Sealed inside a 1980s generator that never ran, 40-year-old strontium-90 waste from a Tennessee federal stockpile is being wired into batteries for the ocean floor, and the assembly site is America’s oldest private nuclear plant
The generator has been sitting in a storage building at Oak Ridge, Tennessee, for nearly four decades.
It was built in the 1980s, never switched on, and never sent anywhere.
Inside it is strontium-90, a radioactive isotope that produces steady heat as it decays.
This approach uses alternative nuclear waste isotopes that are more abundant, and that substitution is what makes the economics work at a scale beyond space probes.
That heat has been doing nothing useful all this time, until now.
So what does a Cold War generator have to do with the floor of the ocean in 2026?
How a decaying isotope becomes a reliable power source
Strontium-90 does not explode or fission. These batteries trap the natural decay heat generated by radioactive isotopes and convert it directly into electricity using Stirling engines or thermoelectric materials. A Stirling engine works on temperature difference alone, using steady warmth on one side and cold ocean water on the other to push a piston back and forth and generate a current.
The physics that makes this compelling is the isotope’s half life of about 29 years. Heat output declines gradually rather than suddenly ending, so a sensor or charging station on the seabed could run for years without a service vessel ever appearing above it.
NASA has relied on similar technology since the 1960s, using rare and costly plutonium-238. This approach uses alternative nuclear waste isotopes that are more abundant, and that substitution is what makes the economics work at a scale beyond space probes.
A Cold War relic finds its location
The site is Vallecitos Nuclear Center, on 1,600 acres of rolling hills in Sunol, California. Vallecitos operated as the country’s first privately owned nuclear power plant from 1957 to 1963, housing a boiling water reactor that received power reactor license number one from the Atomic Energy Commission. Its original reactor was decommissioned long ago, the vessel itself removed in 2023, but the thick walled hot cells built for post irradiation research survived.
The strontium-90 supply traces back to a Cold War era unit called the BUP-500, which was built in the 1980s and has since been sitting in storage at Oak Ridge. Moving that material into production requires exactly the kind of heavily shielded handling rooms that Vallecitos already has, rooms that would cost a prohibitive sum to build from scratch. So the old site and the old waste found each other.
The company is leasing legacy hot cell infrastructure from decommissioning specialist NorthStar Group Services, with non-radiological work already underway and radiological operations awaiting regulatory approval later this year.
The contracts and the evidence behind the plan
The company holds a $7.5 million Department of Defense contract under the DEPTHS program to develop the underwater power station. Beyond that, it has demonstrated a scaled nuclear prototype at a Pacific Northwest national laboratory, secured its first strontium-90 supply from the Department of Energy, raised $70 million in private funding, and locked in over $60 million in government contracts from NASA and the Defense Department.
Those numbers place this well beyond a concept paper. The strontium-90 units are intended to power seabed sensors and deep ocean autonomous vehicles, while a second isotope line using americium-241 sourced through a recycling agreement with a French nuclear operator targets space and lunar missions. Two markets, two waste streams, one set of infrastructure.
The company’s cofounder and chief executive said the facility “is initially supporting production of strontium-90, fueled nuclear batteries for undersea applications,” adding that the company “is also scoping options as it seeks to rapidly scale production of nuclear batteries for space domains.”
What the seabed actually needs and why cables fall short
The ocean floor problem is straightforward to describe and hard to solve. Sensors that monitor submarine cables, autonomous vehicles that patrol pipeline corridors, and acoustic arrays that track shipping lanes all need power that lasts years, not days. Solar panels cannot reach the seabed, and permanent cables can be cut.
A nuclear battery anchored to the seabed removes the cable that an adversary could sever and eliminates the surface ship needed to service a conventional battery bank. “The ocean floor is becoming increasingly important for defense infrastructure, from persistent sensing networks to seabed communications,” the chief executive said. Even so, strontium-90 is biologically hazardous if ingested, and regulators will require demonstration data from real depths before any production unit goes over the side. For context on how private nuclear projects move through qualification, the timelines involved are rarely short.
A site reactivated and wired to the frontier
There is something fitting about pairing the oldest private nuclear plant in America with one of the most unusual applications nuclear energy has ever been asked to serve. Vallecitos lit homes in the Bay Area in 1957, when grid scale nuclear power was still a novelty. Now the same address is being asked to assemble batteries that will sit on the dark seafloor, hundreds of feet below the surface.
The facility will initially fulfill contracts with government customers, with plans to move toward production at scale in 2028, alongside more than 50 local jobs over three years. Those dates are still targets, not receipts, and the gap between a functioning hot cell and a certified seabed unit remains wide. The next milestone is an integrated test showing the strontium heat source and the Stirling converter working together under conditions that simulate years on the seafloor, and that test has not yet happened.
What is already documented is rarer than it sounds: a stockpile of waste once considered a liability has become feedstock for hardware that two demanding customers are paying to receive. Observers tracking nuclear hardware deployments know how carefully each new application is watched, and the strontium-90 sitting in Tennessee started that lesson forty years early.
Hugo is an engineer with strong technical expertise. Multilingual from an early age, his writing combines technical clarity with a strong interest in science and energy.
