To my knowledge risk in insurance speak is probability x cost and insurances don't insure Nuclear reactors.
It is more something like
probability x cost x liquid_captial / (liquid_capital - cost)
Essentially if the cost is so high the insurer would take a massive hit. This is the same reason why normal people by health-insurance. Essentially the 'value' of money is not linear. The less money you have the more value a single dollar has.