Patent Number: 
Section: claims

1. A pressure-tube nuclear reactor comprising:an outer shell having at least one shell side wall and a shell tubesheet that cooperate to define an interior to contain a heavy water moderator at a first pressure;a coolant plenum having a plenum cover, at least one plenum side wall and a plenum tubesheet that cooperate to define a plenum chamber to receive a coolant fluid at a second pressure, the second pressure being greater than the first pressure, wherein the plenum tubesheet seals an open end of the outer shell and is in physical contact with the heavy water moderator;a plurality of pressure tubes received within and extending through the interior of the outer shell from the plenum tubesheet to at least the shell tubesheet, each pressure tube configured to releasably retain at least one fuel bundle and having an outer surface in direct physical contact with the heavy water moderator, a first end of each pressure tube being coupled to the plenum tubesheet in fluid communication with the plenum chamber and a second end of each pressure tube fluidly connected to a coolant conduit to enable the coolant fluid to flow between the coolant plenum and each pressure tube and to flow from the nuclear reactor for further processing; andan insulator liner disposed within each pressure tube to inhibit heat transfer between the coolant fluid and the pressure tube. 2. The nuclear reactor of claim 1, wherein the plurality of pressure tubes extend substantially vertically through the interior of the outer shell. 3. The nuclear reactor of claim 1, wherein each insulator liner is porous and a portion of the coolant fluid is retained within each insulator liner. 4. The nuclear reactor of claim 1, wherein each insulator liner comprises at least one ceramic insulator liner loosely received within each pressure tube. 5. The nuclear reactor of claim 1, wherein the plenum is an inlet plenum fluidly connected to at least one coolant supply conduit to receive the coolant fluid and direct the coolant fluid into the plurality of pressure tubes. 6. The nuclear reactor of claim 5, wherein the first end of each pressure tube defines a pressure tube inlet and the second end of each pressure tube defines a pressure tube outlet, each pressure tube outlet fluidly connected to a coolant outlet conduit. 7. The nuclear reactor of claim 6, wherein the coolant outlet conduit comprises at least one riser, a first end of the at least one riser being fluidly connected to a coolant collection header and a second end of the at least one riser being coupled to the plurality of pressure tube outlets, the at least one riser supported by coupling the first end of the at least one riser to a riser support to allow the second end of the at least one riser to accommodate thermal expansion. 8. The nuclear reactor of claim 6, wherein each pressure tube outlet is fluidly connected to the coolant outlet conduit using expansion joints. 9. The nuclear reactor of claim 1, wherein the moderator is pressurized at a first pressure that is less than 1 MPa and the plenum chamber is configured to receive the coolant fluid at a second pressure that is between 8-15 MPa so that the coolant fluid is a subcritical fluid. 10. The nuclear reactor of claim 1, wherein the moderator is pressurized at a first pressure that is less than 1 MPa and the plenum chamber is configured to receive the coolant fluid at a second pressure that is between 23-28 MPa so that the coolant fluid exiting the second end of each pressure tube is a supercritical fluid. 11. The nuclear reactor of claim 1, wherein the plenum chamber is sized to hold a pre-determined volume of coolant fluid so that in use, substantially all radiation shielding for a portion of the nuclear reactor covered by the coolant plenum is provided by the coolant plenum and the pre-determined volume of coolant fluid. 12. The nuclear reactor of claim 1, further comprising an expansion bellows disposed between the coolant plenum and the outer shell to accommodate thermal expansion of at least one of the coolant plenum, the pressure tubes and the outer shell. 13. The nuclear reactor of claim 1, wherein the plurality of pressure tubes have a neutron absorption cross-section between 150-300 mb. 14. The nuclear reactor of claim 1, wherein the second end of each pressure tube is coupled to the outer shell by a respective tube expansion bellows to accommodate for longitudinal growth of each pressure tube. 15. The nuclear reactor of claim 1, wherein the coolant fluid is heavy water or light water. 16. The nuclear reactor of claim 1, further comprising a second coolant plenum comprising a second plenum tubesheet coupled to the second ends of the plurality of pressure tubes and a second plenum chamber to receive the coolant fluid from the plurality of pressure tubes and direct the coolant fluid to the coolant outlet conduit. 17. The nuclear reactor of claim 1, wherein at least one of the insulator liners is formed from ceramic zirconia. 18. The nuclear reactor of claim 1, wherein the insulator liners have a failure pressure at which the insulator liners will fail, and the second pressure is greater than the failure pressure. 19. The nuclear reactor of claim 1, wherein the plenum tubesheet comprises a plenum surface in contact with the coolant fluid in the plenum chamber and an opposed moderator surface in physical contact with the heavy water moderator. 