Information Notice No. 84-70, Supplement 1: Reliance on Water Level Instrumentation with a Common Reference Leg
SSINS NO.: 6835 IN 84-70 SUPP. 1 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 August 26, 1985 Information Notice No. 84-70 SUPPLEMENT 1: RELIANCE ON WATER LEVEL INSTRUMENTATION WITH A COMMON REFERENCE LEG Addressees: All nuclear power reactor facilities holding an operating license (OL) or a 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. In this regard, this notice supplements and reemphasizes the information contained in IE Information Notice 84-70, Reliance On Water Level Instrumentation With a Common Reference Leg. This notice serves to alert licensees and applicants to the need for operators to recognize normal and abnormal water level instrument behavior under various plant conditions. 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: On February 13, 1985, while performing a reactor startup at TVA's Browns Ferry Nuclear Plant, a half scram was received on low reactor water level. A few minutes before the half scram, the operators had noticed that two of the three narrow-range General Electric measurement and control (GEMAC) water level instruments were reading approximately 40 inches of reactor vessel water level. The other narrow-range instrument was indicating approximately 10 inches of water level. Two wide-range YARWAY instruments were observed by the operators to be indicating approximately 40 inches. At the time of the half scram, reactor pressure was approximately 40 psig and reactor coolant temperature was approximately 286 F. Although four of the instruments observed by the operators indicated nearly normal reactor water level (33 � 5 inches), actual reactor water level was approximately 10 inches. 8508230052 . IN 84-70 SUPP. 1 August 26, 1985 Page 2 of 3 The operators incorrectly concluded that the narrow-range instrument indicating 10 inches was erroneous since four other level instruments were indicating high. The two GEMAC instruments that indicated 40 inches share a common reference leg. This reference leg had lost some of its water inventory, causing all level instrumentation that tapped off that leg to erroneously indicate high. The GEMAC instrument that was reading 10 inches tapped off a different reference column than the aforementioned instruments. The two wide-range YARWAY instruments each have separate reference columns not shared by any of the narrow-range GEMAC instruments. At approximately 40 psig reactor pressure and with actual reactor water level at 33 � 5 inches, the YARWAY instruments should indicate �60 inches, a normal YARWAY level indication. A YARWAY level indication of 40 inches should have alerted the operators that water level was abnormally low. The operators did not check the shutdown vessel flooding range level indication which was available in the control room. This instrument would have confirmed actual low water level conditions since it is calibrated for cold plant conditions. Discussion: The cause of this event was a partial loss of water inventory from a reference leg that is common to several water level instruments, including instrument channels required by the Technical Specifications. From a reactor safety perspective, this event highlights the need for operators to be cognizant of level instruments that share a common reference leg and also to be aware of level instrument behavior subject to various plant conditions. A problem in a reference leg, such as that experienced at Browns Ferry, not only affects level indication, but may also affect the operability of reactor protection instrument channels required by the Technical Specifications. In this event, the most critical technical specification instruments affected were two level switches, one in each reactor protection trip system train. These switches were inoperable since they were common to the faulty reference leg. Had the operators realized earlier what instruments and switches were affected by the faulty reference leg, proper corrective action may have been taken to shut down the plant in a timely manner in accordance with the Technical Specifications. It is important that operators understand level instrument response to various plant conditions. One way to achieve this understanding is through training to emphasize level instrument system design, temperature and/or pressure compensation, instrument calibration, and the purpose of the instruments (i.e., process monitor s. control). Although the YARWAY instruments are designed to provide wide-range accident level indication and are calibrated to be most accurate at normal operating reactor pressure, the operators at Browns Ferry could still have used these instruments as additional level response indication. Had the operators realized that a YARWAY level of 40 inches was abnormally low for the existing low reactor pressure, they might have been alerted earlier to the fact that actual vessel water level was low. . IN 84-70 SUPP. 1 August 26, 1985 Page 3 of 3 Licensees and applicants may wish to 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. Operator awareness of the effects a malfunction in a common reference leg can have on the level instruments and recognition of proper water level indication subject to various plant conditions can enhance plant safety. No specific action or written response is required to this information notice. If you need additional information about this matter, please contact the Regional Administrator of the appropriate NRC regional office or one of the technical contacts listed below. Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contacts: Eric W. Weiss, IE (301) 492-9005 P. D. Wagner, Region II (404) 221-2688 Attachment: List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021