The repayment period for loans with respect to which guarantees are made or interest subsidies paid under this subpart shall be limited to 30 years: Provided, That:

(a) The Secretary may, in particular cases where he determines that a repayment period of less than 30 years is more appropriate to an applicant's total financial plan, approve such shorter repayment period;

(b) The Secretary may, in particular cases where he determines that, because of unusual circumstances, the applicant would be financially unable to amortize the loan over a repayment period of 30 years, approve a longer requirement period which shall in no case exceed 40 years; and

(c) In no case shall a loan repayment period exceed the useful life of the facility to be constructed with the assistance of the loan.