; or (B) 90 percent of the projected useful life of the physical asset to be financed by the loan (as determined by the Secretary). (6) Use of payments Payments of principal and interest on the loan shall— (A) be retained by the Secretary to support energy research and development activities; and (B) remain available until expended, subject to such conditions as are contained in annual appropriations Acts. (7) No penalty on early repayment The Secretary may not assess any penalty for early repayment of a loan provided under this section. (8) Return of unused portion In order to receive a loan under this section, an eligible entity shall agree to return to the general fund of the Treasury any portion of the loan amount that is unused by the eligible entity within a reasonable period of time after the date of the disbursement of the loan, as determined by the Secretary. (9) Comparable wage rates Each laborer and mechanic employed by a contractor or subcontractor in performance of construction work financed, in whole or in part, by the loan shall be paid wages at rates not less than the rates prevailing on similar construction in the locality as determined by the Secretary of Labor in accordance with subchapter IV of chapter 31 of title 40, United States Code. (c) Rules and procedures