Patent Number: 050531905
Section: summary

The present invention primarily 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 PWR's with separate pressurisers. The present invention is particularly suitable for use with light water, the invention is also applicable for use with heavy water moderated water cooled reactor types. A problem associated with integral pressurised water reactors (PWR's) of the saturated self pressurised type is that the reactor cores have a certain amount of boiling in the moderator/coolant to make up for heat losses from the pressuriser and due to non-uniformity in the distribution of cooling across the reactor core. Perturbations in the boiling voidage can cause unwanted disturbances in power level and flow distribution in the reactor core. Also, the transient and steady state pressure of the reactor coolant can be affected by variations in the patterns and levels of boiling voidage. In the prior art voidage has been controlled by means of an external pressuriser. In contrast to pressurised water reactors (PWR's), boiling water reactors (BWR's) are designed to operate with substantial amounts of boiling voidage in their reactor cores. But unlike PWR's in which an intermediate heat exchanger or steam generator is used to raise steam for an indirect turbo-generator Rankine cycle, in most BWR power plants the steam raised in the reactor core is ducted to the turbo-alternator in a direct Rankine cycle arrangement. A disadvantage of the direct cycle arrangement is that the working fluid passing through the turbine, condenser and feed system of the power plant is slightly radioactive. An alternative arrangement is to provide within the steam space of the boiling water reactor pressure vessel, an intermediate heat exchanger or steam generator, as in the integral PWR. However, in the indirect cycle BWR case steam vapour from the reactor core condenses on the primary circuit side of the intermediate heat exchanger steam generator and is returned directly to the reactor core without leaving the reactor pressure vessel. As with a dispersed PWR pressure control and transient coolant inventory control in an integral PWR could be effected by means of an external or integral pressuriser which communicates with the primary circuit through a surge pipeline. However a simple pressuriser/surgeline arrangement could not be employed with an indirect cycle BWR as it is intrinsically unstable. A slight excess of reactor core power over steam demand power would cause the pressuriser to flood. In the case of a PWR the simple pressuriser/surge line arrangement is meta-stable. Here the pressuriser is maintained at a higher temperature than in the reactor pressure vessel and reactor core and a large excess of reactor core power over steam demand is required to cause the pressuriser to flood and the reactor pressure vessel or primary circuit and reactor core to become blanketed in steam. A further problem with water cooled reactors is that under some accident conditions the supply of coolant to the reactor core can be suddenly impaired or lost, resulting in severe reactor core damage in a timescale shorter than can be prevented by engineered safety systems of the prior art. The present invention seeks to provide an integral pressuriser for integral PWR's for controlling the unwanted effects of variation of in core voidage in self pressurised integral PWR's under steady state and transient conditions. The present invention also seeks to provide an integral pressuriser for integral indirect cycle BWR's for controlling primary pressure, primary water level in the steam generator and the degree of boiling in the reactor core under steady state and transient conditions. The present invention also seeks to provide an integral pressuriser for integral PWR's and indirect cycle BWR's which is absolutely stable in normal, upset and accident conditions. The present invention also seeks to provide an external pressuriser for integral PWR's and indirect cycle BWR's which is absolutely stable under normal, upset and accident conditions. The present invention further seeks to provide a reserve supply of coolant immediately and continuously available, to the primary circuit and reactor core under the action of gravity, and a means for preventing steam blanketing of the primary circuit and reactor core during accident conditions. The present invention also seeks to provide a low cost water cooled nuclear reactor power plant in low and moderate power ratings. Accordingly the present invention provides a water cooled nuclear reactor and pressuriser assembly comprising a reactor core, a pressuriser, 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 enclosed by a pressure vessel, the pressuriser having a water space and a steam space, at least a portion of the water space of the pressuriser being positioned above an upper portion of the primary water coolant circuit, at least one 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 portion of the primary water coolant circuit positioned below any normal effective water level range in 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 whereby the at least one 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 to increase the stability of the assembly. The reactor core, the primary coolant circuit and the pressuriser may be arranged as an integral unit enclosed by the pressure vessel, at least one casing 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 coolant circuit being arranged in the second chamber, the pressuriser being arranged in the first chamber, the casing preventing interaction between the water in the primary water coolant circuit and the water in the water space of the pressuriser. The reactor core may be arranged in the lower region of the lower chamber, the primary 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 riser passage may be defined by a hollow cylindrical member, the downcomer passage being defined between the hollow cylindrical member and the pressure vessel. The at least one heat exchanger may be positioned in an upper region of the downcomer passage. The at least one surge port means may comprise a hydraulic diode. The casing may comprise an annular member which extends downwards from the peripheral region thereof, an annular passage being formed between the annular member of the casing and the pressure vessel for the flow of water from the water space of the pressuriser to the primary coolant circuit and from the primary coolant circuit to the steam space of the pressuriser. The pressuriser may form a surge tank positioned in the first chamber, the surge tank being defined by the pressure vessel and the casing. The casing may comprise an annular member which extends downwards from a peripheral region thereof, the annular member being secured to the pressure vessel to form an annular lower portion of the surge tank with the pressure vessel. The casing may comprise a bottom member positioned below the reactor core, the casing dividing the pressure vessel into a first outer chamber and a second inner chamber, the second inner chamber being substantially defined by the casing. A peripheral region of the casing may be secured to the pressure vessel, the casing may be arranged to divide the pressure vessel into a first vertically upper chamber and a second vertically lower chamber. The at least one surge port means may connect a lower portion of the water space of the surge tank with the primary water coolant circuit in the region of the reactor core. The at least one surge port means may connect the lower portion of the water space of the surge tank with the primary water coolant circuit below the reactor core. The riser passage may be defined by a hollow cylindrical member, the downcomer passage being defined between the hollow cylindrical member and the casing. The pressuriser may be a separate pressuriser. The at least one surge port means may connect a lower portion of the water space of the surge tank with a lower portion of the downcomer passage in the region of the heat exchanger. The at least one surge port means may connect a lower portion of the water space of the surge tank with a lower portion of the downcomer passage below the heat exchanger. The at least one surge port means may comprise a re-entrant nozzle. The at least one surge port means may comprise a hydraulic diode. The at least one means which communicates between the pressuriser and the primary coolant circuit may comprise at least one pipe which interconnects at least one port in the casing with the steam space in the pressuriser. The casing may comprise an annular member which extends downwards from a central region thereof, a peripheral region of the casing may be sealingly secured to the pressure vessel, the annular member may be sealed at its lower end to form a lower portion of the surge tank. At least one of the means which communicate between the pressuriser and the primary water coolant circuit may comprise a spray nozzle. At least one of the means which communicate between the pressuriser and the primary water coolant circuit may connect the steam space of the pressuriser with the primary water coolant circuit above the heat exchanger. The water cooled nuclear reactor may be an integral pressurised water reactor. The pressuriser may have heating means to heat the water in the water space. The water cooled nuclear reactor may be an integral indirect cycle boiling water reactor, the at least one means which communicates between the steam space of the pressuriser and the upper portion of the primary water coolant circuit controlling the effective water level in the primary water coolant circuit.