The second finding is phrased unequivocally in terms of “reasonable assurance,” while the first speaks more tentatively of “information sufficient to provide reasonable assurance.” The Commission, furthermore, had good reason to make this distinction. For nuclear reactors are fast-developing and fast-changing. What is up to date now may not, probably will not, be as acceptable tomorrow. Problems which seem insuperable now may be solved tomorrow, perhaps in the very process of construction itself. We see no reason why we should not accord to the Commission's interpretation of its own regulation and governing statute that respect which is customarily given to a practical administrative construction of a disputed provision. Particularly is this respect due when the administrative practice at stake “involves a contemporaneous construction of a statute by the men charged with the responsibility of setting its machinery in motion, of making the parts work efficiently and smoothly while they are yet untried and new.” Norwegian Nitrogen Products Co. v. United States, 288 U. S. 294, 315 (1933). And finally, and perhaps demanding particular weight, this constructtion has time and again been brought to the attention of the Joint Committee of Congress on Atomic Energy, which under § 202 of the Act, 42 TJ. S. C. § 2252, has a special duty during each session of Congress to “conduct hearings in either open or executive session for the purpose of receiving information concerning the development, growth, and state of the atomic energy industry,” and to oversee the operations of the AEC. See, e. g., Hearings on Development, Growth, and State of the Atomic Energy Industry, 84th Cong., 2d Sess., p. 106 (1956); Hearings on Development, etc., 85th Cong., 2d Sess., pp. 119-121 (1958) ; Hearings on Development, etc., 86th Cong., 2d Sess., pp. 103-109, 677-678 (1960); Hearings on Development, etc., 87th Cong., 1st Sess., pp. 29-32 (1961); Hearings on