Climate change has forced us to rethink how we get electricity. Use of renewable sources like solar and wind is rapidly increasing, while nuclear, though long a reliable source of carbon-free electricity, is not. Meanwhile, a number of startups are promising cheap, safe, proliferation-­resistant nuclear energy in the next decade (see “Fail-Safe Nuclear Power”).

Can these startups fulfill their promises? Outside of China, nuclear power is expanding nowhere. China has 21 new reactors under construction; Russia has nine, India six. The U.S. is bringing five new plants online, but since 2012, five other reactors have been retired, with seven more to be shuttered by 2019. California’s Diablo Canyon plant recently announced it will close by 2025. With other plants closing in Japan, Germany, and the U.K., more reactors may be decommissioned than built in the near future.

So why is this happening? Because it’s expensive and time-consuming to design and build a new nuclear plant, and there are cheaper, easier alternatives.

The U.S. Nuclear Regulatory Commission has been waiting since 2014 for applications for design certification licenses for small modular reactors—smaller versions of the large and extra-large operating light-water reactors, with additional safety features. Such plants, which could be factory-built and snapped together on site, hold the promise of providing cheaper nuclear power in a more distributed fashion. Other designs are on the horizon, including molten-salt reactors, which are promising but won’t be ready for decades.

In 2015, the General Accountability Office reported that it takes 20 to 25 years to develop a new reactor in the United States—10 years for the design phase, 3.5 years for a design certification license from the NRC, four years for a combined operating license, and another four years for construction. And that’s only in an ideal world where no unexpected problems occur.

The GAO also found that it’s not cheap to bring a design to fruition: just to reach the design certification point costs somewhere between $1 billion and $2 billion, and only about $75 million of that is NRC fees. There’s a reason it takes so long and costs so much: manufacturers need to confirm that the design is safe and secure.

Some people blame the regulators for holding up the plants. Yet the NRC hasn’t been presented with any applications for new reactors and probably won’t be for years. Data from prototype plants would be helpful, but then many of the “new” designs are not so new at all. Sodium-cooled fast reactors have been built by countries including the U.S., Japan, Russia, Germany, France, and India since the 1950s, but no country has been able to make a plant cheap and reliable enough to even come close to being a viable energy source.

Yes, new nuclear technology can provide carbon-free electricity. But it has to do more than that. It has to be safe, secure, and resistant to proliferation. It has to compete in the marketplace. New nuclear designs are promising, but they’re no short-term solution to the climate problem.

Allison Macfarlane was the chairman of the Nuclear Regulatory Commission from 2012 to 2014.