The Nuclear Regulatory Commission (NRC) last week issued a bulletin to all holders of nuclear plant operating licenses in the U.S., alerting them to a potential design vulnerability discovered at Exelon’s Byron Nuclear Generating Station in January that it says "could have damaged the plant’s emergency core cooling system."

The federal body said that Unit 2 of the Byron plant in Ogle County, Ill., shut down automatically on Jan. 30 due to unbalanced voltage entering the onsite power distribution system from the transmission network. The bulletin requests information about their facilities’ electric power system designs to determine if more regulatory action is needed.

A 345-kilovolt (kV) system provides offsite power (three-phase power–A, B, and C phases) to each Byron unit’s station auxiliary transformer (SAT). According to the NRC, each unit’s set of SATs has sufficient capacity to supply the necessary auxiliary power for the unit when operating at full load. "Each unit’s system auxiliary power supplies are available to all safety auxiliary equipment of both units, and therefore, serve as the second source of offsite power to the other unit," it said.

The engineered safety features (ESF) buses and equipment are protected by two levels of undervoltage protection schemes. By design, in the event of loss of offsite auxiliary power or undervoltage or sustained degraded voltage conditions, the auxiliary power for safe shutdown is supplied automatically from redundant Class 1E diesel-generators located on the site. "All of the equipment relied upon to shut down the reactor safely and to remove reactor decay heat for extended periods of time following a loss of offsite power and/or a loss-of-coolant accident are supplied with ac power from the ESF buses," the NRC said in its July 27 bulletin.

On Jan. 30, Byron’s Unit 2 experienced an automatic reactor trip from full power because the reactor protection scheme detected an undervoltage condition on the 6.9-kV buses that power reactor coolant pumps (RCPs) B and C (one of two phase undervoltage on two of four RCPs initiate a reactor trip). The undervoltage condition was caused by a broken insulator stack of the phase C conductor for the 345-kV power circuit that supplies both SATs. This insulator failure caused the phase C conductor to break off from the power line disconnect switch, resulting in a phase C open circuit and a high impedance ground fault.

"The open circuit created an unbalanced voltage condition on the two 6.9-kV nonsafety-related RCP buses and the two 4.16-kV ESF buses. ESF loads remained energized momentarily, relying on equipment protective devices to prevent damage from an unbalanced overcurrent condition. The overload condition caused several ESF loads to trip," the bulletin said.

Plant operators at the 1987-commissioned reactor diagnosed the problem eight minutes later and manually tripped the necessary circuits.

"The plant’s electric power system’s protection scheme was not designed to sense the loss of one of three power phases and automatically trip circuits to isolate the degraded outside power source and switch to emergency backup power," the NRC said, warning that the "degraded offsite power source potentially could have damaged the plant’s emergency core cooling system."

The current Bulletin requests licensees to provide information on their electric system designs within 90 days.

Sources: POWERnews, NRC