Patent Number: 
Section: claims

1. A power module assembly comprising:a reactor core;a reactor vessel housing the reactor core, wherein the reactor core is submerged in primary coolant contained within the reactor vessel;a containment vessel which substantially surrounds the reactor vessel, wherein the containment vessel is internally dry and forms a containment region which is maintained at a below atmospheric pressure during normal operation of the power module assembly; anda vent configured to controllably release the primary coolant from the reactor vessel into the containment vessel as primarily steam during an over-pressurization event, wherein the containment vessel is configured to retain all of the released primary coolant within the containment vessel, wherein the containment vessel is at least partially surrounded by a heat sink, and wherein a decay heat of the reactor core is transferred to the heat sink primarily through condensation of the released primary coolant on an inner surface of the containment vessel. 2. The power module assembly of claim 1, wherein the vent comprises a flow limiter connected to the reactor vessel and configured to controllably release the primary coolant at a rate that maintains a steady state containment pressure through the condensation of the primary coolant. 3. The power module assembly of claim 1, further comprising an intake configured to circulate the released primary coolant that condenses in the containment vessel back through the reactor core without the primary coolant leaving the containment vessel. 4. The power module assembly of claim 1, wherein the heat sink is configured to passively remove the decay heat for at least three days without any operator intervention. 5. The power module assembly of claim 1, including cooling fins attached to an outside wall of the containment vessel and in contact with the heat sink to further remove the decay heat of the reactor core. 6. The power module assembly of claim 1, wherein the heat sink comprises water or gas. 7. The power module assembly of claim 1, wherein the heat sink comprises rock, soil, or other solid material. 8. The power module assembly of claim 1, wherein a second containment region formed between the reactor vessel and the containment vessel is maintained at atmospheric pressure, and wherein the steam is released into the second containment region during the over-pressurization event. 9. The power module assembly of claim 8, wherein the reactor vessel is insulated by conventional thermal insulation in the containment region, and wherein the reactor vessel is insulated by reflective insulation in the second containment region. 10. The power module assembly of claim 8, further comprising one or more valves connecting the second containment region to the containment region, and wherein the one or more valves are operatively configured to transfer the condensed primary coolant from the second containment region to the containment region. 11. A power module assembly comprising:a reactor core;a reactor vessel housing the reactor core, wherein the reactor core is submerged in primary coolant contained within the reactor vessel;a containment vessel which substantially surrounds the reactor vessel;means for controllably releasing the primary coolant as steam into the containment vessel in response to a high pressure condition within the reactor vessel, wherein an inner surface of the containment vessel is dry prior to releasing the primary coolant as steam into the containment vessel, and wherein steam that condenses on the inner surface of the containment vessel forms a pool of primary coolant in the containment vessel; andmeans for drawing the pool of primary coolant back into the reactor vessel. 12. The power module assembly of claim 11, wherein the condensation of the steam on the inner surface of the containment vessel reduces pressure in the containment vessel at approximately the same rate that the released steam adds pressure to the containment vessel. 13. The power module assembly of claim 11, wherein the steam is released into the containment vessel to remove a decay heat of the reactor core through the condensation of the steam on the inner surface of the containment vessel, and wherein the decay heat is transferred to a heat sink contacting an outer surface of the containment vessel. 14. The power module assembly of claim 13, wherein heat from the power module is removed primarily through conduction from the outer surface of the containment vessel. 15. The power module assembly of claim 13, wherein the heat sink comprises a pool of water which substantially surrounds the containment vessel and is configured to passively remove the decay heat for at least three days without any operator intervention. 16. The power module assembly of claim 11, wherein the containment vessel is maintained at a below atmospheric pressure prior to controllably releasing the primary coolant as steam, and wherein the containment vessel is maintained at an above atmospheric pressure condition after the primary coolant is released into the containment vessel. 17. The power module assembly of claim 16, wherein an outer surface of the reactor vessel comprises a steel housing, and wherein the steel housing is exposed directly to the below atmospheric pressure during normal operation of the power module. 18. The power module assembly of claim 11, wherein a first containment region formed between the containment vessel and the reactor vessel is maintained at atmospheric pressure, and wherein a second containment region formed between the containment vessel and the reactor vessel is maintained at a below atmospheric pressure prior to the over-pressurization event. 19. The power module assembly of claim 18, wherein the reactor vessel is insulated by conventional thermal insulation in the first containment region, and wherein the reactor vessel is insulated by reflective insulation in the second containment region. 20. The power module assembly of claim 11, wherein the pool of primary coolant extends between an outer wall of the reactor vessel and the inner surface of the containment vessel.