“The best way to become good at building nuclear power plants is to build nuclear power plants,” said Sama Bilbao y Léon, the director general of the World Nuclear Association. John Kotek, an executive at the Nuclear Energy Institute, the industry’s American trade group, pointed out that the U.S. Navy builds nuclear-powered submarines and aircraft carriers in a matter of years — suggesting that quick build times for small reactors could be doable.

Perhaps. But the much-vaunted small reactors are still novel, mainly untested technology. In another era, it may have been worth taking a gamble on these systems in order to avert climate disaster.

But Mark Jacobson, a professor of civil and environmental engineering at Stanford and a longtime proponent of renewable energy, told me that such a bet makes less sense today, when wind and solar power keep getting better — because any new money put in nuclear is money you aren’t spending on renewable projects that could lower emissions immediately.

There’s an opportunity cost “of waiting around for a nuclear reactor to be built when you could have spent that money on wind or solar and got rid of emissions much faster,” Jacobson said. This cost may be particularly onerous when you consider the rapid advancement in battery technology, which can help address the main shortcoming of renewable power: its intermittency. The price of lithium-ion batteries has dropped by about 97 percent since they were introduced in 1991, and prices are projected to keep falling.

Jacobson is one of several researchers who have argued that such advances will render nuclear power essentially obsolete. As we build more renewable energy systems — onshore and offshore wind, solar power everywhere — and improve technologies to store energy (through batteries and other ideas), wind and solar can meet most of our energy needs, says Jacobson. In a 2015 paper, he argued that the world can be powered through renewable energy alone. His findings have been hotly disputed, but other researchers have come to similar conclusions.

On the other hand, the International Energy Agency’s projections for reaching net-zero energy still rely on nuclear. The agency says that nuclear capacity will need to double by 2050, with two-thirds of that growth occurring in developing economies. Still, even with nuclear’s doubling, the I.E.A. says nuclear power will contribute less than 10 percent of global electricity in 2050; over the same period, the agency says renewable generation will grow eightfold, contributing 90 percent of electric power in 2050.