Patent Number: 
Section: claims

1. A reactor pressure vessel comprising:a reactor pressure vessel cylindrical wall portion;a lower head closing off a bottom of the cylindrical wall portion;a core barrel substantially coaxially disposed within the cylindrical wall portion and defining a coolant downcomer annulus between the core barrel and the cylindrical wall portion;a lower core support plate capping a bottom of the core barrel; anda cylindrical flow skirt disposed below the lower core support plate in the lower head, the cylindrical flow skirt having a vertical wall with a plurality of holes extending therethrough and an upper edge, the cylindrical flow skirt being supported from spaced support locations around the lower head with a circumferential distance between at least some of the support locations being different than the circumferential distance between other of the support locations and the upper edge being proximate the lower core support plate so that a majority of coolant flow entering the reactor pressure vessel and traveling down the annulus passes through the holes in the vertical wall of the cylindrical flow skirt before passing through the lower core support plate. 2. The reactor pressure vessel of claim 1 wherein the upper edge of the cylindrical flow skirt is spaced from a bottom surface of the lower core support plate. 3. The reactor pressure vessel of claim 1 including a circumferential rib extending radially inward from the interior of the vertical wall of the cylindrical flow skirt to stiffen the wall. 4. The reactor pressure vessel of claim 3 wherein the rib is formed above the center of a height of the vertical wall of the cylindrical flow skirt. 5. The reactor pressure vessel of claim 3 wherein the holes in the vertical wall of the cylindrical flow skirt are formed in a first pattern and a second pattern. 6. The reactor pressure vessel of claim 5 wherein all of the holes in the vertical wall of the cylindrical flow skirt are substantially the same size. 7. The reactor pressure vessel of claim 5 wherein the first pattern is above the rib and the second pattern is below the rib. 8. The reactor pressure vessel of claim 7 wherein the first pattern is substantially circumferentially continuous and the second pattern is not substantially circumferentially continuous. 9. The reactor pressure vessel of claim 8 wherein a plurality of a pair of circumferentially spaced vertical slots extending from a bottom of the cylindrical flow skirt vertically upward, are cut in a lower portion of the vertical wall of the cylindrical flow skirt with each pair of vertical slots forming an attachment leg that is connected to the lower head at the support locations. 10. The reactor pressure vessel of claim 9 wherein a circumferential distance between some of the pairs of spaced vertical slots differs from the distance between others of the pairs of spaced vertical slots. 11. The reactor pressure vessel of claim 9 wherein the vertical slots extend from the bottom of the wall upward substantially to an elevation of the rib. 12. The reactor pressure vessel of claim 9 wherein there are 6 to 8 attachment legs. 13. The reactor pressure vessel of claim 9 wherein the attachment legs are substantially thinner than a circumferential distance between the legs. 14. The reactor pressure vessel of claim 9 wherein the core barrel is attached to an interior of the reactor pressure vessel spaced from a bottom of the lower head, at a plurality of circumferentially spaced locations wherein the attachment legs do not vertically align with the core barrel attachment locations. 15. The reactor pressure vessel of claim 8 wherein the non-continuous pattern of holes of the second pattern has the second pattern circumferentially separated by the attachment legs. 16. The reactor pressure vessel of claim 15 wherein the holes in the first pattern and the holes in the second pattern are substantially aligned in circumferential rows and the number of rows of the second pattern is larger than the number of rows of the first pattern. 17. The reactor pressure vessel of claim 16 wherein each row of the first pattern is nested in another row of the first pattern and each row of the second pattern is nested in another row of the second pattern. 18. The reactor pressure vessel of claim 3 wherein the rib forms a circumferential recess in an outer surface of the vertical wall of the cylindrical flow skirt. 19. The reactor pressure vessel of claim 1 including a circumferential lip on an outer surface of the vertical wall of the cylindrical flow skirt a distance below the upper edge, the lip defining an increase in a thickness of the vertical wall below the lip. 20. The reactor pressure vessel of claim 1 wherein the holes in the vertical wall of the cylindrical flow skirt are arranged so that a first portion of the coolant is directed directly up through a plurality of holes in the lower core support plate and a second portion of coolant is directed downward toward the bottom portion of the lower head. 21. The reactor pressure vessel of claim 20 including a horizontal vortex suppression plate having holes therethrough for the passage of coolant, wherein a substantial part of the second portion of coolant is directed through the vortex suppression plate from an underside upward to the lower core support plate. 22. A reactor having a pressure vessel comprising:a lower head;a lower core support plate; anda cylindrical flow skirt disposed below the lower core support plate in the lower head, the cylindrical flow skirt having a vertical wall with a plurality of holes extending there thru and an upper edge, the cylindrical flow skirt being supported from spaced support locations around the lower head with a circumferential distance between at least some of the support locations being different than the circumferential distance between other of the support locations and the upper edge being proximate the lower core support plate so that a majority of coolant flow entering the reactor pressure vessel passes through the holes in the vertical wall of the cylindrical flow skirt before passing through the lower core support plate.