Patent Number: 
Section: claims

1. A nuclear fuel assembly comprising:a top nozzle;a bottom nozzle;a plurality of elongated guide thimbles extending axially between and attached to the top nozzle and the bottom nozzle, at least some of the plurality of guide thimbles having a tubular sheath that extends a majority of the length of the corresponding guide thimble with a lower end of the sheath capped by a lower end plug having an aperture extending axially there through; anda tube-in-tube dashpot having an axially extending sidewall disposed within a lower portion of the tubular sheath with an opening in the sidewall of the dashpot aligned with an opening in the tubular sheath, wherein the opening in one of the sidewall of the dashpot or the tubular sheath is oblong extending partially around a circumference of the one of the sidewall or the tubular sheath with a greater diameter of the oblong opening extending in the circumferential direction. 2. The nuclear fuel assembly of claim 1 wherein the tube-in-tube dashpot has a lower end cap with an aperture extending axially there through which aligns with the aperture in the lower end plug of the tubular sheath. 3. The nuclear fuel assembly of claim 2 including a fastener that extends through the bottom nozzle, through the lower end plug of the tubular sheath and into the lower end cap of the tube-in-tube dashpot, attaching the guide thimble and the tube-in-tube dashpot to the bottom nozzle. 4. The nuclear fuel assembly of claim 3 wherein the aperture in the lower end cap of the tube-in-tube dashpot is threaded and mates with a corresponding thread on the fastener. 5. The nuclear fuel assembly of claim 1 wherein the opening in the one of the sidewall or the tubular sheath comprises a plurality of circumferentially spaced oblong openings formed approximately at the same elevation. 6. The nuclear fuel assembly of claim 5 wherein the opening in the one of the sidewall or the tubular sheath comprises two circumferentially spaced oblong openings formed approximately at the same elevation. 7. The nuclear fuel assembly of claim 1 wherein the opening in the other of the tubular sheath or the sidewall is circular and overlaps a portion of the oblong opening in the one of the sidewall of the tube-in-tube dashpot or the tubular sheath. 8. A modular reactor having a primary circuit substantially contained within a pressure vessel that houses a core comprising fuel assemblies at least some of which include:a top nozzle;a bottom nozzle;a plurality of elongated guide thimbles extending axially between and attached to the top nozzle and the bottom nozzle, at least some of the plurality of guide thimbles having a tubular sheath that extends a majority of the length of the corresponding guide thimble with a lower end of the sheath capped by a lower end plug having an aperture extending axially there through; anda tube-in-tube dashpot having an axially extending sidewall disposed within a lower portion of the tubular sheath with an opening in the sidewall of the dashpot aligned with an opening in the tubular sheath, wherein the opening in one of the sidewall of the dashpot or the tubular sheath is oblong extending partially around a circumference of the one of the sidewall or the tubular sheath with a greater diameter of the oblong opening extending in the circumferential direction. 9. The modular reactor of claim 8 wherein the tube-in-tube dashpot has a lower end cap with a threaded aperture extending axially there through which aligns with the aperture in the lower end plug of the tubular sheath. 10. The modular reactor of claim 9 including a threaded fastener that extends through the bottom nozzle, through the lower end plug of the tubular sheath and into the lower end cap of the tube-in-tube dashpot, attaching the guide thimble and the tube-in-tube dashpot to the bottom nozzle. 11. The modular reactor of claim 10 wherein the aperture in the lower end cap of the tube-in-tube dashpot is threaded and mates with a corresponding thread on the fastener. 12. The modular reactor of claim 8 wherein the opening in the one of the sidewall or the tubular sheath comprises a plurality of circumferentially spaced oblong openings formed approximately at the same elevation. 13. The modular reactor of claim 11 wherein the opening in the one of the sidewall or the tubular sheath comprises two circumferentially spaced oblong openings formed approximately at the same elevation. 14. The modular reactor of claim 8 wherein the opening in the other of the tubular sheath or the sidewall is circular and overlaps a portion of the oblong opening in the one of the sidewall of the tube-in-tube dashpot or the tubular sheath.