Any contract made under the provisions of this section to acquire plutonium or any interest therein may be at such prices and for such period of time as the Commission may deem necessary: Provided, That with respect to plutonium produced in any reactor constructed under the joint program, no such contract shall be for a period greater than ten years of operation of such reactors or December 31, 1973 (or December 31, 1975, for not more than two reactors selected under section
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(c) of this title, whichever is earlier: And provided further, That no such contract shall provide for compensation or the payment of a purchase price in excess of the Commission’s established price in effect at the time of delivery to the Commission for such material as fuel in a nuclear reactor.