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Information Notice No. 91-73: Loss of Shutdown Cooling During Disassembly of High Pressure Safety Injection System Check Valve
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 November 21, 1991 NRC INFORMATION NOTICE 91-73: LOSS OF SHUTDOWN COOLING DURING DISASSEMBLY OF HIGH PRESSURE SAFETY INJECTION SYSTEM CHECK VALVE Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to an event where loss of reactor shutdown cooling (SDC) occurred because of inadequate reviews of work instructions that affected plant conditions and systems configured to establish and maintain SDC. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On May 5, 1991, SDC for the Waterford Steam Electric Station, Unit 3, was lost for 19 minutes when the bonnet of check valve SI 512A (Attachment 1) in the high pressure safety injection (HPSI) system was opened. This allowed air to enter the suction piping of the connecting SDC train that was in service. During this period, the temperature of the reactor coolant increased from about 100�F to 110�F. At the time of the event, the plant operators were refilling the reactor coolant system following refueling. The reactor vessel head had been installed, but the containment equipment hatch was still open. The "A" train of the SDC was aligned to fill the reactor coolant system from the refueling water storage pool, leaving the operating "B" SDC train as the only immediately available means to provide shutdown cooling. When the vessel level reached the 19 foot elevation, the operators stopped filling the system so that maintenance personnel could remove the bonnet from check valve SI 512A to replace a bonnet seal. Check valve SI 512A is a hot leg injection check valve in the "A" train of the HPSI system. This valve is located at an elevation of 23 feet 10 inches and connects to a high point in the "B" train of the SDC system at the 23 foot elevation. The opening of the check valve permitted air to enter the SDC suction piping, breaking the siphon that was holding the water in the high section of the SDC system and allowing water to run out of the affected SDC train and into the reactor vessel. This raised the vessel level to 21.3 feet. Since this level was below the level of 9111150071 . IN 91-73 November 21, 1991 Page 2 of 3 the high point in the "B" SDC train suction piping, the "B" low pressure safety injection (LPSI) pump became air bound (the LPSI pumps are also used for SDC pumping). After unsuccessfully attempting to vent this pump, the plant operators shut it down. The licensee evacuated all unnecessary personnel from the containment and started the process of installing the containment equipment hatch. The licensee recovered shutdown cooling within 20 minutes by realigning the "A" SDC train, venting the "A" LPSI pump (a standard precaution in the procedure for starting up an idle shutdown cooling loop), and placing the "A" LPSI pump into service. Within a half hour of the start of the event, the licensee had closed the containment. Discussion The root cause of this event was a change in the work procedure that had not received adequate review. The licensee determined that check valve SI 512A needed repair about a month prior to the event. The licensee's initial plans included the use of a freeze seal to isolate the valve with the reactor coolant level at 44 feet. A freeze seal was established in a horizontal section of piping (Attachment 1), using CO2, but it failed when plant personnel attempted to open the bonnet of the check valve. Another freeze seal was established later using liquid nitrogen, but it too failed. The licensee found that the freeze seals failed because this section of pipe had a slight slope, allowing a small amount of air to be trapped at the elevation of the freeze seal, which was about an inch above the piping's vent path. The path left by the trapped air permitted water to leak through and melt the seal. The licensee then changed the procedure to accomplish the repair without the use of a freeze seal at a lower reactor coolant level of 19 feet. The change was discussed and concurred in by the Maintenance, Operations and Planning, and Scheduling Departments. However, the licensee made the change without reviewing isometric drawings and system configurations that would have shown that opening the HPSI check valve would break the suction of operating LPSI pump "B". The licensee did not perform an adequate review and did not obtain the required approvals. Some special controls that would have been applied at a level lower than 18 feet (previously defined by the licensee as reduced inventory requiring special controls) were not applied at this level. Careful review and performance of work procedures, including changes to previously approved work procedures, are particularly important during operations which involve opening a partially drained reactor coolant system when SDC is required. The NRC has discussed loss of SDC in a number of generic communications including Information Notice 87-23, "Loss of Decay Heat Removal During Low Reactor Coolant Level Operation," May 27, 1987, and Generic Letter 88-17, "Loss of Decay Heat Removal," October 14, 1988. . IN 91-73 November 21, 1991 Page 3 of 3 This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: Donald C. Kirkpatrick, NRR (301) 492-1176 Thomas F. Westerman, RIV (817) 860-8145 Attachments: 1. Waterford Unit 3 Partial Safety Injection & Shutdown Cooling Systems 2. List of Recently Issued NRC Information Notices .
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