Unrelated Incidents Cut Power Output at Two Palo Verde Reactors

Two of the three units at the country's largest nuclear plant cut their power output Monday following unrelated mechanical problems.

Unit 2 at the Palo Verde plant operated by Arizona Public Service shut down completely just before 6 p.m. According to a report filed with the Nuclear Regulatory Commission, the pressurized water reactor tripped automatically from full power. An initial investigation suggested the shutdown resulted from a tripped power supply circuit breaker for a reactor coolant pump. According to the report, operators noted that the 1A RCP was not running during the shutdown.

At about the same time at unit 3, a control rod dropped unexpectedly when an associated circuit breaker tripped open, according to another NRC report. The other 88 control rods were unaffected, but operators began to power down the reactor as a precaution. Early Tuesday morning, with the unit at 48 percent power, workers cancelled the shutdown after replacing and testing a power switch for the dropped control rod and realigning it with the other control elements, according to the NRC.

Neither incident posed a threat to public or worker safety. 

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