Patent Number: 
Section: claims

1. An apparatus, comprising:a containment vessel having a first, fluid-containing region and a second, vacuum region, the second region housing a reactor pressure vessel of a nuclear reactor module, and the first region positioned above the second region;a control rod drive mechanism (CRDM) partially housed in the first region to control movement of control rods relative to the containment vessel;a fluid-tight barrier positioned around the CRDM, the fluid-tight barrier and the CRDM partitioning the first and second regions from each other wherein at least a portion of the CRDM is positioned in the second region and wherein one or more CRDM drive motors of the CRDM are thermally coupled with fluid in the first region;a fluid inlet positioned to direct the fluid into the first region, anda fluid outlet positioned to direct the fluid out of the first region. 2. The apparatus of claim 1, wherein the fluid inlet is positioned to direct air into the first region. 3. The apparatus of claim 2, further comprising an active device located outside the containment vessel to force the air outside the containment vessel through the fluid inlet into the first region. 4. The apparatus of claim 2, wherein the first region comprises air at a higher density than any air located in the second region. 5. The apparatus of claim 1, further comprising a heat exchanger positioned in thermal communication with the drive motors to remove heat generated by the drive motors from the containment vessel. 6. The apparatus of claim 1, wherein the fluid-tight barrier comprises an attachment section sealably coupled with the CRDM, and an expansion section coupled between the CRDM and the containment vessel to permit the attachment section to move with the CRDM relative to the containment vessel. 7. The apparatus of claim 6, wherein the CRDM is one of multiple CRDMs, and wherein individual CRDMs include a CRDM housing, and wherein the attachment section comprises a plate with an opening for each CRDM housing. 8. The apparatus of claim 6, wherein the expansion section comprises a bellows. 9. The apparatus of claim 6, wherein the attachment section is welded to the CRDM. 10. The apparatus of claim 1, wherein the at least the portion of the CRDM comprises one or more CRDM magnetic coils. 11. The apparatus of claim 1, wherein the fluid comprises air.