The NRC has no regulatory authority over the expenditure of decommissioning funds. The licensees submit a Decommissioning Cost Plan to the NRC for review, but not for approval.
One authority the NRC does have is to allow some decommissioning funds to be used for dry storage. If the NRC determines there are sufficient funds to do both decommissioning and spent fuel storage, they give an exemption to use some funds for dry storage.
By law the decommissioning fund is only for dismantling the facility. In California our Public Utilities Commission required an addition ratepayer fee be collected for spent fuel management, but this isn’t typical. The money is commingled with the decommissioning fund.