removal tank being positioned above the full pressure emergency core cooling and residual heat removal system, a third pipe means to interconnect a lower portion of the second low pressure emergency core cooling and residual heat removal tank with the full pressure emergency core coolant and residual heat removal system or with the primary circuit, a fourth pipe means to interconnect the steam space of the pressuriser with the low pressure emergency core coolant tank, the third pipe means having a non return valve and a control valve, the fourth pipe means having a control valve. At least one second residual heat removal means may be arranged to remove heat from the water in the second low pressure emergency core cooling and residual heat removal tank. The water in low pressure emergency core cooling and residual heat removal tank may form a heat sink for the full pressure emergency core cooling and residual heat removal system. The full pressure residual heat removal cooler may be located in the low pressure emergency core cooling and residual heat removal tank. The full pressure emergency core cooling and residual heat removal system may be located in the low pressure emergency core cooling and residual heat removal tank. The pressure vessel may be located within a dry chamber defined by a cylindrical walled member, the cylindrical walled member being positioned in the low pressure emergency core cooling and residual heat removal tank, a vent interconnects an upper region of the dry chamber with a lower region of the emergency core cooling and residual heat removal tank. A containment building may contain, the reactor pressure vessel, the reactor core, the primary water coolant circuit, the pressuriser, the full pressure emergency core cooling and residual heat removal system and the second low pressure emergency core cooling and residual heat removal tank, a fifth pipe means may interconnect a pump means with the low pressure emergency core cooling and residual heat removal tank, the pump means being arranged to pump any split water coolant above a predetermined level in the containment building to the low pressure emergency core cooling and residual heat removal tank, the fifth pipe means may have a non return valve. The at least one second residual heat removal circuit may comprise a third heat exchanger positioned in the second, low pressure, emergency core cooling and residual heat removal tank, a fourth heat exchanger positioned outside of the containment building, ducting means interconnecting the third and fourth heat exchangers to convey working fluid therebetween. A fifth heat exchanger may be positioned substantially at the uppermost region of the containment building, ducting means interconnecting the fifth heat exchanger and the fourth heat exchanger to convey working fluid therebetween, a collecting vessel positioned below the fifth heat exchanger and above the second, low pressure emergency core cooling and residual heat removal tank for collecting vapour condensed by the fifth heat exchanger, pipe means to supply condensed vapour from the collecting vessel to the second, low pressure emergency core cooling and residual heat removal tank. Ducting means may interconnect an intermediate heat exchanger and the fourth heat exchanger to convey working fluid therebetween, the second heat exchanger exchanging heat to the intermediate heat exchanger, the intermediate heat exchanger and second heat exchanger being positioned inside the containment building. The emergency core cooling and residual heat removal tanks may contain a neutron absorbing agent, dissolved in the water. The neutron absorbing agent may be boron, in the form of boric acid. At least a portion of the water space of the pressuriser may be positioned above an upper portion of the primary water coolant circuit, at least one vent means which communicates between the pressuriser and the primary water coolant circuit to connect the steam space of the pressuriser with the upper portion of the primary water coolant circuit, at least one surge port means which communicates between the pressuriser and the primary water coolant circuit to connect the water space of the pressuriser with a lower portion of the primary water coolant circuit, the at least one surge port means being arranged to have relatively low flow resistance for water from the water space of the pressuriser to the primary water coolant circuit and relatively high flow resistance for water from the primary water coolant circuit to the water space of the pressuriser, the at least one vent means which communicates between the steam space of the pressuriser and the upper portion of the primary water coolant circuit allows excess vapour formed in the primary water coolant circuit to flow to the steam space of the pressuriser. The reactor core, the primary water coolant circuit and the pressuriser may be arranged as an integral unit enclosed by the pressure vessel, at least one casing being arranged in the pressure vessel to substantially divide the pressure vessel into a first chamber and a second chamber, the reactor core and the primary water coolant circuit being arranged in the second chamber, the pressuriser being arranged in the first chamber, the casing preventing mixing interaction between the water in the primary water coolant circuit and the water in the water space of the pressuriser. The first pipe means may interconnect the water space of the pressuriser with the full pressure reactor core cooler means. The reactor core may be arranged in the lower region of the pressure vessel, the primary water coolant circuit comprising a riser passage to convey relatively hot water and steam to at least one heat exchanger, and a downcomer passage to convey relatively cool water from the at least one heat exchanger to the reactor core. The at least one heat exchanger may be a steam generator. The heat exchanger may be positioned in the pressure vessel. The primary water coolant circuit may comprise at least one pump to assist the circulation of primary water coolant. The pressuriser may be a separate pressuriser. The water cooled nuclear reactor may be an integral pressurised water reactor. The water cooled nuclear reactor may be an integral indirect cycle boiling water reactor.