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Information Notice No. 93-58: Nonconservatism in Low-Temperature Overpressure Protection for Pressurized-Water Reactors
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 July 26, 1993 NRC INFORMATION NOTICE 93-58: NONCONSERVATISM IN LOW-TEMPERATURE OVERPRESSURE PROTECTION FOR PRESSURIZED-WATER REACTORS Addressees All holders of operating licenses or construction permits for pressurized- water reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to a nonconservatism in the low-temperature overpressure protection (Ltop) setpoint calculation for Westinghouse facilities. 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 Circumstance On October 29, 1992, the Texas Utilities Electric Company, the licensee for Comanche Peak Steam Electric Station, reported that its existing low temperature overpressure protection may not have provided the required margins of safety against reactor vessel brittle fracture under certain overpressure transients. The same concern was later reported by the licensees for Byron, Zion, Diablo Canyon, Kewaunee, Sequoyah and Point Beach nuclear stations. Discussion In reactor units designed by Westinghouse, overpressure protection of the reactor vessel at low temperature conditions is provided by a cold overpressure mitigation system (COMS). This system compares pressure and temperature inputs against a preset setpoint curve and relieves the pressure when the setpoint is reached. This protection is necessary because, while at low temperatures during plant startup and shutdown conditions, certain transients could cause the reactor coolant system pressure to exceed the reactor vessel pressure-temperature (P-T) limitations established for protection against brittle fracture. A spurious start of a safety injection pump, reactor coolant pump, or other operational errors could activate this system. During such events, the P-T limitations are maintained by opening the pressurizer power-operated relief valves (PORVs) or safety relief valves in the residual heat removal (RHR) suction lines to relieve system pressure. 9307190131. IN 93-58 July 26, 1993 Page 2 of 3 The transmitters that provide pressure signals to the COMS are located at the primary system hot leg piping of the reactor vessel. During low temperature operation of the reactor coolant pumps, dynamic pressure in the reactor vessel would be higher (by the amount of flow loss in the core and vessel outlet) than that sensed in the hot leg. Additionally, the static head correction for the difference in elevation of the sensor to the core region was not considered. The resulting pressure difference between the sensor and the vulnerable location in the reactor vessel could be as high as 790 kPa (100 psig), depending on the number of reactor coolant pumps in operation and the location of the pressure-sensing taps. The Ltop setpoint curve that was originally developed by Westinghouse did not take these factors into consideration. Westinghouse has sent a letter to licensees recommending one of the following methods to compensate for this pressure increase: 1) reduce the maximum allowable relief valve setpoint by an amount equivalent to the plant-specific calculated difference in pressure, 2) maintain RCS pressure below the heatup/cooldown curves by a value equal to the plant specific difference in pressure from both flow loss and elevation difference when the reactor coolant pumps are in operation, 3) restrict the number of reactor coolant pumps and, therefore, the flow loss error that can be operated below a defined RCS temperature without drawing a steam bubble in the pressurizer, or 4) demonstrate that the available margin in the Ltop calculation, taking into account instrumentation uncertainty, is sufficient to offset the plant- specific pressure difference. The Westinghouse letter describes interim administrative controls as well as calculational methods to verify setpoint adequacy for addressing the Ltop concern. The staff notes that the administrative restrictions described in approaches (2) and (3) are intended by Westinghouse to provide interim actions to address the concern only until Ltop setpoints are verified to be adequate or are revised appropriately in technical specifications. Although the information in this notice addresses the cold overpressure mitigation system at Westinghouse designed plants, aspects of this issue may also be applicable to other PWRs. . IN 93-58 July 26, 1993 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. ORIGINAL SIGNED BY Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: Thomas Koshy (301) 504-1176 Chu-Yu Liang (301) 504-2878 Attachment: List of Recently Issued NRC Information Notices .
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