Burying nuclear reactors might make them cleaner and cheaper

Sep 3rd 2025|Berkeley, California|4 min read

Sometimes an idea is so elegant that it really deserves to work. One such is a proposal put forward by the boss of Deep Fission, an aspiring nuclear-power firm in Berkeley, California. Elizabeth Muller’s brainwave is to build a reactor at the bottom of a mile-deep shaft drilled into Earth’s crust, and then fill the shaft with water. This would, in one fell swoop, minimise the risk of radioactive leaks, dispose of the “hot” waste reactors generate and eliminate much of the paraphernalia that make them expensive to build and run.

Most nuclear power stations generate power from pressurised-water reactors (PWRs). The water concerned has two roles: cooling the reactor’s core (with the heat thus removed being used to drive steam turbines to generate electricity) and neutron-moderation (slowing down the neutrons released by nuclear fission so that they are more easily captured by other nuclei, causing fission in those as well). To stop this water boiling, however, it has to be kept at high pressure. That requires a pressuriser; a strong pressure vessel to surround the core; and a further containment vessel in case the system springs a leak.

The optimum pressure inside a PWR is approximately 155 times atmospheric pressure at sea level: equivalent to the pressure at the bottom of a column of water 1.6km or (in old money) one statute mile deep. Hence Ms Muller’s idea of a stripped-down reactor core at the bottom of a shaft of such depth, letting the water replace the pressuriser and the surrounding rocks stand in for the containment vessel.

This idea did not come from nowhere. For the past decade Ms Muller has been collaborating with her physicist father Richard, a former academic at the University of California, Berkeley, to run Deep Isolation, a firm that proposes burying reactor waste in deep shafts. About three years ago she realised such an approach could also be applied to the reactors.

Deep Fission’s tasks, then, are to design a reactor core narrow enough to fit down a shaft and to drill shafts wide enough to accommodate such cores. Ms Muller’s calculations converge on 75cm as being the diameter for which to aim. That is a target reckoned feasible by the firm’s (currently anonymous) drilling partner, since wider (and, in at least one case, deeper) boreholes were dug routinely from the 1950s to the 1970s, to accommodate nuclear warheads being tested underground. It is also a realistic size for a reactor core.

Once in place, the buried core would be treated as if it were a source of geothermal power. Water heated by it would be brought to the surface through a pipe in the shaft, used to produce turbine-turning steam, and then returned to the shaft to keep up the pressure. The core itself would be pre-loaded with enough uranium fuel to last for two years, after which another would be lowered on top of it, then another, and so on, for a working life of 50-60 years (the estimated lifetime of the casing of the first core). The shaft would then be pumped dry and sealed with concrete, obviating the waste-disposal problem.

Ms Muller says the firm’s engineers are close to completing a preliminary core design and expect to have a fully worked-out version ready for submission for licensing by next year. On August 12th the Department of Energy picked Deep Fission as one of ten firms that will be part of its Nuclear Reactor Pilot Programme, intended to speed up how new designs are tested.

Each unit (borehole plus initial core) will cost about $30m and produce 15MW of electricity at a cost of 5-7 cents a kWh, which is competitive with other power sources. Ms Muller sees an immediate market serving the power-hungry data centres popping up across America and threatening to destabilise the country’s grids. She has already signed a deal with Endeavour, an American data-centre company.

A grid-scale facility would require more holes, but would still be cheaper than an equivalent stand-alone PWR on the surface. With attitudes to nuclear power softening (in America, at least), there has been an efflorescence of ideas for small reactors such as this. Inevitably, many will fail and be quietly buried. For Ms Muller’s, though, burial would be just the start. ■

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