Helium Leak Mitigation

Heavy water is a moderator, reflector, and coolant for the NIST reactor. A blanket of helium gas flowing across the liquid surface is used to help maintain isotopic purity of the D₂O inside the vessel, and to minimize the tritium concentration outside the vessel. Although the vessel is also mechanically sealed, the seal is not hermetic and helium leakage is expected during normal operation. However, over the years, the helium leak rate has gradually increased. Whereas the elevated leak rate was previously deemed a minor inconvenience, primarily due to the costs associated with helium, however it now represents a more significant issue that needs attention due to the presence of the remaining fuel debris in the primary system. 

Nuclear fission within the remaining fuel debris generates fission products, some of which are gaseous and are released into the helium blanket during reactor operation. Since each leakage pathway represents a potential avenue for these products to exit the vessel, minimizing leaks has gained newfound significance. Consequently, NCNR will undertake a comprehensive leak detection and sealing initiative slated for the fall of 2024. The initiative will concentrate on the two major components responsible for sealing the vessel top. These components not only provide radiation shielding but also accommodate passages for experimental devices, ports for helium system circulation, refueling tools, and various other features. Every port or mechanical joint requires sealing to avert helium leaks, and the condition of many of these seals remains unknown.  NCNR staff will inspect all seals and replace or repair them as required.