An aspiring nuclear-power firm based in Berkeley, California, founded by Elizabeth Muller. The company proposes building reactors at the bottom of mile-deep shafts drilled into the Earth's crust, then filling the shafts with water. The depth provides the pressure equivalent of a conventional pressurised-water reactor's pressuriser and containment vessels, stripping away much of the cost of surface-built plants.
Ms Muller previously spent a decade running Deep Isolation, a firm she co-founded with her physicist father Richard Muller, a former academic at the University of California, Berkeley, which proposes burying reactor waste in deep shafts.
The target diameter for the reactor core is 75cm. Once lowered into place, heated water would be brought to the surface through a pipe in the shaft, used to produce turbine-turning steam, and returned to the shaft. Each core would be pre-loaded with enough uranium fuel for two years, after which another would be lowered on top of it for a total working life of 50-60 years. The shaft would then be pumped dry and sealed with concrete, disposing of the waste.
Each unit (borehole plus initial core) would cost about $30m and produce 15MW of electricity at 5-7 cents per kWh. Ms Muller sees an immediate market serving the power-hungry data centres threatening to destabilise America's grids. The company has signed a deal with Endeavour, an American data-centre company.
On August 12th 2025 the Department of Energy selected Deep Fission as one of ten firms in its Nuclear Reactor Pilot Programme, intended to speed up how new designs are tested.
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