Subject to subsection (c) and to section 1513(b) of this title, any loan under subsection (a) shall be repayable out of revenue from production of the geothermal energy reservoir with respect to which the loan was made, at a rate, in any year, not to exceed 20 per centum of the gross revenue from the reservoir in that year; except that if any disposition of the geothermal rights to the reservoir is made to one or more other persons by the borrower, the full amount of the loan balance outstanding, or so much of the loan balance outstanding as is equal to the full amount of the compensation realized by the borrower upon such disposition, whichever is less, shall be repaid immediately. In any case where the reservoir is confirmed (as determined by the Secretary), the Secretary may impute a reasonable revenue for purposes of determining repayment if—