Shares of Oklo, NuScale Power and Nano Nuclear Energy rose Friday after the Treasury Department and Internal Revenue Service revised rules for hydrogen production tax credits. The three companies all are working on small modular nuclear reactors. Oklo was up 25%, to $27.25, while NuScale gained 17%, to $20.69. Nano rose 13%, to $26.97.

Hydrogen startups are widely seen as a promising way to eliminate fossil fuels from heavy industry and long-haul transportation. But they have been stuck in limbo for the last couple years, waiting for official guidance from the U.