Information Notice No. 84-70: Reliance on Water Level Instrumentation with a Common Reference Leg
SSINS No.: 6835 IN 84-70 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 September 04, 1984 Information Notice No. 84-70: RELIANCE ON WATER LEVEL INSTRUMENTATION WITH A COMMON REFERENCE LEG Addressees: All nuclear power reactor facilities holding an operating license (OL) or construction permit (CP). Purpose: This information notice is provided to alert licensees and applicants of the potential for degradation of safety associated with operator reliance on level instruments that share a common reference leg. Recipients are expected to review the information for applicability to their facilities and consider actions, if appropriate, to preclude similar problems occurring at their facilities. However, suggestions contained in this information notice do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: At Maine Yankee, on June 12, 1984, while preparing for startup from the cold shutdown condition following a 10 week refueling outage, the licensee discovered that the pressurizer had been inadvertently drained and that a bubble of air and nitrogen had entered the vessel head. Although the event was primarily caused by inadequate filling and venting operations and pressurizer level irregularities, the event highlights the need for operators to avoid total reliance on level instruments that share a single common reference leg. There were three conditions that made the initiation of this event possible. First, no form of evacuation was used during the initial loop fills. Second, all the cold-calibrated, wide-range level instruments have a common reference leg. Third, the common reference leg was slowly drained because of a small leak. The leak was associated with a modification to permit wide-range level to be indicated at a remote shutdown panel. At approximately 5:00 a.m., 100 psi air and later 150 psi nitrogen was applied to the top of the pressurizer as part of the normal filling and venting procedures. Operators were relying upon cold-calibrated, wide-range level indications to control pressurizer level. The operators did not use or pay attention to the narrow-range level instruments, which each have independent reference legs, because the narrow-range instruments are not calibrated for use at power. Thus, because of the slow leak in the single reference leg, the operators maintained an apparent level while the actual level decreased, 8408310070 . IN 84-70 September 04, 1984 Page 2 of 2 thereby causing the pressurizer to drain and a bubble to enter the top of the head. Sometime during this event, both channels of narrow-range level went offscale low. At about 7:30 p.m., there was an unexplained rise in the wide-range indicated level, and the plant shift supervisor initiated an investigation. The unexplained rise in level was subsequently found to be associated with a routine calibration of one of the wide-range instruments and the previously mentioned slow leak. Examination of the shutdown cooling flow and temperature readings verified that the level was still above the core, but realizing what had happened, operators had to use 8,000 gallons of water to refill the vessel head and pressurizer. Discussion: Situations where a common reference leg can cause multiple level instruments to have erroneous indication are common to most light water nuclear power plants. Boiling water reactors, for example, typically share a reference leg between narrow-range vessel instruments used for feedwater control and wide-range instruments used for emergency core cooling system (ECCS) actuations and ECCS trips such as turbine trip of high pressure coolant injection. For pressurized water reactors that use cold-calibrated, wide-range instruments with a common reference leg, the narrow-range, hot-calibrated instruments can still provide a valuable source of information to the operators during cold conditions. Hot-calibrated instruments should generally read higher than cold-calibrated instruments. If the operators at Maine Yankee had questioned why cold-calibrated instruments were reading higher than the hot-calibrated instruments, or if the operators had followed the narrow-range instruments for general trending of pressurizer level as opposed to precise measurement of level, perhaps the draining of the pressurizer would have been recognized earlier. Licensees and applicants should review their system descriptions, operating procedures, and operator training programs to ensure that a common reference leg shared by multiple level instruments is adequately addressed. Appropriate operator awareness of the effect of draining a common reference leg on multiple level instruments can enhance plant safety. No written response to this information notice is required. If you need additional information about this matter, please contact the Regional Administrator of the appropriate NRC regional office or this office. Edward l. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: Eric Weiss, IE (301) 492-4973 Attachment: List of Recently Issued IE Information Notices
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Page Last Reviewed/Updated Tuesday, March 09, 2021