So you speak about $400B, lets consider it upper bound and even up to $500B by adding the cost Three Miles island incident and the various minor ones around the world.
Spread over 60 years of the humanity nuclear power usage, that makes us an insurance cost of 8.3 billions dollar per year at world scale.
So the risk cost $8.3B per year at world scale for a source of energy that generate 25% of the world low-carbon electricity [1] and make us save around 470 millions tons of CO2 per year [2].
To put things into perspective: the US spend $150B in natural disaster cost coverage per year alone [3]
The EnergyWende of Germany alone cost around $40B per year.
And the cost of the death related to fossil fuel emissions in modern history is estimated at $1.7T in the OECD alone [4].
So I think it is fair to qualify $8.3B/year for a nuclear energy insurance cost to be pretty cheap deal at humanity scale.
I would even qualify it to be ridiculously cheap.
Your sentence "Nuclear power plants are nowhere near insured to risk" is pretty typical of the anti-nuclear argumentation: Mainly based on feelings not on facts.
Based on facts, it is currently plain wrong.
[1]: https://world-nuclear.org/information-library/current-and-fu...
[2]: https://world-nuclear.org/information-library/current-and-fu...
[3]: https://www.climate.gov/news-features/blogs/beyond-data/2022...
[4] https://www.oecd-ilibrary.org/docserver/d1b2b844-en.pdf?expi...