The Commission may determine from time to time that other material is source material in addition to those specified in the definition of source material. Before making such determination, the Commission must find that such material is essential to the production of special nuclear material and must find that the determination that such material is source material is in the interest of the common defense and security, and the President must have expressly assented in writing to the determination. The Commission’s determination, together with the assent of the President, shall be submitted to the Energy Committees and a period of thirty days shall elapse while Congress is in session (in computing such thirty days, there shall be excluded the days on which either House is not in session because of an adjournment of more than three days) before the determination of the Commission may become effective: , That the Energy Committees, after having received such determination, may by resolution in writing waive the conditions of or all or any portion of such thirty-day period.Provided, however