Leak at the generator of a nuclear power plant

A leak started on the hydrogen coolant circuit of one of the power generators of the plant. It was indirectly detected thanks to the increased need to generate new hydrogen for cooling. The trend kept growing in the following days. After attempts to isolate the leak, the operator decided to shut down the turbine-generator unit and replaced all the hydrogen coolers.

This is a more detailed sequence of events:
28 August – First signal of an increased make-up rate of hydrogen cooling gas to the main generator. The normal make-up rate of hydrogen was approximately 600 standard cubic feet per day (SCFD). This increase was an indication of hydrogen losses due to a leak.
Technicians sampled areas of the turbine building confirming absence of an explosive mixture. They verified through periodic tours of the turbine building that no welding, cutting, or grinding was being conducted that could result in an ignition source for any potential hydrogen concentration build-up in the turbine building.
10 September - Make-up rate of hydrogen increased to approximately 4,000 SCFD by September 10, 2002. The operator tried unsuccessfully to isolate the hydrogen cooler suspected to leak.
11 September – After unsuccessful attempts to isolate the hydrogen leak, the operator decided to take the unit off-line to implement a repair., The power was reduced to approximately 10% and the generator was taken off-line.
13 September - All four generator hydrogen coolers were replaced, and the unit was restored to full power operations.

hydrogen cooler

The nuclear power plant consisted in two four-loop Westinghouse pressurized water reactors .
• Units 2 was completed in 1974 with capacity of 1,032 MWe.
• Units 3 was completed in 1976, with a of 1,051 MWe.

DESCRIPTION OF THE GENERATOR COOLING SYSTEM
In a typical H2 coolant system, the hydrogen circulates in a loop between the generators and the coolers, which function are heat exchangers which bring the temperature of the hot hydrogen exiting the generator loop back to near room temperature.
Due to unavoidable hydrogen minor losses, the hydrogen in the circuit has to be topped up by make-up hydrogen, which can be produced onsite and/or supplied by an onsite storage system o be regularly replaced via delivery. The NRC report does not specify which make-up system was present in this plant. The daily need for make-up hydrogen was approximately 1.5 kg. During the reported event, the daily losses were amounting at almost one order of magnitude more (10 kg/d). A New York Times news was reporting a value of 25 kg/d.

The fact that all four were replaced induce to conclude that the loss of hydrogen was occurring there.

The damage was limited to the loss of electricity production suring few days, and the reapclements of 4 hydrogen coolers.

The operator regularly monitored the evolution of the hydrogen concentration in the turbine hall, to ensure that the value was remaining below the safe flammability threshold, and eventually replaced all hydrogen cooling systems, because unable to locate and isolate the leak.

A nuclear authority performed an inspection of the plant after the notification of the incident. The inspection scope covered the fire protection measures, the training of personnel, the maintenances procedures, the risk assessment and the foreseen emergency actions. The findings were all positive and no corrective action was issued.