Resolution of Generic Safety Issues: Issue 19: Safety Implications of Non-Safety Instrument and Control Power Supply Bus ( NUREG-0933, Main Report with Supplements 1–34 )
On November 10, 1979, an event occurred at the Oconee Power Station, Unit 3, that resulted in loss of power to a non-Class 1E 120-volt AC single-phase power panel that supplied power to the integrated control system (ICS) and the nonnuclear instrumentation (NNI) system. This loss of power resulted in control system malfunctions and significant loss of information to the control room operator. Specifically, an NNI-channel inverter tripped due to blown fuses, with consequent failure to automatically transfer loads from the DC power source to the regulated AC power source. Loss of power to the NNI rendered control room indicators and recorders for the reactor coolant system (except one pressure recorder) and most of the secondary plant systems inoperable, causing loss of indication for systems used for decay heat removal and water addition to the reactor vessel and steam generators. The loss of power existed for approximately three minutes, until an operator could reach the equipment room and manually switch the inverter to the regulated AC source.
The incident caused concern about the implications of loss of non-Class 1E (i.e., nominally non-safety) power supply buses for the ability to achieve cold shutdown.
IE Bulletin No. 79-27,24 issued on November 30, 1979, called on licensees and applicants to review the Class 1E and non-Class 1E buses supplying power to safety and nonsafety-related instrumentation and control systems which could affect cold shutdown capability. The bulletin also directed preparation of pertinent emergency procedures (or review and possible modification of existing ones).
Generic consideration of the licensee and applicant responses to IE Bulletin No. 79-2724 is included in Sub-task 4.2 of the task action plan for USI A-47, "Safety Implications of Control Systems" (May 1982) as part of broader efforts to evaluate the effects of loss of power supply to control systems (Task 4).
This issue is included in the scope of the broader efforts on USI A-47, "Safety Implications of Control Systems" and isolating it for separate consideration is not warranted.