Resolution of Generic Safety Issues: Item D-2: Emergency Core Cooling System Capability for Future Plants (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
This issue was documented in NUREG-04713 and stemmed from an ACRS recommendation that the staff explore diverse means of obtaining ECCS capability for future plants. The ACRS also recommended the staff explore the issue in the context of the revision to 10 CFR 50, Appendix K. This issue does not affect currently operating plants since it specifically addresses future plants.
ECCS is an important safety feature for all plants and this system is reviewed by the staff before issuing an OL. In the past, specific staff concerns with ECCS performance and reliability have been raised and resolved through the generic issue process. Specifically, the generic issues in this area were:
|II.E.2||Emergency Core Cooling System|
|B-8||Locking Out of ECCS Power Operated Valve|
|B-61||Allowable ECCS Equipment Outage Periods|
|B-69||ECCS Leakage Ex-Containment|
Currently, ECCS reliability and performance on existing plants has not been identified as a major contributor to risk (based upon the review of many plant specific PRAs). However, certain initiating events of high frequency, which involve LOCA sequences that challenge the ECCS, remain as significant contributors to overall risk. Accordingly, these initiating events are currently being addressed in the resolution of Issue 23, "Reactor Coolant Pump Seal Failure," Issue 70, "Power Operated Relief Valve and Block Valve Reliability,"and Issue 24, "Automatic ECCS Switch to Recirculation."
In addition, as part of implementing the Commission's Severe Accident Policy Statement, the staff is evaluating the need for additional requirements and/or assessments for future plants to reduce the risk from severe accidents. It is expected that as part of satisfying these requirements, future reactor designs will, as a minimum, be assessed for vulnerabilities to low probability events (common mode failure, multiple failure, etc) and, if a significant vulnerability
attributable to the ECCS is found, it will have to be corrected. Therefore, separate additional requirements directed toward future plant ECCS diversity are not justified since currently identified ECCS vulnerabilities have been or are being addressed, and ECCS vulnerabilities on future plants will be addressed on a design specific basis.
The question of diversity was not explored by the staff in its proposed revision to 10 CFR 50 Appendix K. The proposed revision to the ECCS Rule contained in 10 CFR 50.46 and Appendix K only addresses more realistic analyses of ECCS performance. The staff has published a draft regulatory guide, "Best-Estimate Calculations of Emergency Core Cooling System Performance," (RS 701-4) which contains: (1) features of ECCS evaluation codes that are acceptable to produce realistic calculations of ECCS performance; (2) models, data, and model evaluation procedures that are acceptable to the staff; and (3) descriptions of methods acceptable to the staff for performing uncertainty calculations.1076 Much of this guidance should be applicable to future as well as existing plants.
Requirements exist for future plants that propose to use ECCS designs similar to those of currently operating reactors. These ECCS requirements have been proven satisfactory by extensive analytical and experimental work and specific vulnerabilities addressed through the generic issue process. In addition, as part of implementing the Commission's Severe Accident Policy,1118 the staff is developing guidelines for the treatment of severe accidents for future LWRs and, as a minimum, it is expected that future ECCS designs will have to be evaluated for vulnerabilities consistent with these guidelines. Accordingly, since the need for improvements in the ECCS for future LWRs will be determined as part of implementing the Commission's Severe Accident Policy, this issue was DROPPED from further consideration as a separate issue.