20. The nuclear reactor of claim 1, wherein the plenum tubesheet is convexly curved toward the outer shell. 21. The nuclear reactor of claim 1, further comprising at least one flow regulating element disposed within the plenum chamber to distribute the coolant fluid amongst the pressure tubes. 22. The nuclear reactor of claim 1, wherein a total coolant flow rate divided by the number of pressure tubes in the reactor defines a mean flow rate and wherein a flow rate of coolant fluid through each pressure tube is within 25% of the mean flow rate. 23. The nuclear reactor of claim 1, wherein the insulator liners are removably disposed within the pressure tubes. 24. The nuclear reactor of claim 1, wherein the plenum tubesheet has a wall thickness of between about 40 cm and about 50 cm. 25. A coolant containment system for a nuclear reactor having an outer shell containing a liquid moderator, the coolant containment system comprising:a plenum connectable to an outer shell of a nuclear reactor and having a fluid connection for connecting to a coolant processing system, the plenum comprising a plenum tubesheet and a plenum sidewall extending from the plenum tubesheet to define a plenum chamber and when the plenum is connected to the outer shell the plenum tubesheet seals an open end of the outer shell and is in physical contact with a liquid moderator contained within the outer shell;a plurality of pressure tubes connected at first ends thereof to the plenum tubesheet, the pressure tubes being adapted to receive nuclear fuel bundles and to be mounted within the outer shell and in physical contact with the liquid moderator, and second ends of the pressure tubes fluidly connected to the coolant processing system, the plenum chamber being openable to provide simultaneous access to an interior of the plenum chamber and the plurality of pressure tubes;whereby coolant can be circulated through the coolant processing system, the plenum and the pressure tubes and wherein the moderator is pressurized at a first pressure and the plenum chamber is configured to receive the coolant fluid at a second pressure that is at least 7 MPa greater than the first pressure and the coolant fluid exiting the second end of each pressure tube is a supercritical fluid. 26. The coolant containment system of claim 25, wherein the plenum tubesheet is a pressure barrier between the coolant fluid in the plenum chamber and the moderator and can resist pressure differentials of at least 22 MPa. 27. The coolant containment system of claim 25, further comprising an insulator liner disposed within each pressure tube to inhibit heat transfer between the coolant fluid and the pressure tube. 28. The coolant containment system of claim 27, wherein at least one of the insulator liners is formed from ceramic zirconia. 29. The coolant containment system of claim 25, wherein the plenum chamber is configured to receive the coolant fluid at a pressure that is between 23-28 MPa. 30. The coolant containment system of claim 25, wherein the plenum tubesheet has a wall thickness of between about 40 cm and about 50 cm. 31. A pressure-tube nuclear reactor comprising:an outer shell having, at least one shell side wall and a shell tubesheet that cooperate to define an interior to contain a heavy water moderator at a first pressure;a coolant plenum having a plenum cover, at least one plenum side wall and a plenum tubesheet that cooperate to define a plenum chamber to receive a coolant fluid at a second pressure, the second pressure being greater than the first pressure and the plenum tubesheet is a pressure barrier between the coolant fluid in the plenum chamber and the heavy water moderator contained in the outer shell and can resist pressure differentials of at least 22 MPa; anda plurality of pressure tubes received within and extending through the interior of the outer shell from the plenum tubesheet to at least the shell tubesheet, each pressure tube configured to releasably retain at least one fuel bundle and having an outer surface in direct physical contact with the heavy water moderator, a first end of each pressure tube being coupled to the plenum tubesheet in fluid communication with the plenum chamber and a second end of each pressure tube fluidly connected to a coolant conduit to enable the coolant fluid to flow between the coolant plenum and each pressure tube and to flow from the nuclear reactor for further processing. 32. The nuclear reactor of claim 31, wherein the first end of each pressure tube defines a pressure tube inlet and the second end of each pressure tube defines a pressure tube outlet, each pressure tube outlet fluidly connected to a coolant outlet conduit. 33. The nuclear reactor of claim 31, further comprising an insulator liner disposed within each pressure tube to inhibit heat transfer between the coolant fluid and the pressure tube. 34. The nuclear reactor of claim 33, wherein the insulator liners are removably disposed within the pressure tubes. 35. The nuclear reactor of claim 31, wherein the plenum tubesheet comprises a plenum surface in physical contact with the coolant fluid in the plenum chamber and an opposed moderator surface in physical contact with the heavy water moderator. 36. The nuclear reactor of claim 31, wherein the plenum tubesheet is convexly curved toward the outer shell. 37. The nuclear reactor of claim 31, wherein each insulator liner comprises at least one ceramic insulator liner loosely received within each pressure tube.