Why is nuclear power controversial in climate terms?

Because it scores extremely well on the metric that matters most for climate and poorly on several others, and people weight those differently.

The case for it:

Very low lifecycle emissions, comparable to wind and generally lower than solar per unit of electricity generated.

High capacity factor. Nuclear plants run around 90% of the time, versus roughly 25–40% for wind and solar depending on location. It provides firm power that does not depend on weather.

Small land and material footprint per unit of energy.

Safety record per unit of energy is strong. Analyses consistently rank nuclear alongside wind and solar as among the safest sources, and far safer than fossil fuels — which cause enormous mortality through air pollution that is diffuse and therefore under-noticed.

The case against:

Cost and construction time. Recent Western projects have run massively over budget and years late. Hinkley Point C and Vogtle are the standard examples. Meanwhile solar and wind costs fell dramatically. This is the strongest current objection, and it is economic rather than ideological.

Waste. High-level waste remains hazardous for many thousands of years. The technical solution — deep geological disposal — is broadly agreed among specialists, but very few repositories exist, and the political difficulty of siting one is real.

Accident consequences. Accidents are rare and their consequences are severe, long-lasting and geographically concentrated. People weight low-probability high-consequence risks differently from diffuse ongoing harm, and that is a legitimate difference in values rather than a factual error.

Proliferation concerns around fuel cycle technology.

Speed. Plants take a decade or more, which matters when the timescale is tight.

Where the debate is heading: small modular reactors promise lower cost through factory production, unproven at scale. Several countries have reversed phase-out plans as energy security concerns rose.