The Majorana Demonstrator has an impenetrable shield made up of six layers of various materials designed to block out minute traces of radiation. Extra care needs to be taken with the last layer. 12 inches of polyethylene, a combustible material that requires a defense-in-depth fire protection plan to mitigate fire hazards.
"Every prevention measure added, is a significant increase in fire protection," said Kathy Carney from Oak Ridge National Laboratory and the environmental, health and safety manager for the Majorana Demonstrator Project. "We took a look at all potential failure modes and from that analysis developed an in-depth fire protection defense."
Covering all aspects of a fire and thermal protection plan isn't as simple as installing fire alarms and fire extinguishers in the cleanroom. Vince Guiseppe, assistant physics professor at the University of South Carolina and a researcher with the Majorana experiment, helped create the fire prevention plan, which looks at all the different ways to prevent a fire.
"We believe that our defense-in-depth strategy minimizes the risk of fire as much as possible," Guiseppe said. "The redundant nature of our fire protection strategy ensures that the risk is low even if some of our protection systems are lost or not in service. Our strategy is to prevent a fire from occurring inside the shield, rather than just relying on fire suppression should one start."
During the fire prevention planning process, the Majorana team covered all potential fire hazards. "The amount of work and the amount of effort you need to put into fire protection is not something you can take lightly and hope for the best," Guiseppe said.
"We made it our mission to create a worst-case-scenario, a situation in which a fire could begin and last long enough to do major damage. Then we developed a plan to ensure that a fire wouldn't happen."
What they came up with was a five-step plan.
Purge the enclosed areas of oxygen using nitrogen gas. Without oxygen, a fire can't survive.
Cool the enclosed area to prevent thermal overload.
Encase the poly shield with aluminum.
Add redundant heat and smoke detection units in the enclosed area.
Place equipment in such a way as to reduce likely ignition sources.
"Once fully enclosed, the detector is physically inaccessible yet it contains vital systems to operate the experiment," Guiseppe said. "By implementing our fire and thermal management plan, we have confidence that the system can run safely without the risk of a thermal overload or a fire event. This plan is one of many detector safety and hazard mitigation strategies employed for long term operation of the Majorana detector."