Daily Rules, Proposed Rules, and Notices of the Federal Government



Draft Regulatory Guide: Issuance, Availability

AGENCY: Nuclear Regulatory Commission.
ACTION: Notice of Issuance and Availability of Draft Regulatory Guide, DG-1216, "Plant-Specific Applicability of Transition Break Size Specified in 10 CFR 50.46a."
FOR FURTHER INFORMATION CONTACT: Robert L. Tregoning, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, telephone: (301) 251-7662,,or, Richard Jervey, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, telephone: (301) 251-7404,

The U.S. Nuclear Regulatory Commission (NRC) is issuing for public comment a draft guide in the agency's “Regulatory Guide” series. This series was developed to describe and make available to the public such information as methods that are acceptable to the NRC staff for implementing specific parts of the NRC's regulations, techniques that the staff uses in evaluating specific problems or postulated accidents, and data that the staff needs in its review of applications for permits and licenses.

The draft regulatory guide (DG), entitled “Plant-Specific Applicability of Transition Break Size Specified in 10 CFR 50.46a,” is temporarily identified by its task number, DG-1216, which should be mentioned in all related correspondence. DG-1216 is a proposed new regulatory guide written to support implementation of proposed rulemaking setting forth an alternate approach for evaluating the performance of an emergency core cooling system (ECCS). The proposed rule, 10 CFR 50.46a, “Risk-Informed Changes to Loss-of-Coolant Accident Technical Requirements,” was published in theFederal Registeron August 10, 2009, (74 FR 40006). The NRC regulatory framework for nuclear power plants consists of a number of regulations and supporting guidelines, including, but not limited to, General Design Criterion (GDC) 35, “Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors,” as set forth in Appendix A, “General Design Criteria for Nuclear Power Plants,” to 10 CFR part 50, “Domestic Licensing of Production and Utilization Facilities” and 10 CFR 50.46a. GDC 35 states, in part, that the licensee must calculate ECCS cooling performance in accordance with an acceptable evaluation model. Furthermore, the licensee must calculate ECCS cooling performance for a number of postulated loss-of-coolant accidents (LOCAs) of different sizes, locations, and other properties sufficient to provide assurance that the evaluation considered the most severe postulated LOCAs. The proposed 10 CFR 50.46a would provide an alternative to the existing, conservatively-set deterministic requirements for evaluating the performance of ECCS systems.

Section 50.46a would contain alternative requirements for ECCS at nuclear power reactors established by using risk information based on the likelihood of pipe breaks of different sizes. The rule would divide all coolant piping breaks currently considered in emergency core cooling requirements into two size groups: breaks up to and including a “transition break size,” and breaks larger than the transition size up to the largest pipe in the reactor coolant system. Selection of the transition size was based upon pipe break frequency estimates, the associated uncertainties, and the need to provide regulatory stability to guard against changes resulting from any future increases in the LOCA frequency estimates. Because pipe breaks smaller than the transition break size are considered more likely they would be analyzed using existing criteria for ensuring the reactor core stays cool during and after an accident. Larger breaks are considered less likely and would be analyzed with less conservative methods, but plants would still have to mitigate the effects of failure of the largest pipe and maintain core cooling. After the final rule is issued, power plant operators could make plant design changes that could enhance safety and/or provide operational benefits. The rule also specifies risk acceptance criteria to ensure that modified designs would continue to provide adequate protection of public health and safety.

This draft guide describes a method that the staff of the NRC considers acceptable for demonstrating that the generic transition break size (TBS) specified in the proposed 10 CFR 50.46a is applicable to a specific plant. The proposed rule would require a licensee to conduct the evaluation described herein either before, or as part of, the initial application to modify a nuclear power plant under the proposed rule. The proposed rule would also require a more limited evaluation to demonstrate the continued applicability of the TBS after each subsequent plant modification. The entire evaluation is greatly simplified for plants that the NRC has approved for license renewal. The evaluation is also simplified for plants that the NRC has approved for leak before break (LBB) or that have applied for license renewal.

This guide only applies to light-water reactor designs that have received a construction permit or operating license prior to January 1, 2000. This guide does not apply to new light-water (i.e.,evolutionary and passive) or to non-light water (i.e.,high temperature gas or liquid metal) reactor designs. Supplemental guidance for applying 10 CFR 50.46a to these reactor designs will be developed at a later date as needed.

The NRC staff is currently soliciting feedback on whether a pilot program should be conducted to demonstrate the use of this draft guide. Information gained from a pilot program would be used in the development of the final regulatory guide and the final 10 CFR 50.46a rule. The NRC staff is also seeking one or more pilot plants to participate in such a program. One or more public meetings may be arranged to discuss a possible pilot program and support public input to the guidance development process. Comments related to the need for, or suggestions for, pilot plants are encouraged at this time.

II. Further Information

The NRC staff is soliciting comments on DG-1216. Comments may be accompanied by relevant information or supporting data and should mention DG-1216 in the subject line. Comments submitted in writing or in electronic form will be made available to the public in their entirety through the NRC's Agencywide Documents Access and Management System (ADAMS).

Because your comments will not be edited to remove any identifying or contact information, the NRC cautions you against including any information in your submission that you do not want to be publicly disclosed.

The NRC requests that any party soliciting or aggregating comments received from other persons for submission to the NRC inform those persons that the NRC will not edit their comments to remove any identifying or contact information, and therefore, they should not include any information in their comments that they do not want publicly disclosed. You may submit comments by any of the following methods:

1.Mail comments to:Rules, Announcements, and Directives Branch Mail Stop: TWB-05-B01M, Office of Administration, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001.

2.Federal e-Rulemaking Portal:Go tohttp://www.regulations.govand search for documents filed under Docket ID [NRC-2010-0229] Address questions about NRC dockets to Carol Gallagher, 301-492-3668;

3.Fax comments to:Rules, Announcements, and Directives Branch, Office of Administration, U.S. Nuclear Regulatory Commission at (301) 492-3446.

Comments would be most helpful if received by August 25, 2010. Comments received after that date will be considered if it is practical to do so, but the NRC is able to ensure consideration only for comments received on or before this date. Although a time limit is given, comments and suggestions inconnection with items for inclusion in guides currently being developed or improvements in all published guides are encouraged at any time. Requests for technical information about DG-1216 may be directed to the NRC contact: Robert L. Tregoning, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, telephone: (301) 251-7662,,or, Richard Jervey, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, telephone: (301) 251-7404,

Electronic copies of DG-1216 are available through the NRC's public Web site under Draft Regulatory Guides in the “Regulatory Guides” collection of the NRC's Electronic Reading Room at copies are also available in ADAMS (, under Accession No. ML100430356. The regulatory analysis may be found in ADAMS under Accession No. ML101530472.

In addition, regulatory guides are available for inspection at the NRC's Public Document Room (PDR) located at 11555 Rockville Pike, Rockville, Maryland. The PDR's mailing address is USNRC PDR, Washington, DC 20555-0001. The PDR can also be reached by telephone at (301) 415-4737 or (800) 397-4205, by fax at (301) 415-3548, and by e-mail

Regulatory guides are not copyrighted, and Commission approval is not required to reproduce them.

Dated at Rockville, Maryland, June 17, 2010.

For the Nuclear Regulatory Commission.

Andrea D. Valentin, Chief, Regulatory Guide Development Branch, Division of Engineering, Office of Nuclear Regulatory Research.