A US company sees reasons to try it in 2026

The nuclear industry has been looking for years for the moment of SMRs, smaller, cheaper and more versatile fission reactors. A Californian startup called Deep Fission believes it has the key to getting them off the ground: bury them.

160 free atmospheres. Most of the world’s commercial reactors run on pressurized water. To do this, the water that cools the core must remain liquid at more than 300ºC, which requires an immense pressure, around 150 to 160 atmospheres. On the surface, this translates into steel vessels of enormous thickness and cost.

The Deep Fission proposal harness the brute force of gravity to eliminate that problem. Placing the reactor a mile underground, inside a well filled with water, the column of liquid itself exerts a natural hydrostatic pressure of 160 atmospheres. There is no need for a complex pressure vessel: the reactor water is kept in a liquid state without wasting energy or exotic materials.

There is another advantage. The second key point is the mineral environment. Instead of building reinforced concrete domes to contain radiation in the event of an accident, Deep Fission takes advantage of the environment. The solid rock at that depth acts as a natural and inexhaustible retaining wall.

Petroleum engineering. What Deep Fission proposes is to use standard fuel (low-enriched uranium), but with fracking and oil drilling techniques, extracting heat as if it were geothermal.

Its Gravity reactor is a 15 MW module narrow enough to fit into a drill hole about 76 centimeters in diameter. But the economic promise is immense: a cost of 50-70 dollars per MWh and an 80% reduction in civil works, which would be completed in months.

There is a but. Although Deep Fission has already announced a portfolio of potential clients in Texas and Kansas, its design has an Achilles heel. At the same time that burying the reactor protects it from tornadoes, plane crashes or terrorism, it creates a logistical nightmare for its maintenance.

In a normal plant, if a valve fails or a sensor breaks, technicians can access auxiliary areas by taking precautions. Here, everything would be 1.6 km deep. To refuel or repair a breakdown, the entire module would have to be hoisted to the surface using cables, as if it were a miniature submarine.

Today there is no regulatory framework for “deep well reactors” anywhere in the world. Still, Deep Fission promises to have a pilot ready by July 2026.