The footprint of a nuclear power plant is pretty huge. That machinery is generally not optional even if doing a "fire and forget" design. Underground space woul

The footprint of a nuclear power plant is pretty huge. That machinery is generally not optional even if doing a "fire and forget" design. Underground space would come at a premium cost, likely far more than could be saved on reactor design. Also nuclear power plants obviously generate an absolutely massive amount of heat that needs to be dissipated, a task that would be difficult and expensive underground.