"Nuclear energy is back," declared French President Emmanuel Macron at the UN climate summit in Dubai last week, summoning its revival after decades of decline.

France, a leader in nuclear energy, is one of more than 20 nations — including the US, UK, United Arab Emirates and Japan — to have signed a pledge at COP28 to triple nuclear energy capacity by 2050.

Although non-renewable, nuclear is considered a clean energy source because it produces relatively low greenhouse gas emissions responsible for planetary heating, compared to oil, coal and gas.

Yet, the difficulty of dealing with nuclear waste, which can remain radioactive for tens of thousands of years, and the potential for a Chernobyl or Fukushima-style disaster, makes it a contentious source of energy.

There are more than 430 reactors around the world — which collectively produce around 10% of global electricity — and 57 more under construction. This new pledge seeks to up that percentage at a time when countries such as Germany have turned their back on nuclear altogether.

What exactly does the declaration involve?

The declaration states a nuclear revival is critical for reaching net-zero emissions, keeping to the Paris Accord goal of limiting planetary heating to 1.5 degrees Celsius (2.7 Fahrenheit), and ensuring a stable, low-carbon energy supply as the world transitions to renewables.

Countries signed up to the pledge said they would work together to triple nuclear capacity on 2020 levels by extending the lifetimes of existing plants and building new reactors, including new small modular reactors (SMRS), which are potentially cheaper, quicker to build and safer than conventional ones.

That would mean a jump from 400 gigawatts (GW) annual nuclear capacity to almost 1200 GW and likely hundreds of new plants, said William D Magwood, director general of the Paris-based intergovernmental Nuclear Energy Agency, whose analysis of nuclear's role in curbing emissions is cited in the declaration.

Can nuclear power realistically be tripled in 27 years?

Criticisms of nuclear energy largely boil down it being too expensive and slow to contribute to rapid emissions cuts. High up-front costs, construction times of at least a decade, and a reputation for projects mired in delays make tripling capacity an unrealistic option for critics.

Mycle Schneider, an independent international energy analyst, who has long been critical of nuclear, believes the goal is "strictly impossible from an industrial point of view," pointing to a decline in nuclear power generation of 4% globally.

According to Schneider, the industry would need to "build and start an additional 270 reactors or 230 GW" on top of current reactors under construction, simply to keep pace with plant closures in the run up to 2050.

"There is no evidence that this is possible let alone to triple the current operating capacity by then," said Schneider, who also publishes the yearly World Nuclear Industry Status Report.

And while proponents say SMRs could be a game-changer for nuclear, with the likes of Microsoft cofounder Bill Gates investing in them, they are still in the early stages of development, with just three deployed so far.

Still, Magwood, who previously served as Director of Nuclear Energy at the US Department of Energy, says the goals can be met if governments and industry are committed.

"If we're really serious about climate change, then it will be pretty straightforward to triple capacity."

He points to the pace of construction seen in countries like the US, France, and Sweden in the 1970s and 1980s as a sign it is "doable and realistic." France gets around 70% of its electricity from nuclear.

Is nuclear a distraction from renewables?

According to the UN Emissions Gap Report published in November, predicted global emissions must fall 42% by 2030 to have a chance of limiting global average temperature rise to 1.5 C and avoiding the worst impacts of climate change.

More than 100 countries also pledged to triple renewable energy capacity by 2030 at COP28. According to the Paris-based intergovernmental International Energy Agency (IEA), to reach net zero emissions by 2050, annual clean energy investment, including nuclear, will need to triple by 2030 to around $4 trillion per year.

But given the urgency to cut greenhouse gas emissions from fossil fuels to curb planetary heating, climate campaign group 350.org say expanding nuclear would detract from the drive to cheaper and faster climate protection measures.

"We already have cheaper, safer, democratic, and faster solutions to the climate crisis, and they are renewable energy and energy efficiency," said Masayoshi Iyoda, 350.org Japan campaigner, calling nuclear a "dangerous distraction."

Still, Aidan Rhodes, research fellow at Energy Futures Lab, Imperial College London, says the push for nuclear expansion would not create a funding battle with renewables.

"Renewables, like wind and solar, are relatively quick to build and quick to provide a return [on investment]. If you want to build a nuclear power plant today it is probably going to be 10-15 years before you get any power out and that very much changes the financial equation of who is going to invest in it," said Rhodes.

While Germany's federal environment agency the UBA published a study this week stating a tripling of nuclear wasn't "needed to achieve climate targets according to the Paris Agreement," Rhodes says it doesn't need to be a choice of one or the other.

"All of these technologies have a place in the system," said Rhodes. "They all have disadvantages. They all have advantages. Ruling out one means that we unavoidably slow down the transition. And I think that's the real danger."

Edited by: Jennifer Collins