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Information Notice No. 93-44: Operational Challenges During a Dual-Unit Transient
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON D.C. 20555 June 15, 1993 NRC INFORMATION NOTICE 93-44: OPERATIONAL CHALLENGES DURING A DUAL-UNIT TRANSIENT Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform addressees of an event during which an atypical shift staffing level and lack of familiarity with the tasks to be performed in that shift configuration affected the ability of the control room operating shift to respond to the event. 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 December 31, 1992, a dual-unit reactor trip initiated by an electrical fault in the switchyard occurred at the Sequoyah Nuclear Plant. Units 1 and 2 at Sequoyah share a common control room. At the time of the event the control room reactor operator staffing was the minimum allowed by the plant Technical Specifications (TS); shift staffing consisted of five licensed senior operators (the shift operations supervisor, one assistant shift operations supervisor for each unit, the shift technical advisor, and an additional senior operator) and three reactor operators - two assigned to Unit 1 and one assigned to Unit 2. The NRC inspection team investigating the event concluded that the Unit 2 response to the event, with one reactor operator, was hampered by lack of adherence to procedures, operator errors, and an unnecessary cooldown of the reactor coolant system (RCS) after the reactor trip. The Unit 1 response to the same event, with two reactor operators, was normal. Discussion An NRC inspection team reviewed the effects of the Unit 2 staffing level during the event with regard to the crew's ability to stabilize the unit after the reactor trip occurred. The inspection team concluded that the Unit 2 staffing level delayed the recovery of plant systems, contributed to erroneous decisions made by the assistant shift operations supervisor (ASOS), and 9306080117. IN 93-44 June 15, 1993 Page 2 of 3 overburdened and challenged the crew's ability to perform tasks in the manner in which it was trained. Control room simulator training scenarios had not been conducted using only one reactor operator. Although the normal shift staffing and training consisted of two reactor operators for each unit, licensee operational staffing practices resulted in the frequent use of one reactor operator for one of the units. The licensee used this reduced staffing configuration 12 times in December 1992. In this configuration, with the dual-unit transient, the single reactor operator performed all of the recovery evolutions for the unit as directed by the ASOS in charge of the unit. Because of delays caused by the one reactor operator performing primary and secondary system recovery evolutions, as required by the applicable recovery procedures, an RCS cooldown resulted. This was a direct result of the crew not adequately controlling auxiliary feedwater after the reactor trip. The atypical staffing level also contributed to problems involving the decision of the assistant shift operations supervisor to use the normal-batch borate process in response to the cooldown rather than the emergency borate process as required by the emergency procedure. The decision not to use the emergency borate process was made, in part, because of the reluctance of the assistant shift operations supervisor to commit the one reactor operator to the emergency borate process, which restricts the operator's attention to monitoring only that activity. As a corrective action to prevent recurrence, the licensee implemented revised administrative controls to ensure that the control room staffing will be maintained at two reactor operators for each operating unit. The need for additional training at minimum staffing levels is being evaluated by the licensee. Related Generic Communications 1. IN 85-80, "Timely Declaration of an Emergency Class, Implementation of an Emergency Plan, and Emergency Notifications," dated October 15, 1985 2. IN 91-77, "Shift Staffing at Nuclear Power Plants," dated November 26, 1991 . IN 93-44 June 15, 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: S. M. Shaeffer, RII (615) 842-8001 D. R. Desaulniers, NRR (301) 504-1043 Attachment: List of Recently Issued NRC Information Notices .
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