Resolution of Generic Safety Issues: Issue 124: Auxiliary Feedwater System Reliability (Rev. 3) ( NUREG-0933, Main Report with Supplements 1–34 )
In 1985, operating experience as well as staff and industry studies indicated that AFW systems continued to fail at a high rate. These studies also indicated that plants with similar AFW system reliabilities (as calculated in accordance with the SRP11 guidance) did not necessarily exhibit similar AFW system availabilities. Based on these studies and on engineering judgment, the staff concluded that the PWR AFW system reliabilities calculated in accordance with the SRP11 guidance may have represented the relative reliability of AFW system hardware configurations for various plants, but did not represent the real availability of these crucial safety systems.914
In order to ascertain a high level of AFW system reliability and availability, the staff proposed a requirement that all operating plants demonstrate by PRA that their AFW systems had a minimum reliability of 10-4 unavailability/demand after accounting for: AFW system support systems, common cause failures, or operator errors. As input to the PRAs, each utility was expected to use its plant-specific data, if available. It was believed that such plant-specific data would reflect design faults, poor maintenance practices, and inadequate testing and surveillance and would indicate how well a particular plant was being operated, thereby identifying those plants that needed improvements.
The following issues were integrated into the resolution of Issue 124:
(1) Issue 68, "Postulated Loss of Auxiliary Feedwater System Resulting from Turbine-Driven Auxiliary Feedwater Pump Steam Supply Line Rupture"
(2) Issue 122.1.a, "Failure of Isolation Valves in Closed Position"
(3) Issue 122.1.b, "Recovery of Auxiliary Feedwater"
(4) Issue 122.1.c, "Interruption of Auxiliary Feedwater Flow"
(5) Issue 125.II.1.b, "Review of Existing AFW Systems for Single Failures."
Because of the significance of the AFW system in reducing core-melt frequency, the staff determined that all PWRs should meet the reliability criterion specified in SRP11 Section 10.4.9 which was not applied to reactors in operation at the time this issue was identified. In order to achieve and maintain a high degree of reliability for the AFW system or alternate decay heat removal, the following was completed: (1) PWR licensees and applicants demonstrated, using reliability analyses, that their AFW systems were of high reliability (10-4 to 10-5 unavailability/demand); and (2) the staff reviewed the reliability analyses and/or any necessary system modifications and procedural or maintenance changes.
As a result of (1) and (2) above, the staff determined whether it was necessary to require that plants upgrade their AFW systems to the safety-related standards.
Based on the staff evaluation of the AFW systems of W, CE, and B&W plants (NUREG-0611,93 NUREG-0635,95 B&W plant SERs), the staff initially determined that the AFW system of the following plants were not sufficiently reliable and should be upgraded: Prairie Island 1, Prairie Island 2, ANO-1, ANO-2, Fort Calhoun, Crystal River, and Rancho Seco.
In resolving this issue, the staff concluded that substantial improvement in plant safety could be achieved by provision of an additional means of water supply, e.g., startup feedpump, to the steam generators. The staff determined that the two-pump AFW systems at ANO-2 and Rancho Seco needed to be upgraded and took the necessary steps to inform the affected licensees of this decision. Thus, this issue was RESOLVED and requirements were issued to two plants.1203