Daily Rules, Proposed Rules, and Notices of the Federal Government
Publicly available NRC documents created or received in connection with this rulemaking are also available electronically via the NRC's Electronic Reading Room at
In a staff requirements memorandum dated July 1, 2004, the Commission directed the staff to propose a risk-informed alternative rule to the current requirements in 10 CFR 50.46. The NRC is making preliminary versions of the draft rule conceptual basis and draft rule language available to inform stakeholders of the current status of the NRC's activities to risk-inform 10 CFR 50.46. This draft rule conceptual basis may be subject to significant revisions during the rulemaking process. To meet the Commission's schedule, the NRC is not soliciting early public comments on this draft rule conceptual basis and draft rule language. No stakeholder requests for a comment period will be granted at this stage in the rulemaking process. Stakeholders will have an opportunity to comment on the rule conceptual basis and rule language when it is published as a proposed rule.
Under this risk-informed alternative, the NRC would establish requirements, in a new § 50.46a, which would divide the existing spectrum of LOCA pipe break sizes up to the double-ended rupture of the largest reactor coolant system pipe into two regions. Each region will be subject to different ECCS analysis requirements, commensurate with likelihood of the break. Loss-of-coolant accidents in the smaller break size region (up to and including a “transition break size”) will be analyzed by the methods, assumptions and criteria currently used for LOCA analysis; accidents in the larger break size region (from the transition break size up to the double-ended rupture of the largest reactor coolant system pipe) may be analyzed by less stringent methods based on their lower likelihood. Although loss-of-coolant accidents for breaks larger than the transition break size will become beyond design-basis accidents, the NRC will promulgate regulations ensuring that licensees maintain the ability to mitigate pipe breaks up to the double-ended rupture of the largest reactor coolant system pipe. Since LOCAs in the larger break size region would be required to be mitigated, such accidents would remain separate from severe accidents, which are addressed by voluntary industry guidelines.
Licensees who perform new LOCA analyses using the new risk-informed alternative requirements may find that their plant designs are no longer limited by certain parameters associated with previous analyses. Changing these limitations could enable licensees to
The NRC intends to periodically evaluate LOCA frequency information. If estimated LOCA frequencies significantly change, the NRC may revise the transition break size. In such a case, the backfit rule (10 CFR 50.109) would not apply. Similarly, if future evaluations of LOCA frequency invalidate the bases for a design change made by a licensee, that licensee would be required to change the facility and/or procedures or make other compensatory changes elsewhere to reduce facility risk to acceptable levels. In such cases, the backfit rule (10 CFR 50.109) also would not apply.
The NRC's current concept regarding the rule framework, the associated technical bases, and early draft rule language will be posted on the NRC's rulemaking Web site at
At the public meeting on August 17, 2004, the NRC would like to obtain information about the potential costs and benefits of the above rule changes in order to complete the regulatory analysis for the proposed rule. After licensees and other stakeholders review the draft rule conceptual basis and draft rule language posted on the NRC Web site (
1. Estimate the number and type of plants that might pursue this voluntary option. Estimate the costs of performing the ECCS reanalyses at these plants.
2. Provide the estimated number and types of plant design changes that would be permitted by the ECCS reanalyses at these plants (on a per unit basis) and the estimated costs of any decision analyses associated with such design changes.
3. Estimate the costs of additional analyses (apart from the ECCS reanalyses) required by the proposed rule to determine the acceptability of the above design changes. These costs could include but may not be limited to (1) updating probabilistic risk assessments (PRAs) to reflect the new design and to meet the PRA quality and scope requirements and (2) analyses to determine compliance with the risk acceptance criteria and the defense-in-depth criteria.
4. Estimate the number and types of plant design changes (on a per unit basis) that would meet the acceptance criteria for the additional analyses.
5. Estimate the costs of implementing the plant design changes that meet the acceptance criteria for the additional analyses.
6. Estimate any operational costs and/or savings resulting from implementing the above design changes.
7. Estimate any anticipated changes in licensee information collection, reporting, and retention burden that could result if this rulemaking is implemented.
For the Nuclear Regulatory Commission.