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Information Notice No. 92-86: Unexpected Restriction to Thermal Growth of Reactor Coolant Piping
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 December 24, 1992 NRC INFORMATION NOTICE 92-86: UNEXPECTED RESTRICTION TO THERMAL GROWTH OF REACTOR COOLANT PIPING Addressees All holders of operating licenses and construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to problems that may be caused by restricted thermal growth of reactor coolant system (RCS) piping and components. 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 February 28, 1992, the licensee for the Wolf Creek Nuclear Generating Station (Wolf Creek) was heating the RCS to prepare for a plant startup. The RCS was at 15.51 MPa [2250 psia] and 280.6�C [537�F] when plant personnel heard a loud metallic noise and felt vibration in the reactor containment. Both the seismic and the loose parts and vibration monitoring (LPVM) system alarms annunciated in the control room. The licensee halted the heat-up and initiated an incident investigation team to investigate and evaluate the cause of the noise. Discussion The initial investigation by the licensee included (1) a system walkdown, (2) interviews of plant personnel and (3) a review of relevant plant data. Additionally, the licensee team began to search past events that may have caused similar effects. The walkdown inspection did not find any piping or pipe support discrepancies attributable to the noise event. However, the search of past records found two events with similar effects. These events occurred on January 9, 1992, and on May 10, 1990. Later investigation showed that the event on May 10, 1990 was not related. Based on the interviews and the review of plant data, the licensee team concluded that the noise event that occurred on February 28 was the result of a thermal expansion event caused by restrictions to RCS thermal growth. The licensee initiated a 9212180008. IN 92-86 December 24, 1992 Page 2 of 4 program to closely monitor RCS component behavior during plant heat-up. The licensee placed instrumentation to measure the pressure, temperature, and displacement of portions of RCS piping and components on each of the four RCS loops. The licensee monitoring program addressed many possible causes for the noise event, such as the upper and lower steam generator (SG) supports and the reactor coolant pump (RCP) tie rods; however, as the investigation progressed, the licensee team began to focus on the movement of the RCS crossover leg saddle blocks in relation to their individual thrust block supports. The saddle blocks are attached to the crossover leg elbows beneath each SG and each RCP (see Figures 1 and 2). On March 8, the licensee began a controlled plant heat-up to normal operating temperature. Personnel were stationed in the containment building to perform system walkdowns and to record data. The heat-up was halted at selected plateaus to allow time for recording data. As the heat-up progressed, the licensee team found that, except for the saddle block on the SG side of loop D, all of the RCS crossover leg saddles (both SG and RCP sides) came into contact with their supports at approximately 260 to 282�C [500 to 540�F]. The saddle block on the SG side in loop D made contact with its support at 226.7�C [440�F]. On March 16, with the plant at 15.51 MPa [2250 psia] and 288.3�C [551�F], the licensee experienced another noise event. Seismic alarms and LPVM alarms annunciated in the control room and personnel in the containment building heard a metallic noise and felt vibration. After reviewing the data from this heat-up, the licensee concluded that the mechanism for the noise events was (1) the restrained thermal growth of the RCS piping caused by binding between the RCS crossover leg saddle blocks and the support blocks and (2) the sudden release of energy that resulted when the restrained thermal growth overcame the frictional resistance. This conclusion was supported by visual inspection of the surfaces of the shim plates installed on the support blocks. After reaching this conclusion, the licensee initiated actions to increase the clearance between the crossover leg saddle blocks and the support blocks. With the performance of a RCS integrity analysis, the support block shim plates were removed and machined to allow for a minimum clearance between the saddle blocks and the support blocks at normal operating temperature and pressure. On March 23, the licensee conducted another heat-up of the RCS. As the saddle blocks approached the support blocks because of thermal growth in the piping, the licensee temporarily removed shims from the support block surfaces to ensure there would not be any contact between the saddles and the support blocks. The licensee observed that the crossover leg piping was free to expand in a uniform manner with no unusual movement. The licensee concluded . IN 92-86 December 24, 1992 Page 3 of 4 that this further demonstrated that the cause of the previous noise events was binding between the saddle blocks and the shims on the support blocks. The licensee has noted no unusual noise or vibration during subsequent plant heatups. As stated in the licensee team Report 92-01, the licensee has performed other corrective actions and is considering implementing additional actions to preclude recurrence. Although the root cause for the insufficient clearance was not determined, the following potential contributors were identified: 1. Quality assurance did not verify the clearances for the saddle block to thrust block support during the initial hot functional testing. 2. No inspections were performed to monitor the clearances. 3. Housekeeping deficiencies from previous modifications in this area may have resulted in debris build up in the gap. 4. The shim plates may have been deformed during previous heat ups. Unanticipated restriction to thermal growth of the RCS may result in support damage and levels of piping stress and fatigue that were not considered in the original design of the system. Excessive stress and fatigue can lead to functional impairment of piping and components. The staff addressed similar concerns in NRC Information Notice (IN) 88-80, "Unexpected Piping Movement Attributed to Thermal Stratification," October 7, 1988, and in IN 91-38, "Thermal Stratification in Feedwater System Piping," June 13, 1991. In its Report 92-01, the licensee team recommended, "perform required analysis and remove shims and saddles from crossover legs if possible." However, these components can only be removed after the NRC has approved the piping for leak before break criteria in accordance with the requirements of General Design Criterion 4 of Appendix A to 10 CFR Part 50. The licensee subsequent action was to machine down and reinstall the shim plates in a manner that returned the installation to its original design. . IN 92-86 December 24, 1992 Page 4 of 4 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 (NRR) project manager. ORIGINAL SIGNED BY Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: Cheng-Ih Wu, NRR (301) 504-2764 Terence Chan, NRR (301) 504-2169 Attachments: 1. Figure 1. Crossover-Leg Saddle Supports Overview 2. Figure 2. Crossover-Leg Saddle Supports 3. List of Recently Issued NRC Information Notices.
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