Patent Number: 050531905
Section: claims

1. A water cooled reactor and pressuriser assembly comprising a reactor core, a primary coolant circuit, a pressuriser, a pressure vessel, at least one vent conduit and at least one surge conduit, the reactor core and at least a portion of the primary water coolant circuit being enclosed by the pressure vessel, the primary water coolant being arranged to cool the reactor core, the primary water coolant circuit having an upper portion and a lower portion, the lower portion being positioned below any normal effective range of water levels of the primary water coolant circuit, the upper portion being positioned above the lower portion,  the pressuriser having a water spaced and a steam space, at least a portion of the water space of the pressuriser being positioned above the upper portion of the primary water coolant circuit.  the at least one surge conduit communicating between the pressuriser and the primary water coolant circuit to connect the water space of the pressuriser with the lower portion of the primary water coolant circuit, the at least one surge conduit being arranged to have a relatively low flow resistance for water from the water space of the pressuriser to the primary water coolant circuit and a relatively high flow resistance for water from the primary water coolant circuit to the water space of the pressuriser,  the at least one vent conduit communicating 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 whereby the at least one vent conduit 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 and at least a portion of the primary water coolant circuit being enclosed by the pressure vessel, the primary water coolant circuit being arranged to cool the reactor core, the primary water coolant circuit having an upper portion, and a lower portion, the lower portion being positioned below any normal effective range of water levels of the primary water coolant circuit, the upper portion being positioned above the lower portion,  the pressuriser having a water space and a steam space, at least a portion of the water space of the pressuriser being positioned above the upper portion of the primary water coolant circuit,  the at least one surge conduit communicating between the pressuriser and the primary water coolant circuit to connect the water space of the pressuriser with the lower portion of the primary water coolant circuit, the at least one surge conduit being arranged to have a relatively low flow resistance for water from the water space of the pressuriser to the primary water coolant circuit and a relatively high flow resistance for water from the primary water coolant circuit to the water space of the pressuriser,  the plurality of vent conduits communicating 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, each of the plurality of vent conduits extending into the primary water coolant circuit to a different elevation, the plurality of vent conduits allowing 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 plurality of vent conduits controlling the effective water level in the primary water coolant circuit. 2. A water cooled nuclear reactor as claimed in claim 1 in which the pressure vessel has a lower region, and an upper region, the reactor core is arranged in the lower region of the pressure vessel, the primary coolant circuit comprising a riser passage and a downcomer passage, at least one heat exchanger, the riser passage conveys relatively hot water and steam to the at least one heat exchanger, the downcomer passage conveys relatively cool water from the at least one heat exchanger to the reactor core. 3. A water cooled nuclear reactor as claimed in claim 2 in which the at least one heat exchanger is positioned in the upper region of the downcomer passage. 4. A water cooled nuclear reactor as claimed in claim 3 in which the at least one heat exchanger is a steam generator. 5. A water cooled nuclear reactor as claimed in claim 1 in which the primary water coolant circuit comprises at least one pump to assist the circulation of primary water coolant. 6. A water cooled nuclear reactor as claimed in claim 3 in which the reactor core, the primary coolant circuit and the pressuriser are 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 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. 7. A water cooled nuclear reactor as claimed in claim 6 in which the pressuriser forms a surge tank positioned in the first chamber, the surge tank being defined by the pressure vessel and the casing. 8. A water cooled nuclear reactor as claimed in claim 7 in which the casing comprise a peripheral region and an annular member, the annular member extends downwards from the peripheral region of the casing, 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. 9. A water cooled nuclear reactor as claimed in claim 7 in which the casing comprise a peripheral region and an annular member, the annular member extends downwards from the peripheral region of the casing, the annular member being secured to the pressure vessel to form an annular lower portion of the surge tank with the pressure vessel. 10. A water cooled nuclear reactor as claimed in claim 7 in which the casing comprises a peripheral region, a central region and an annular member, the annular member extends downwards from the central region of the casing, the peripheral region of the casing being secured to the pressure vessel, the annular member having a lower end, the annular member being sealed at its lower end to form a lower portion of the surge tank. 11. A water cooled nuclear reactor as claimed in claim 7 in which a peripheral region of the casing is secured to the pressure vessel. 