Patent Number: 
Section: claims

1. A reactor pressure vessel assembly, comprising:a reactor core;steam separators over the reactor core;a chimney between the reactor core and the steam separators, an inner surface of the chimney and an inner surface of the reactor core defining a conduit for transport of a gas-liquid two phase flow stream from the reactor core through the chimney to the steam separators;a housing surrounding the reactor core, the chimney, and the steam separators,the housing defining at least one feedwater opening,an inner surface of the housing, an outer surface of the chimney, and an outer surface of the reactor core defining an annulus in fluid communication with the at least one feedwater opening and the conduit;at least one feedwater sparger in the housing, each feedwater sparger being connected to a corresponding one of the at least one feedwater opening, each feedwater sparger being configured to deliver a sub-cooled feedwater into the annulus; anda flow barrier structure in the housing, the flow barrier structure being over the chimney and below the steam separators, the flow barrier structure being spaced apart from the chimney and the steam separators in a vertical direction, the flow barrier structure being configured to force mixing between the sub-cooled feedwater and a downcomer fluid from the steam separators, and at least one of the steam separators being directly vertically over a portion of the flow barrier structure in a plan view. 2. The reactor pressure vessel assembly of claim 1, whereinthe flow barrier structure has a ring shape, anda height of the flow barrier structure in the housing is about level with the at least one feedwater sparger. 3. The reactor pressure vessel assembly of claim 1, wherein the flow barrier structure includes stainless steel. 4. The reactor pressure vessel assembly of claim 1, whereinthe at least one feedwater sparger is a plurality of feedwater spargers arranged in a circular pattern over the chimney,the at least one feedwater opening is a plurality of feedwater openings defined by the housing, andthe feedwater spargers are connected to the feedwater openings. 5. The reactor pressure vessel assembly of claim 1, further comprising:a dam plate in the housing between the chimney and the steam separators, whereina distance between an outer edge of the dam plate and the inner surface of the housing is greater than or equal to a distance between an outer edge of the flow barrier structure and the inner surface of the housing, anda vertical distance between the dam plate and a top of the chimney is different than a vertical distance between the flow barrier structure and the top of the chimney. 6. The reactor pressure vessel assembly of claim 5, wherein the vertical distance between the dam plate and the top of the chimney is less than the vertical distance between the flow barrier structure and the top of the chimney. 7. The reactor pressure vessel assembly of claim 5, wherein the vertical distance between the dam plate and the top of the chimney is greater than the vertical distance between the flow barrier structure and the top of the chimney. 8. The reactor pressure vessel assembly of claim 7, wherein a diagonal distance between the outer edge of the dam plate and an inner surface of the flow barrier structure is equal to or greater than a width of the annulus. 9. The reactor pressure vessel assembly of claim 1, further comprising:a backflow dam on the at least one feedwater sparger, whereinthe backflow dam covers a top of the annulus. 10. The reactor pressure vessel assembly of claim 9, wherein an edge of the backflow dam is one of spaced apart from the inner surface of the housing and connected to the inner surface of the housing. 11. The reactor pressure vessel assembly of claim 1, wherein the flow barrier structure is configured to force the downcomer fluid past and over the at least one feedwater sparger. 12. A reactor pressure vessel assembly, comprising:a reactor core;steam separators over the reactor core, an inner surface of the reactor core defining a conduit for transport of a gas-liquid two phase flow stream from the reactor core to the steam separators;a housing surrounding the reactor core and the steam separators,the housing defining at least one feedwater opening,an inner surface of the housing and an outer surface of the reactor core defining an annulus in fluid communication with the at least one feedwater opening and the conduit;at least one feedwater sparger in the housing, each feedwater sparger being connected to a corresponding one of the at least one feedwater opening, each feedwater sparger being configured to deliver a sub-cooled feedwater into the annulus; anda flow barrier structure in the housing below the steam separators, the flow barrier structure being configured to force mixing between the sub-cooled feedwater and a downcomer fluid from the steam separators, a top of the flow barrier structure in the housing being about level with the at least one sparger, and at least one of the steam separators being directly vertically over a portion of the flow barrier structure in a plan view. 13. The reactor pressure vessel assembly of claim 12, wherein the flow barrier structure has a ring shape. 14. The reactor pressure vessel assembly of claim 12, further comprising:a dam plate in the housing between the reactor core and the steam separators, whereina distance between an outer edge of the dam plate and the inner surface of the housing is equal to or greater than a distance between an outer edge of the flow barrier structure and the inner surface of the housing, anda vertical distance between the dam plate and a top of the reactor core is different than a vertical distance between the flow barrier structure and the top of the reactor core. 15. A method of manufacturing a reactor pressure vessel assembly, comprising:disposing steam separators over a reactor core;disposing a chimney between the reactor core and the steam separators, an inner surface of the chimney and an inner surface of the reactor core defining a conduit for transport of a gas-liquid two phase flow stream from the reactor core through the chimney to the steam separators;disposing a housing surrounding the reactor core, the chimney, and the steam separators,the housing defining at least one feedwater opening,an inner surface of the housing, an outer surface of the chimney, and an outer surface of the reactor core defining an annulus in fluid communication with the at least one feedwater opening and the conduit;disposing at least one feedwater sparger in the housing;connecting each feedwater sparger to a corresponding one of the at least one feedwater opening, each feedwater sparger being configured to deliver a sub-cooled feedwater into the annulus; anddisposing a flow barrier structure in the housing, the flow barrier structure being over the chimney and below the steam separators, the flow barrier structure being spaced apart from the chimney and the steam separators in a vertical direction, the flow barrier structure being configured to force mixing between the sub-cooled feedwater and a downcomer fluid from the steam separators, and at least one of the steam separators being directly vertically over a portion of the flow barrier structure in a plan view. 16. The method of claim 15, whereinthe flow barrier structure has a ring shape, andthe disposing the flow barrier structure includes arranging the flow barrier structure at a height in the housing that is about level with the at least one feedwater sparger. 17. The method of claim 15, wherein the flow barrier structure includes stainless steel. 18. The method of claim 15, wherein the flow barrier structure is configured to force the downcomer fluid past and over the at least one feedwater sparger. 19. The method of claim 15, further comprising:placing a dam plate in the housing between the chimney and the steam separators, whereina distance between an outer edge of the dam plate and the inner surface of the housing is equal to or greater than a distance between an outer edge of the flow barrier structure and the inner surface of the housing, anda vertical distance between the dam plate and a top of the chimney is different than a vertical distance between the flow barrier structure and the top of the chimney. 20. The method of claim 19, whereinthe vertical distance between the dam plate and the top of the chimney is greater than the vertical distance between the flow barrier structure and the top of the chimney, anda diagonal distance between the outer edge of the dam plate and an inner surface of the flow barrier structure is greater than a width of the annulus.