variable gap conductance (including effects of densification), clad oxidation (two-sided oxidation, heat generation by oxidation), decay heat from actinides, flow redistribution. The sensitivity of calculated results to variations in noding and input parameters must be explored in repeated computer runs. Calculations will be required for a spectrum of pipe break sizes, including splits as well as double-ended breaks. At least three values of the discharge coefficient must be used for each break examined, spanning the range from 0.6 to 1.0. This large number of calculations will now be needed for all of the newer power reactors, as well as for other reactors currently under licensing review. Only the reactor vendors will have the capability of performing the analyses for the plants they have sponsored. Each vendor will suddenly be inundated by needs to provide calculations for all the plants it has designed. We reject at the outset any schedule which does not take proper account of the steps needed for well-founded implementation. It is evident that the vendors cannot develop the models and produce the necessary calculations in the four-month period proposed by the Staff in its Concluding Statement. This is so even though prudent manufacturers may well have done advance calculations upon the assumption that the Staff's proposal would ultimately be the rule. Inasmuch as our rule is somewhat different from that proposed by the Staff, the calculation process will have to begin anew. It is also unlikely that the AEC four-month period. We do not wish to require a schedule for implementation that makes impossible demands on either the vendors or the Regulatory Staff or that might lead to taking hasty steps on the basis of unconsidered analysis, perhaps requiring retraction or revision later. Rather, we wish to ensure that the calculations be thorough as well as timely. A requirement of immediate compliance would be tantamount to an order shutting down or substantially derating all reactors until requisite calculations were complete. We would not hesitate to take that action if circumstances so warranted. But the record shows, and we find, that the Interim Acceptance Criteria will provide reasonable assurance of protection for the public health and safety during the relatively brief transitional period which will culminate in compliance with the new rule. In addition, there is not sufficient basis for presently accepting the view, espoused primarily by the utility and industry participants, which would defer implementation for as long as three years. The evidence has demonstrated that a number of improvements could be made in the models used to evaluate emergency core cooling systems, and these have been incorporated into the rule we announce today. These proceedings have reduced the number of unknowns, and thus reduced the degree to which unknowns must be bridged by conservative safety assumptions. In short, the new rule provides a more objective basis for Criteria, we have permitted them to remain in effect throughout our reevaluation. We also conclude that they will continue to provide reasonable assurance for a transitional period. However, since we now have an improved rule, supported by an evidentiary record, we choose to implement it at a rate which meets the following principles: 1. The schedule should allow for thorough development of the complex evaluation models necessary to comply with the new rule; 2. The schedule should seek to effectuate the new rule's incremental increase in safety at the earliest practicable time; 3. If possible, this goal should be accomplished without unwarranted disruption in the nation's production of electric energy. Guided by these principles, we adopt the following implementation schedule with respect to all facilities for which operating licenses have previously been issued and for which operating licenses may issue during a period of one year from the date of this decision: 1. The rule shall become effective for the purpose of computing the time within which required or permitted actions must be done 30 days following its publication in the Federal Register. 2. Within six months following said effective date, all licensees shall submit the requisite evaluation to the Director of Regu proposed changes in technical specifications or license amendments as may be necessary to bring reactor operation in conformity with the rule. 3. Any licensee may request an extension of the six-month period for good cause. Any such request shall be submitted not less than 45 days prior to the expiration of the six-month period, and shall be accompanied by affidavits showing precisely why the evaluation is not complete and the minimum time believed necessary to complete it. The Director of Regulation shall cause notice of such a request to be published promptly in the Federal Register; such notice shall provide for the submission of comments by interested persons within a time period to be established by the Director of Regulation. If, upon reviewing the foregoing submissions, the Director of Regulation concludes that good cause has been shown for an extension, he may extend the six-month period for the shortest additional time which in his judgment will be necessary to enable the licensee to comply with paragraph 2 above. Requests for extensions of the six-month period, submitted under this paragraph, shall be ruled upon by the Director of Regulation prior to the expiration of that period. 4. Upon the submission of the matters specified in paragraph 2 above, (or under paragraph 3 above, if the six-month period is extended), the facility may continue or commence operation within the limits of both the proposed technical specifications or license amendments submitted under the above procedure, and all technical specifications 5. Further restrictions on reactor operation will be imposed by the Director of Regulation if his review of the evaluations submitted under paragraphs 2 and 3 above so warrants. 6. Exemptions from the operating requirements of paragraph 4 above may be granted by the Commission for good cause. Requests for such exemption shall be submitted not less than 45 days prior to the date upon which the plant would otherwise be required to operate in accordance with the procedures of paragraph 4 above. Any such request shall be filed with the Secretary, who shall cause notice of its receipt to be published promptly in the Federal Register; such notice shall provide for the submission of comments by interested persons within 14 days following Federal Register publication. The Director of Regulation shall submit his views as to any requested exemption within five days following expiration of the comment period. 7. Any request for an exemption submitted under paragraph 5 above must show, with appropriate affidavits and technical submissions, that it would be in the public interest to allow the licensee a specified additional period of time within which to alter the operation of the |