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Accounts Receivable
 3 Months Ended
Mar. 31, 2013
Accounts Receivable  
Accounts Receivable
5. ACCOUNTS RECEIVABLE
March 31,
2013
December 31,
2012
(millions)
Utility customers
$ 106.1 $ 118.3
Sale of NAC (see Note 2)
3.3 -
DOE pro-rata share of RD&D program funding
 33.0 4.4
Contract services, primarily DOE:
Billed revenue
8.7 10.5
Unbilled revenue
2.1 1.6
10.8 12.1
$ 153.2 $ 134.8

Additional details regarding DOE's pro-rata share of funding for American Centrifuge expenditures under the RD&D program are provided in Note 4.

Billings for contract services related to DOE are generally invoiced based on provisional billing rates approved by DOE. Unbilled revenue represents the difference between actual costs incurred, prior to incurred cost audit and notice by DOE authorizing final billing, and provisional billing rate invoiced amounts. USEC expects to invoice and collect the unbilled amounts as billing rates are revised, submitted to and approved by DOE.

Current accounts receivable are net of valuation allowances and allowances for doubtful accounts totaling $2.4 million at March 31, 2013 and $2.1 million at December 31, 2012. Certain receivables from DOE of $25.8 million, net of valuation allowances, are included in other long-term assets as of March 31, 2013 and December 31, 2012 based on the extended timeframe expected to resolve claims for payment filed by USEC under the Contract Disputes Act. USEC believes DOE has breached its agreement by failing to establish appropriate provisional billing and final indirect cost rates on a timely basis.