A software error that went uncorrected for over a year at the Wolf Creek nuclear power plant in Kansas which could have overestimated iodine emissions in the event of an accident ended in a white finding, U.S. regulators said.

Considered to be of low to moderate safety significance, the issue first came to light from Nuclear Regulatory Commission (NRC) inspectors during a scheduled emergency preparedness exercise at the single-unit pressurized water reactor plant in Kansas last November.

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The exercise simulated the effects of an accident caused by an earthquake leading to an automatic plant trip, reactor coolant system leak and other system failures. These would have resulted in radiological release to the environment.

In normal operation, levels of noble gases, radioactive iodine and radioactive particulates in the plant’s main ventilation stack end up monitored by separate detectors. In an emergency situation, the detectors for iodine and particulates would not be available, so the plant’s radiological assessment software (Electronic Dose Calculation Program, or EDCP) calculates assumed release rates from the measured rates of noble gases using a default noble gas to iodine ratio of 10:1. Wolf Creek’s EDCP was not applying the default ratio correctly when the monitor was in accident mode, overestimating the concentration of iodine and particulates by a factor of 10.

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Later, NRC inspectors found the licensee, the Wolf Creek Nuclear Operating Corporation, knew since November 2012 the ECDP was potentially inaccurate and had initiated a service request to its own information systems department to investigate the problem, but no one had acted on the request.

The NRC said the incident was a “white finding,” the second level of the regulator’s four-stage color-coded classification of safety significance. The first and least serious stage is green.

“This finding is more than minor because it affected the licensee’s ability to implement adequate measures to protect the health and safety of the public,” the NRC ruled in its preliminary report dated April 2. “The licensee failed to verify the existence of a safety-significant problem and subsequently failed to resolve the problem within a timeframe appropriate to its safety significance,” it said.

Following the exercise in November, Wolf Creek Nuclear Operating Corporation put in place interim measures to address the problem and they corrected the issue in an upgrade to the EDPC in February.

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