TRIPWIRE: Leading the way for radiation detection

IDAHO FALLS – As nations explore ways to reduce carbon emissions, nuclear energy is increasingly recognized as a safe, reliable option for coping with the increase in electricity use.

Further, tomorrow’s advanced nuclear reactors are smaller and more flexible than today’s reactor fleet, which allows them to integrate more renewables with the grid, provide heat for industry, and power remote applications, such as mines and data centers.

Just as the world needs the next generation of nuclear reactors to meet climate goals, it needs new options for safely and securely storing radioactive materials. As of 2022, there were approximately 400,000 metric tons of spent fuel being stored globally, with roughly 90,000 tons in the U.S. alone.

Nuclear experts say deep geological repositories are a necessary long-term solution for securely storing nuclear waste. While no commercial nuclear repositories currently exist, Finland plans to open the world’s first deep geological repository by the middle of this decade. Switzerland, Canada and France are licensing their own disposal facilities.

In the U.S., consolidated spent fuel storage facilities will bridge the gap between on-site storage and a national repository.

Likewise, nations need to secure nuclear materials for arms control purposes, to meet treaty obligations and to prevent proliferation of nuclear materials in an increasingly volatile world.

These nuclear storage scenarios will need reliable detection equipment to ensure public trust and the nonproliferation of nuclear materials.

At Idaho National Laboratory, researchers developed TRIPWIRE to enable detecting radiation over large and inaccessible areas like nuclear material repositories. TRIPWIRE won a 2024 R&D 100 Award and provides governments and industry with an inexpensive, durable, one-of-a-kind option for monitoring radioactive materials.

INL’s Laboratory Directed Research and Development program initially funded the work. TRIPWIRE has also received funding from the National Nuclear Security Administration.

Scintillating fibers

TRIPWIRE uses thin scintillating fibers to detect radiation. Made of plastic, big blocks of scintillating materials are sometimes used to detect radiation at above-ground locations like border crossings.

“The radiation excites the molecules in the scintillating fibers” said David Chichester, a nuclear engineer and directorate fellow at INL. “When they de-excite, they will give off visible photons, there’s a sensor to detect that light.”

With TRIPWIRE, the scintillating plastic is drawn out to fibers that are about 1 mm thin. At that thickness, the fibers exhibit a similar property to glass fiber optics called total internal reflection. Total internal reflection means the light will reflect down the length of the fiber until it reaches the end.

“With these plastic scintillators, the maximum length that will transmit light effectively is something less than 50 meters,” Chichester said. “But we can couple the scintillating fiber to a glass fiber optic, then the sensor can be kilometers away.”

That 50 meters — the length of an Olympic swimming pool — allows detection over a much greater length and orders of magnitude longer than current gamma radiation detectors, which are typically limited to just a few inches.

The upshot of TRIPWIRE is that the technology eliminates the need to bury electronic equipment within the repository. Instead, the electronic equipment remains on the surface, where it is easy to repair or replace, if necessary.

“Originally our sponsor was interested in detecting undeclared activities,” Chichester said. “Imagine a repository that is sealed, and a host nation attempts to get back into it. Having a real-time sensor underground with the spent fuel provides a strong deterrent against theft or diversion.”

Spread out like a spiderweb

With underground facilities and tunnels, TRIPWIRE can spread out like a spiderweb and cover a three-dimensional area.

TRIPWIRE’s optical fibers are low-cost and readily available, which makes the technology a good option to significantly reduce personnel exposure to potentially dangerous radiation and maintenance costs in long-term nuclear storage facilities.

The scintillating fibers are sheathed in protective materials that resist abrasion, chemical degradation, and changes in temperature and humidity. The fibers can withstand being stepped on or struck by falling rocks, which is essential for long-term deployment in a sealed repository where the fibers may need to function unattended for decades — or even centuries.

TRIPWIRE also has applications beyond repositories, including monitoring the natural background radiation along oil and gas pipelines, monitoring radiation at ports of entry for national security, and perimeter awareness for emergency response.

Researchers have even considered using TRIPWIRE inside the gloves used for moving and examining radioactive materials as a radiation safety monitor.

“TRIPWIRE is a versatile cost-effective technology for radiation monitoring, it has the potential for tremendous impact in several fields, and we look forward to its potential deployment in the coming years,” Chichester said.