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Information Notice No. 87-05: Miswiring in a Westinghouse Rod Control System
SSINS NO.: 6835 IN 87-05 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 February 2, 1987 IE INFORMATION NO. 87-05: MISWIRING IN A WESTINGHOUSE ROD CONTROL SYSTEM Addressees: All Westinghouse nuclear power reactor facilities holding an operating license or a construction permit. Purpose: This notice is to alert addressees to the miswiring of circuit card interconnections in the Westinghouse-supplied rod control system of a Westinghouse reactor. It is suggested that recipients 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: Beaver Valley Unit 1 On December 1 1986, with the plant at full power, the bank selector switch in manual, and all rods fully withdrawn, the operator inserted control rods consistent with a minor power redution. Although control bank 'D' rods were expected to move, the operator observed that control bank 'A' rods inserted two steps. Subsequent investigation revealed that two circuit boards in the bank overlap unit (BOU) of the rod control system had been wired together improperly. This miswiring was determined to have existed since initial installation of the rod control system when the plant was new. Rod control systems of this design have been supplied for approximately 60 other Westinghouse nuclear units. Discussion: With the rod control system in manual or automatic, the BOU automatically selects the sequence and overlap of the control banks. To keep track of the process, each in or out step of the control banks is recorded by a counter in the BOU. The BOU count increases from 000 to 999 as control bank rods are stepped out. Shutdown bank motion does not affect BOU count. To support special evolutions such as troubleshooting and repair, the BOU is fitted with "+1", "-1" and "reset" push buttons which permit advancing, retarding, or zeroing, respectively, the BOU count. Depressing any of these buttons does not 8701220053 . IN 87-05 February 2, 1987 Page 2 of 2 result in rod motion, but only in an adjustment of the BOU count memory. The nature of the incorrect wiring was such that depressing the +1 button caused the BOU count to reset to 000, as would depressing the reset button. With the BOU count at or near 000, and the bank selector switch in manual or automatic, any demand for rod motion would result in movement of bank 'A' rods, as was observed in this case. This is a violation of technical specification rod bank insertion limits which require all control bank 'A' rods to be fully withdrawn when the plant is operating in the power range. This is to ensure that adequate negative reactivity due to rods is available to safely shut down the reactor in the event of a reactor trip. After testing eliminated the possibility of a failed BOU electronics card, it was concluded that the BOU counter had been zeroed by operation of the BOU +1 button. In a properly wired BOU, depressing the +1 and -1 buttons an equal number of times would have no net effect on BOU count, i.e. , the final count would be the same as it had been before manipulating the buttons. In the miswired configuration, the BOU count used to supervise rod motion was reduced to 000. Investigation showed, however, that the visual display of the counter incremented and decremented in step with the push buttons. Thus, although the internal counter which controls rod sequence and overlap was reset to 000, the display indicated normal function. In addition, since the BOU count automatically resets to 000 on a scram signal, any existing counter misalignments are erased at that time. These factors may have masked the problem. This problem was rectified at Beaver Valley Unit 1 by rewiring the BOU circuit board interconnections as prescribed by the original drawings, which were correct. The unit remained at power during repairs. The Beaver Valley Unit 2 rod control system was inspected and found not to be affected. No specific action or written response is required by this information notice. If you have any questions about this matter, please contact the Regional Administrator of the appropriate regional office or this office. Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: Kevin Wolley, IE (301) 492-9428 Attachment: List of Recently Issued IE Information Notices .
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