Patent Number: 
Section: claims

1. A power module assembly comprising:a reactor core immersed in a primary coolant;a reactor vessel housing the primary coolant and the reactor core, wherein an inner surface of the reactor vessel is exposed directly to the primary coolant during normal operation of the power module assembly;an internal dry containment vessel in direct contact with and completely submerged in liquid, wherein the containment vessel substantially surrounds the reactor vessel in a below atmospheric pressure condition, wherein an outer surface of the reactor vessel is exposed directly to the below atmospheric pressure condition during the normal operation of the power module assembly, and wherein the containment vessel is configured to prohibit a release of the primary coolant out of the containment vessel; anda vent configured to controllably release the primary coolant into the containment vessel as primarily steam during an over-pressurization event. 2. The power module assembly according to claim 1, wherein the vent is connected to the reactor vessel and controllably releases the primary coolant at a rate that maintains a steady state containment pressure through condensation of the primary coolant on an inner surface of the containment vessel. 3. The power module assembly according to claim 1, wherein an inner surface of the containment vessel is dry prior to the over-pressurization event. 4. The power module assembly according to claim 1, wherein during the over-pressurization event, the vent controllably releases the primary coolant into the containment vessel and removes a decay heat of the reactor core through condensation of the primary coolant on an inner surface of the containment vessel located adjacent the liquid. 5. The power module assembly according to claim 4, further comprising cooling fins attached to an outside wall of the containment vessel and within the liquid to further remove the decay heat of the reactor core, wherein the condensation on the inner surface occurs on a portion of the containment wall opposite the cooling fins. 6. The power module assembly according to claim 4, wherein the primary coolant that condenses in the containment vessel is circulated back through the reactor core without leaving the containment vessel. 7. The power module assembly according to claim 4, wherein a pool of the condensed primary coolant forms in the containment vessel and surrounds a lower portion of the reactor vessel. 8. The power module assembly according to claim 1, wherein the primary coolant remains substantially confined within the containment vessel during the over-pressurization event. 9. The power module assembly according to claim 1, wherein the containment vessel is maintained at the below atmospheric pressure condition during the normal operation of the power module assembly, and wherein the containment vessel is maintained at an above atmospheric pressure condition after the primary coolant is released into the containment vessel. 10. The power module assembly according to claim 1, wherein substantially all thermal insulation of the reactor vessel is provided by the below atmospheric pressure condition. 11. The power module assembly according to claim 1, further comprising reflective insulation provided between the reactor vessel and the containment vessel, wherein substantially all thermal insulation of the reactor vessel is provided by a combination of the below atmospheric pressure condition and the reflective insulation. 12. The power module assembly according to claim 1, wherein the below atmospheric pressure condition provides a reduction in convective heat transfer between the reactor vessel and the containment vessel. 13. The power module assembly according to claim 1, wherein substantially all of the primary coolant contained in the containment vessel is housed in the reactor vessel during the normal operation of the power module assembly. 14. A power module comprising:a containment vessel in contact with and substantially submerged in a liquid, wherein during normal operation of the power module, the containment vessel is configured to prohibit a release of a primary coolant out of the containment vessel, and wherein during normal operation of the power module, the containment vessel is maintained at a below atmospheric pressure condition;a reactor vessel mounted inside the containment vessel, wherein during normal operation of the power module, substantially all thermal insulation of the reactor vessel is provided by the below atmospheric pressure condition;a reactor core submerged in the primary coolant; anda steam vent connected to the reactor vessel, wherein when the reactor core becomes over-heated the steam vent controllably releases the primary coolant as primarily steam into the containment vessel, wherein the steam condenses on an inner surface of the containment vessel and forms a pool of primary coolant extending between the inner surface of the containment vessel and an outer surface of the reactor vessel, and wherein the entire inner surface of the containment vessel is dry during the normal operation of the power module and prior to the steam being vented into the containment vessel. 15. The power module according to claim 14, 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 steam released by the steam vent adds pressure to the containment vessel. 16. The power module according to claim 14, wherein the steam vent is configured to release the steam 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 removed by the liquid located opposite the condensed steam on an outer surface of the containment vessel. 17. The power module according to claim 14, wherein heat from the power module is removed primarily through conduction from an outer surface of the containment vessel. 18. The power module according to claim 14, wherein the steam vent controllably releases the steam into the containment vessel when a pressure within the reactor vessel exceeds a maximum pressure threshold value. 19. The power module according to claim 14, wherein the pool of primary coolant fills the containment vessel outside of the reactor vessel to a level that is greater than a top of the reactor core. 20. The power module according to claim 19, wherein the level of the pool of primary coolant outside of the reactor vessel is maintained at approximately the same level as the primary coolant that remains within the reactor vessel. 21. The power module according to claim 14, wherein the pool of primary coolant within the containment vessel comprises the primary coolant that is released from the reactor vessel. 22. The power module according to claim 14, wherein the outer surface of the reactor vessel comprises a steel housing, and wherein the steel housing is exposed directly to the below atmospheric pressure condition during the normal operation of the power module. 23. The power module according to claim 22, wherein no insulating material is placed on the steel housing. 24. The power module according to claim 14, wherein the below atmospheric pressure condition provides a reduction in convective heat transfer through gases located between the reactor vessel and the containment vessel. 25. The power module according to claim 14, wherein during normal operation of the power module, substantially all of the primary coolant contained in the containment vessel is housed within the reactor vessel. 26. A power module assembly, comprising:means for circulating primary coolant through a reactor core, wherein the reactor core and the primary coolant are housed in a reactor vessel;means for controllably releasing the primary coolant as steam into a containment vessel substantially surrounding the reactor vessel in response to a high pressure condition within the reactor vessel, wherein an entire inner surface of the containment vessel is dry prior to releasing the primary coolant as steam into the containment vessel, wherein the steam condenses on the inner surface of the containment vessel, and wherein the condensed steam forms a pool of primary coolant extending between an outer wall of the reactor vessel and the inner surface of the containment vessel; andmeans for circulating the pool of primary coolant back into the reactor vessel and through the reactor core, wherein the pool of primary coolant is made up entirely of the condensed steam. 27. The power module assembly according to claim 26, wherein a surface level of the pool of primary coolant is higher than a top of the reactor core, and wherein the pool of primary coolant did not exist prior to releasing the primary coolant as steam into the containment vessel. 28. The power module assembly according to claim 26, wherein the containment vessel is maintained at a below atmospheric pressure condition prior to controllably releasing the primary coolant as steam, and wherein substantially all thermal insulation of the reactor vessel is provided by the partial vacuum. 29. The power module assembly according to claim 26, wherein prior to controllably releasing the primary coolant as steam:the containment vessel is maintained at a below atmospheric pressure condition; andthe outer surface of the reactor vessel is exposed to the below atmospheric pressure condition, wherein the below atmospheric pressure condition provides a reduction in convective heat transfer through gases located between the reactor vessel and the containment vessel. 30. The power module assembly according to claim 26, wherein the containment vessel is configured to prohibit a release of the primary coolant out of the containment vessel, and wherein the containment vessel is maintained at a below atmospheric pressure condition prior to controllably releasing the primary coolant as steam.