A liquid hydrogen neutron moderator developed a leak between the canister that contains liquid hydrogen and the insulating vacuum jacket.

The moderator assembly consists of an exterior metal vacuum jacket with an interior metal transfer line and canister that contain liquid hydrogen. The moderator canister is constructed of aluminum and is approximately five inches wide, five inches high, and two inches deep. The liquid hydrogen supply lines to the moderator canister are constructed of stainless steel. The operating temperature of the moderator varies from -420 degrees Fahrenheit to a possible 300 degrees Fahrenheit. Mechanical operators discovered a leak following a cleaning operation on the moderator. The cleaning operation was performed to remove impurities that could freeze and reduce flow in the transfer line. Since the moderator is constrained and not readily accessible by personnel, inspections could not be performed to determine the exact location or cause of the leak.

Possible causes for the leak were:

Material failure from the cycling of the unit between the working temperature of -420 degrees Fahrenheit to 300 degrees Fahrenheit.
Changes in material caused by possible overheating of the unit. The existing moderator did not have a thermocouple so that temperatures could be monitored and operations modified, if necessary.
Over-pressurization of the unit from possible overheating.

Incident Date
Sep 14, 1995
  • Hydrogen Storage Equipment
  • Vessel
Damage and Injuries
Probable Cause
Contributing Factors
When Incident Discovered
Lessons Learned

Designs for high-tech systems/components evolve based on operating experience. The design changes should resolve identified deficiencies and are part of a continuous improvement process to increase reliability and productivity.