SC 13G 1 usu13g.txt USU13G05 CUSIP NO. 90333E108 13G 1 Name of Reporting Person / IRS Identification Number: Advisory Research, Inc. / 36-2831881 ------------------------------------------------------------------ 2 Check the Appropriate Box if a Member of a Group (a) [ ] (See Instructions) (b) [X] ------------------------------------------------------------------ 3 SEC Use Only ------------------------------------------------------------------ 4 Citizenship or Place of Organization Delaware ------------------------------------------------------------------ Number of 5 Sole Voting Power Shares 0 ----------------------------------------------- Beneficially 6 Shared Voting Power Owned By 0 Shares ----------------------------------------------- Each 7 Sole Dispositive Power Reporting 0 ----------------------------------------------- Person 8 Shared Dispositive Power With 0 Shares ------------------------------------------------------------------ 9 Aggregate Amount Beneficially Owned by Each Reporting Person 0 ------------------------------------------------------------------ 10 Check if the Aggregate Amount in Row (9) Excludes Certain Shares [ ] (See Instructions) ------------------------------------------------------------------ 11 Percent of Class Represented by Amount in Row (9) 0 ------------------------------------------------------------------ 12 Type of Reporting Person IA ------------------------------------------------------------------ Item 1 (a) Name of Issuer: USEC INC Item 1 (b) Name of Issuer's Principal Executive Offices: 2 DEMOCRACY CENTER 6903 ROCKLEDGE DRIVE BETHESDA, MD 20817 Item 2 (a) Person Filing - Advisory Research, Inc. Item 2 (b) Address - 180 North Stetson St., Suite 5500 Chicago, IL 60601 Item 2 (c) Citizenship - Advisory Research, Inc.is a Delaware Corporation Item 2 (d) Title of Class of Securities - Common Item 2 (e) CUSIP Number - 90333E108 Item 3 If this statement is filed pursuant to Rules 13d-1(b) or 13d-2(b), check whether the person filing is a: (a) [ ] Broker or Dealer registered under Section 15 of the Act (b) [ ] Bank as defined in Section 3(a)(6) of the Act (c) [ ] Insurance Company as defined in Section 3(a)(19) of the Act (d) [ ] Investment Company registered under Section 8 of the Investment Company Act (e) [X] Investment Advisor in accordance with section 240.13d-1(b)(1)(ii)(E) (f) [ ] Employee Benefit Plan or Endowment Fund in accordance with section 240.13d-1(b) (1)(ii)(F) (g) [ ] Parent Holding Company or Control Person in accordance with secton 13d-1(b) (1)(ii)(G) (h) [ ] A savings association as defined in section 3(b) of the Federal Deposit Insurance Act (i) [ ] A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 Item 4 Ownership (a) Amount Beneficially Owned: Advisory Research, Inc. 0 Shares (b) Percent of Class 0 (c) Number of shares as to which reporting person has: (i) Sole Voting Power 0 Shares (ii) Shared Voting Power 0 Shares (iii) Sole Dispositive Power 0 Shares (iv) Shared Dispositive Power 0 Shares Item 5 Ownership of Five Percent or Less of a Class: If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following [ ] Item 6 Ownership of More than Five Percent on Behalf of Another Person: Not Applicable Item 7 Identification and Classification of the Subsidiary Which Acquired the Security being Reported on by the Parent Holding Company: Not Applicable Item 8 Identification and Classification if Members of the Group: Not Applicable Item 9 Notice of Dissolution of Group: Not Applicable Item 10 Certification By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of such securities and were not acquired and are not held in connection with or as a participant in any transaction having such purposes or effect. SIGNATURE After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. 02/14/2006 ---------------------------------- Date /s/ Brien M. O'Brien ---------------------------------- Signature Brien M. O'Brien, President ---------------------------------- Name/Title