10 C.F.R. § 766.106
In the case of a late payment by a domestic utility of its Special Assessment, the domestic utility shall pay interest at the per annum rate (365-day basis) established by DOE for general application to monies due DOE and not received by DOE on or before a designated due date. Interest shall accrue beginning the date of the designated payment except that, whenever the due date falls on a Saturday, Sunday, or a United States legal holiday, interest shall commence on the next day immediately following which is not a Saturday, Sunday, or United States legal holiday. Late payment provisions for the Special Assessment to the Fund shall be based on the Treasury Current Value of Funds Rate (which is published annually by the Treasury and used in assessing interest charges for outstanding debts on claims owed to the United States Government), plus six (6) percent pro rata on a daily basis. The additional six (6) percent charge shall not go into effect until five (5) business days after payment was originally due. Late payment fees shall be invoiced within two days of receipt of utility payment of the special assessment when delinquency is less than 30 days. For longer periods of delinquency, DOE will submit additional invoices, as appropriate. Late payment fees will be due 30 days from the date of invoice.
10 C.F.R. § 766.106