U.S. particle detectors go underground to safeguard fusion plant water supplies

To protect drinking water supplies near a planned commercial fusion plant, optical fibers and tiny silicon sensors are being sent underground on a new mission. The devices, called the Subsurface Continuous Radioisotope Environmental Monitor, can monitor soil and groundwater in real time; their primary job is detecting tritium leaks. Tritium, a rare hydrogen isotope used as fuel for fusion reactions, can seep directly into local aquifers if it escapes. The system uses a two-step detection chain to spot radiation. Thin scintillating fibers are buried in the ground around the plant; radioactive beta particles strike the fibers and trigger tiny light pulses.

Those photons travel along the fibers to compact silicon photomultipliers (SiPMs), which, as the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility notes, "convert photons into electrical signals." A connected computer then processes the pulses, detecting and tracking subsurface radiation levels in real time.

Technical advantages of SiPMs

After overcoming the limitations of vacuum tubes, Brian Kross, of the lab's detector development group, said, "Jefferson Lab was the first institution in the world to use SiPMs on a large scale." These devices are light sensors, but the reason we need them for experiments is that they can operate in magnetic fields. Traditional radiation monitors have long relied on conventional glass vacuum tubes — bulky, fragile instruments that require high voltage and can malfunction when exposed to stray electromagnetic fields. Solid-state SiPMs eliminate those constraints entirely: they need very little power, deliver strong readout signals despite their small size, and, crucially, they work flawlessly in powerful magnetic fields.

That makes them ideal for directly monitoring ground conditions beneath fusion reactors, which use massive magnetic coils to confine plasma.

The technology was developed at the Thomas Jefferson National Accelerator Facility in Virginia, where inventors Jack McKisson, Brian Kross, and John McKisson first deployed SiPMs at scale in the early 2000s. The team built them into the Continuous Electron Beam Accelerator Facility during an upgrade of the "GlueX" particle detector. Before turning to groundwater, the team had tested similar sensors for guiding breast-cancer surgery and imaging root systems in plant biology.

Commercial partnership and public trust

A private company is now bringing that government hardware into the power sector. Maryland startup Canary Instruments has signed a formal research agreement with Jefferson Lab. The company was founded by engineer Walter Xu and regulatory attorney Adil Ahmed, who teamed up through the Boost platform, driven by Sandia National Laboratories and FedTech, to commercialize federal research.

Public trust will play a decisive role in the development of fusion energy; plant operators must prove to nearby towns that their water remains uncontaminated. Canary Instruments is currently conducting soil tests and pre-production hardware tests with lab researchers while meeting with private fusion companies now designing the first grid-connected facilities. Real-time subsurface detection gives operators instant alerts, helping keep communities safe as clean fusion energy moves closer to the grid.

The importance of real-time monitoring technology for fusion development

As fusion technology advances, protecting drinking water sources becomes critical. The real-time monitoring capability of the Subsurface Continuous Radioisotope Environmental Monitor not only detects tritium leaks but also strengthens public trust in fusion plants. Through partnerships with private companies, this technology can commercialize faster, ensuring community safety and promoting the widespread adoption of fusion energy. As the technology matures, future fusion plant operations will become more transparent, laying the foundation for sustainable energy development.