In the US at least, the cost for all decommissioning and fuel handling are built into the cost of electricity sold by law.

That was the plan, but it has failed. The mechanism varies but the end result is the same - the operator doesn't pay the actual costs, so they don't charge the actual costs from the customers. The actual costs are paid by taxpayers from some anonymized bucket of money, like a government entity.

As an example, the DOE has so far paid over 100 million USD to SMUD for the decommissioned Rancho Seco plant and will continue to pay for decades more. The operator doesn't pay - the government does.

This has been proven to be true in best-case scenarios like the US, France, Germany and Sweden, and of course once anything deviates from best-case the costs become astronomical almost immediately (Japan).