US startup Deep Fission has raised $4m (€3.5m) in pre-seed funding to develop nuclear reactors designed to be built a mile underground.

The Berkeley, California-based company, which has achieved milestones including a conceptual design and regulatory engagements, said it aims to take advantage of natural underground conditions to reduce costs and improve safety compared with traditional reactor pressure vessels and containment structures.

The company said it can do this while using conventional low-enriched uranium fuel and an existing supply chain, sidestepping a significant source of delay and concern for other advanced reactor designs.