The U.S. nuclear power industry is at an impasse. Since 2012, 11 of the 104 light-water reactors in operation at the time have closed, mainly as a result of aging infrastructure and the inability to compete with natural gas, wind and solar, which are now the cheapest sources of electricity in the U.S. and most other countries worldwide.

One way the industry is trying to reverse the trend is by looking to what it likes to call “advanced” reactors. Despite the name, these designs are largely based on unproven concepts from more than 50 years ago. Unlike conventional light-water reactors, these rely on sodium or molten salt or gas for cooling, and their proponents claim they will be less expensive, safer and more secure than their predecessors. Some claim that these innovative devices will be ready for prime time by the end of this decade.

This has naturally attracted the attention of Biden administration officials and some key members of Congress, who are looking for ways to curb carbon emissions. But an analysis of non-light-water reactor concepts in development by the Union of Concerned Scientists (UCS) has found that these designs are no better—and in some respects significantly worse—than the light-water reactors in operation today. The report’s author, UCS physicist Edwin Lyman, took a close look at the claims developers have been making: that these new devices will burn uranium fuel more efficiently and produce less radioactive waste than existing plants; will reduce the risk of nuclear proliferation; and will be commercialized relatively soon. Those claims, however, do not hold up to scrutiny.

One contender, for example, TerraPower’s 345-megawatt Natrium reactor, received considerable media attention earlier this year when company founder Bill Gates touted it during interviews about his new book, How to Avoid a Climate Disaster. According to the UCS report, however, sodium-cooled fast reactors such as Natrium would likely be less uranium-efficient and would not reduce the amount of waste that requires long-term isolation. They could also experience safety problems that are not an issue for light-water reactors. Sodium coolant, for instance, can burn when exposed to air or water, and the Natrium’s design could experience uncontrollable power increases that result in rapid core melting.

In June, TerraPower announced that it would build the first Natrium reactor in Wyoming as part of a 50–50 cost-share program with the Department of Energy. The DOE program originally required the company to have the reactor, still in its early design stage, up and running by 2027. That was recently pushed back a year, but it is still a completely unrealistic timetable. According to the UCS report, if federal regulators require the necessary safety demonstrations, it could take at least 20 years—and billions of dollars in additional costs—to commercialize such reactors, their associated fuel-cycle facilities, and other related infrastructure.

The Nuclear Regulatory Commission (NRC) may have to adapt some regulations when licensing reactor technologies that differ significantly in design from the current fleet. Lyman says that should not mean weakening public health and safety standards, finding no justification for the claim that “advanced” reactors will be so much safer and more secure that the NRC can exempt them from fundamental safeguards. On the contrary, because there are so many open questions about these reactors, he says they may need to meet even more stringent requirements.

Lyman’s report recommends that the DOE suspend its advanced reactor demonstration program until the NRC determines whether it will require full-scale prototype tests before any designs are licensed for commercial deployment, which the report argues are essential. It also calls on Congress to require the NRC to convene an independent commission to review the technical merits of non-light-water reactors and approve only those projects that have a high likelihood of commercialization and are clearly safer and more secure than the current fleet.

Finally, it recommends that the NRC and Congress consider spending more research and development dollars on improving the safety and security of light-water reactors rather than on commercializing immature, overhyped non-light-water reactor designs. Any federal appropriations for R&D and deployment of these reactor designs, Lyman says, should be guided by a realistic assessment of the likely benefits and not based on wishful thinking.