Patent Number: 
Section: claims

1. A nuclear reactor comprising:a nuclear reactor core comprising a fissile material;a pressure vessel containing the nuclear reactor core immersed in primary coolant disposed in the pressure vessel; andan isolation valve assembly including:an isolation valve vessel having a single open end with a flange;a spool piece having a first flange secured to a wall of the pressure vessel and a second flange secured to the flange of the isolation valve vessel;a fluid flow line passing through the spool piece to conduct fluid flow into or out of the first flange wherein a portion of the fluid flow line is disposed in the isolation valve vessel; andat least one valve disposed in the isolation valve vessel and operatively connected with the fluid flow line,wherein the fluid flow line both enters and exits the isolation valve vessel through the single open end. 2. The nuclear reactor of claim 1 wherein the spool piece and the isolation valve vessel cooperatively define a sealed volume capable of withstanding an operating pressure of the pressure vessel of the nuclear reactor. 3. The nuclear reactor of claim 1, wherein the at least one valve is a check valve preventing fluid flow out of the pressure vessel. 4. The nuclear reactor of claim 3, wherein the fluid flow line is a makeup line for supplying reactor coolant to the pressure vessel and the at least one valve is a check valve preventing primary coolant from flowing out of the pressure vessel through the fluid flow line. 5. The nuclear reactor of claim 1, wherein the at least one valve comprises at least two valves arranged in series on the fluid flow line. 6. The nuclear reactor of claim 1, wherein the at least one valve includes an actuator for moving the valve between open and closed positions. 7. The nuclear reactor of claim 6, wherein the fluid flow line is a letdown line that removes reactor coolant from the pressure vessel responsive to the actuator opening the at least one valve. 8. The nuclear reactor of claim 1, wherein an end of the fluid flow line is disposed coaxially inside the spool piece. 9. The nuclear reactor of claim 1, further comprising a redundant valve disposed outside of the isolation valve vessel and operatively connected with the fluid flow line. 10. A nuclear reactor comprising:a nuclear reactor core comprising a fissile material;a pressure vessel containing the nuclear reactor core immersed in primary coolant disposed in the pressure vessel; andan isolation valve assembly including:a valve cover having a single open end with a flange;a spool piece including a first flange and a second flange secured with the flange of the valve cover to define a sealed volume enclosed by the valve cover;a fluid flow line passing through the spool piece and flowing fluid into or out of the first flange wherein a portion of the fluid flow line is disposed in the valve cover; anda valve supported in the sealed volume and operatively connected with the fluid flow line,wherein the fluid flow line both enters and exits the valve cover through the single open end. 11. The nuclear reactor of claim 10 comprising:reactor coolant inventory and purification system (RCIPS);wherein the fluid flow line is a makeup line supplying makeup coolant water from the RCIPS to the pressure vessel and the valve is a check valve preventing backflow of coolant water from the pressure vessel to the RCIPS. 12. The nuclear reactor of claim 10 comprising:reactor coolant inventory and purification system (RCIPS);wherein the fluid flow line is a letdown line and the valve is an actuated valve that is opened by an actuation signal to initiate flow of coolant water through the letdown line from the pressure vessel to the RCIPS. 13. The nuclear reactor of claim 1, wherein the portion of the fluid flow line disposed in the isolation valve vessel is U-shaped. 14. The nuclear reactor of claim 10, wherein the portion of the fluid flow line disposed in the isolation valve vessel is U-shaped.