There shall be paid into the fund such part of the amounts appropriated pursuant to
section 2514 of this title as the Secretary of Energy deems necessary to carry out the purposes of this chapter and such amounts as may be returned to the United States pursuant to subsection (g) of this section, and the amounts in the fund shall remain available until expended, except that after the expiration of the 7-year period established by subsection (h) of this section such amounts in the fund as are not required to secure outstanding guarantee obligations shall be paid into the general fund of the Treasury.