The Commission shall establish guaranteed purchase prices for plutonium produced in a nuclear reactor by a person licensed under section 2134 of this title and delivered to the Commission before January 1, 1971. The Commission shall also establish for such periods of time as it may deem necessary, but not to exceed ten years as to any such period, guaranteed purchase prices for uranium enriched in the isotope 233 produced in a nuclear reactor by a person licensed under section 2133 or section 2134 and delivered to the Commission within the period of the guarantee. Guaranteed purchase prices established under the authority of this section shall not exceed the Commission’s determination of the estimated value of plutonium or uranium enriched in the isotope 233 as fuel in nuclear reactors, and such prices shall be established on a nondiscriminatory basis: Provided, That the Commission is authorized to establish such guaranteed purchase prices only for such plutonium or uranium enriched in the isotope 233 as the Commission shall determine is produced through the use of special nuclear material which was leased or sold by the Commission pursuant to section 2073 of this title.