Can a nuclear plant operator get insurance against the costs of a nuclear meltdown?
The costs of the Fukushima meltdown are reportedly $200 billion. Offering a policy with a downside that big would basically be gambling the entire company.
An insurance company can't pay out more money than they have.
Imagine I'm the CEO of an insurance company with $50 billion in assets, and you're a customer wanting to insure against a risk with a 0.1% chance of happening.
If you want the insurance to cover a payout $10 million, the expected value is $10k so if you're willing to pay a premium of $11k I'm happy to do business with you.
If you want the insurance to cover a payout of $100 billion, the expected value is $10 million. But it doesn't matter if you're willing to pay a premium of $11 million, $20 million, $100 million. Because I don't have $100 billion, I can't tell you I'll pay out $100 billion.
Sure, reinsurance exists - but that relies on there being some other insurance company around that can eat a $100 billion loss. Not many fools who'd take a gamble with that much downside.
In your follow-up example, they could reinsure much of the $200B so that they're only liable for a small(er) part of the losses.
The calculation is thus whether they can pay the premiums on the excess (and accept reinsurers' contractual terms) and still make money.