12. A water cooled nuclear reactor as claimed in claim 6 in which the casing is arranged to divide the pressure vessel into a first vertically upper chamber and a second vertically lower chamber. 13. A water cooled nuclear reactor as claimed in claim 8 in which the casing comprises 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. 14. A water cooled nuclear reactor as claimed in claim 7, in which the water space of the surge tank has a lower portion, the at least one surge conduit connects the lower portion of the water space of the surge tank with a portion of the downcomer passage in the region of the heat exchanger. 15. A water cooled nuclear reactor as claimed in claim 7, in which the water space of the surge tank has a lower portion, the at least one surge conduit connects the lower portion of the water space of the surge tank with the primary water coolant circuit in the region of the reactor core. 16. A water cooled nuclear reactor as claimed in claim 15 in which the at least one surge conduit connects the lower portion of the water space of the surge tank with the primary water coolant circuit below the reactor core. 17. A water cooled nuclear reactor as claimed in claim 7, in which the water space of the surge tank has a lower portion, the at least one surge conduit connects the lower portion of the water space of the surge tank with a lower portion of the downcomer passage below the heat exchanger. 18. A water cooled nuclear reactor as claimed in claim 2 in which the riser passage is defined by a hollow cylindrical member, the downcomer passage being defined between the hollow cylindrical member and the at least a part of the pressure vessel. 19. A water cooled nuclear reactor as claimed in claim 13 in which the riser passage is defined by a hollow cylindrical member, the downcomer passage being defined between the hollow cylindrical member and the casing. 20. A water cooled nuclear reactor as claimed in claim 1 in which the at least one surge conduit comprises a re-entrant nozzle. 21. A water cooled nuclear reactor as claimed in claim 1 in which the at least one surge conduit comprises a hydraulic diode. 22. A water cooled nuclear reactor as claimed in claim 1 in which the at least one vent conduit which communicates between the pressuriser and the primary coolant circuit comprises at least one pipe which interconnects at least one port in the casing with the steam space in the pressuriser. 23. A water cooled nuclear reactor as claimed in claim 1 in which at least one of the vent conduits which communicate between the pressuriser and the primary water coolant circuit comprises a spray nozzle. 24. A water cooled nuclear reactor as claimed in claim 2 in which at least one of the means which communicates between the pressuriser and the primary water coolant circuit connects the steam space of the pressuriser with the primary water coolant circuit above the heat exchanger. 25. A water cooled nuclear reactor as claimed in claim 1 in which the water cooled nuclear reactor is an integral pressurised water reactor. 26. A water cooled nuclear reactor as claimed in claim 25 in which the pressuriser has heating means to heat the water in the water space. 27. A water cooled nuclear reactor as claimed in claim 1 in which the water cooled nuclear reactor is an integral indirect cycle boiling water reactor, the at least one vent conduit which communicates between the steam space of the pressuriser and the upper portion of the primary water coolant circuit controls the effective water level in the primary water coolant circuit. 28. A water cooled nuclear reactor as claimed in claim 1 in which the pressuriser is a separate pressuriser. 29. A water cooled nuclear reactor as claimed in claim 28 in which the water cooled nuclear reactor is an integral indirect cycle boiling water reactor, the at least one vent conduit which communicates between the steam space of the pressuriser and the upper portion of the primary water coolant circuit controls the effective water level in the primary water coolant circuit. 30. A water cooled nuclear reactor as claimed in claim 28 in which the water cooled nuclear reactor is an integral pressurised water reactor. 31. A water cooled nuclear reactor as claimed in claim 2 in which the at least one vent conduit extends into the pressuriser steam space by at least a distance equal to the head loss due to flow in the downcomer passage between the normal effective range of water levels of the primary water coolant circuit and the elevation at which the at least one surge conduit communicates with the lower portion of the primary water coolant circuit. 32. A water cooled nuclear reactor as claimed in claim 31 in which the at least one surge conduit communicates with the lower portion of the primary water coolant circuit at the highest practical evaluation below the normal effective range of water levels of the primary water coolant circuit to minimize the distance that the at least one vent conduit extends into the pressuriser steam space. 33. A water cooled nuclear reactor as claimed in claim 27 in which a plurality of vent conduits communicate between the steam space of the pressuriser and the upper portion of the primary water coolant circuit, the vent conduits extending into the primary water coolant circuit to different elevations. 34. A water cooled nuclear reactor as claimed in claim 29 in which a plurality of vent conduits communicate between the steam space of the pressuriser and the upper portion of the primary water coolant circuit, the vent conduits extending into the primary water coolant circuit to different elevations. 35. A water coolant integral indirect cycle boiling reactor and pressuriser assembly comprising a reactor core, a primary coolant circuit, a pressuriser, a pressure vessel, a plurality of vent conduits and at least one surge conduit,