Only over the course of many decades do they net reduce carbon emissions, at the cost of much higher carbon emissions during the construction phase. Right now we need to be implementing the fastest carbon reductions possible and this does the exact opposite; for the next fifteen years, these plants will be a massive carbon emissions increase, and then it will take many years of operation to "pay that back." By then it will be far too late.
Nuclear is one of the most expensive electricity generating technologies and it has only gotten more expensive, while solar, wind, and battery technologies are following expected plunging cost paths. Nuclear power plants are difficult to site geologically, difficult to site grid-wise (because the only way they are economical is through massive scale, and you can't just drop 5, 10, or more gigawatts onto the grid just anywhere), there is no need for base load which is the only thing nuke can provide (either in terms of capability, or economically), they don't like fussy climates/extreme weather (and if they use natural bodies of water, are vulnerable to invasive species, increasingly a problem around the world), they have very deep and exacting supply chains when countries have experienced significant supply chain problems for numerous different reasons, and you can't just snap your fingers and have qualified staff to run the plant. They require significant socio-political stability (functioning education, security, accountable political leadership_, and generate material that is extremely dangerous to life for generations, and have the potential to render entire geographical regions uninhabitable, again for generations.
Guess what has none of these problems? Solar. Wind. Hydro. Energy storage.
Nuclear power also doesn't solve the fundamental problems: the environmental impact of staggeringly large militaries, the amount of 'disposable' packaging used in almost everything, the cross-planet shipping of consumer goods most of which are, frankly, useless, heavy use of low occupancy vehicle travel, poor building efficiency, and proof-of-work cryptocurrency schemes.