Very unlikely. In this case 'the market' is not WallStreetBets, it's experts around the world who do nothing else but assess and estimate the risks.
There are two explanations I can think of:
> Compare that with nuclear, where the negative impact is risk of meltdown - high cost but low risk - and insurers are the ones taking it on.
This may be one reason. The risk of a 'meltdown' is actually so high cost but very low risk, that the corresponding distribution has no first moment. The 'expected cost' is infinite. I've heard folks use it as a real-world example of a Cauchy-type distribution, for more info see [1].
Another explanation might be that there is some sort of recency bias. But that would mean the cost was estimated correctly for the past - which is not what you would want to hear either.
You make a good point that externalities for fossil fuels are not priced in either, but that doesn't explain the phenomenon here. And I'm not arguing for or against the use of nuclear power - societies have already agreed to socialise the cost. I'm pointing out it's not as black and white ("nuclear power is and has always been safe") as some HN commenters may think.
[1] https://en.wikipedia.org/wiki/Cauchy_distribution