Patent Number: 
Section: claims

1. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel,a nuclear reactor core disposed in the pressure vessel,a baffle plate disposed in the pressure vessel and separating the pressure vessel into an internal pressurizer volume disposed above the baffle plate and an operational PWR volume disposed below the baffle plate wherein the baffle plate includes a pressure transfer passage having a lower end in fluid communication with the operational PWR volume and an upper end in fluid communication with the internal pressurizer volume at a level below an operational pressurizer liquid level range, anda vent pipe having a lower end in fluid communication with the operational PWR volume and an upper end in fluid communication with the internal pressurizer volume at a level above the operational pressurizer liquid level range. 2. The apparatus of claim 1, wherein the vent pipe is transverse to and passes through the baffle plate. 3. The apparatus of claim 1, wherein the pressure transfer passage comprises at least one surge pipe passing through the baffle plate and having a lower end in fluid communication with the operational PWR volume and an upper end in fluid communication with the internal pressurizer volume at a level below the operational pressurizer liquid level range. 4. The apparatus of claim 1, wherein a fluid flow path in the pressure transfer passage is transverse to and passes through the baffle plate. 5. The apparatus of claim 1 wherein the baffle plate comprises first and second spaced apart plates. 6. The apparatus of claim 5, wherein the first and second spaced apart plates define an unsealed volume that fills with water when the baffle plate is immersed in water. 7. The apparatus of claim 1, wherein the baffle plate comprises first and second spaced apart plates defining a sealed volume. 8. The apparatus of claim 1, wherein the baffle plate comprises:first and second spaced apart plates defining a sealed volume; anda gas disposed in the sealed volume. 9. The apparatus of claim 1, wherein the baffle plate comprises a thermally insulating barrier between the internal pressurizer volume disposed above the baffle plate and the operational PWR volume disposed below the baffle plate. 10. The apparatus of claim 1, wherein the PWR further comprises:a central riser disposed in the pressure vessel and having a lower end arranged to receive upwardly flowing primary coolant from the nuclear reactor core and an upper end arranged to discharge the upwardly flowing primary coolant toward the baffle plate. 11. The apparatus of claim 10, wherein the PWR further comprises:a perforated screen surrounding the upper end of the central riser. 12. The apparatus of claim 11, wherein the perforated screen extends from the central riser to the baffle plate such that all upwardly flowing primary coolant discharging at the upper end of the central riser passes through the perforated screen. 13. The apparatus of claim 11, wherein the lower end of the pressure transfer passage is disposed outside the perforated screen. 14. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel configured to contain a nuclear reactor core, anda baffle plate disposed in the pressure vessel and separating the pressure vessel into an internal pressurizer volume disposed above the baffle plate and an operational PWR volume disposed below the baffle plate, the baffle plate comprising first and second spaced apart plates. 15. The apparatus of claim 14, wherein the first and second spaced apart plates define an unsealed volume that fills with water when the baffle plate is immersed in water. 16. The apparatus of claim 14, wherein the baffle plate comprises first and second spaced apart plates defining a sealed volume. 17. The apparatus of claim 16, wherein the baffle plate further comprises:a gas disposed in the sealed volume. 18. The apparatus of claim 14, wherein the baffle plate comprises a thermally insulating barrier between the internal pressurizer volume disposed above the baffle plate and the operational PWR volume disposed below the baffle plate. 19. The apparatus of claim 14, wherein the PWR further comprises:a vent pipe having a lower end in fluid communication with the operational PWR volume and an upper end in fluid communication with the internal pressurizer volume at a level above an operational pressurizer liquid level range. 20. The apparatus of claim 19, wherein the baffle plate includes a pressure transfer passage having a lower end in fluid communication with the operational PWR volume and an upper end in fluid communication with the internal pressurizer volume at a level below an operational pressurizer liquid level range. 21. The apparatus of claim 14, wherein the baffle plate includes a pressure transfer passage having a lower end in fluid communication with the operational PWR volume and an upper end in fluid communication with the internal pressurizer volume. 22. The apparatus of claim 21, wherein the pressure transfer passage comprises at least one surge pipe passing through the baffle plate. 23. An apparatus comprising:a baffle plate configured to be disposed in a pressurized water reactor (PWR) with a first side of the baffle plate facing an internal pressurizer volume and an opposite second side of the baffle plate facing an operational PWR volume; anda vent pipe passing through the baffle plate and having first end in fluid communication with the first side of the baffle plate and an opposite second end in fluid communication with the second side of the baffle plate;wherein the first end of the vent pipe, is relatively closer to the baffle plate and the second end of the vent pipe is relatively further away from the baffle plate. 24. The apparatus of claim 23, wherein a distance of the second end of the vent pipe from the baffle plate is effective to position the second end of the vent pipe at a level above an operational pressurizer liquid level range. 25. The apparatus of claim 23, wherein the baffle plate comprises first and second spaced apart plates defining an unsealed volume that fills with water when the baffle plate is immersed in water. 26. The apparatus of claim 23, wherein the baffle plate comprises first and second spaced apart plates defining a sealed volume. 27. The apparatus of claim 23, wherein the baffle plate comprises:first and second spaced apart plates defining a sealed volume; anda gas disposed in the sealed volume. 28. The apparatus of claim 23, wherein:the baffle plate includes a pressure transfer passage different from the vent pipe, the pressure transfer passage passing through the baffle plate and having first end in fluid communication with the first side of the baffle plate and an opposite second end in fluid communication with the second side of the baffle plate, andthe second end of the pressure transfer passage is relatively closer to the baffle plate than the second end of the vent pipe. 29. The apparatus of claim 28, wherein:a distance of the second end of the vent pipe from the baffle plate is effective to position the second end of the vent pipe at a level above an operational pressurizer liquid level range, anda distance of the second end of the pressure transfer passage from the baffle plate is effective to position the second end of the pressure transfer passage at a level below the operational pressurizer liquid level range.