Nine Mile Point Nuclear Generating Station - 1991 Site Area Emergency

1991 Site Area Emergency

On August 13, 1991 a "site area emergency" was declared at the plant. According to Time magazine, this was the third occasion an SAE had been declared at a US nuclear plant.

The emergency was due to an electrical fault which caused a momentary loss of electrical power to the reactor and control room. The operators shut down the reactor in accordance with the emergency plan requirements. There were no injuries or release of radiation as a result of the incident. The reactor came back online 13 hours after the incident.

From NRC Information Notice 91-64, Supplement 1:

The facility was operating at full power when a phase-to-ground electrical fault occurred on phase B of the Unit 2 main power transformer. This resulted in a trip of the main generator, main turbine, and reactor. The fault also caused the voltage on electrical distribution phase B buses to drop to about half of its nominal value for approximately 200 milliseconds, after which it returned to normal.

This momentary voltage drop resulted in the simultaneous loss of power output from each of the five Non-Class 1E uninterruptible power supplies. Exide's UPS units have internal continuously charged back-up batteries to prevent a loss of control logic power. Exide's UPS control logic circuitry receives, processes, generates, and sends electrical signals essential for proper UPS operation. However, in this incident the back-up power battery packs were apparently past their useful life and were completely discharged.

The loss of power from the UPS units caused a loss of all the following: the control room annunciators, the safety parameter display system computer, control rod position indication, the plant process computer, the core thermal limits computer, the feedwater control system, some of the lighting for the plant, the plant radio and paging systems, some instrumentation for balance-of-plant systems, and some instrument recorders.

The loss of control room annunciators concurrent with the plant transient resulting from automatic tripping of the main generator, main turbine, and reactor caused the licensee (Niagara Mohawk) to declare a site area emergency in accordance with the site emergency plan. The loss of control rod position indicators and other equipment losses burdened the operators in implementing the emergency procedures. However, the operators shut down the plant in accordance with emergency procedures. About 13 hours after the plant trip, the reactor was placed in a cold shutdown condition; and approximately 1 hour later, the licensee ended the site area emergency.