Listening in real time - the case of Diablo Canyon

With the aid of neutron noise analysis, a number of problems have been resolved in nuclear power plants over the years. For example, the Diablo Canyon PWR plant in California began to experience an increasing number of neutron signal alarms at the end of one of its operating cycles. As a result, the regulators ordered the plant to identify the source of the alarms and to shut the plant down if the alarms revealed a significant safety issue. The regulators, however, gave the plant permission to continue to operate while conducting an investigation into the cause of the alarms.

It was postulated that the alarms could be due to excessive vibration of the core barrel or other reactor internal components. Therefore, using the output of the ex-core neutron detectors, the vibration of reactor internals was measured. The results showed normal vibration levels for the reactor internals. Next, neutron signals were cross correlated with core-exit thermocouples (CETs) and other sensors. This helped narrow the problem down to core flow anomalies caused by new fuel assemblies installed in the plant in a previous refuelling outage. Further analysis of the noise data demonstrated that the flow anomalies were not significant and the plant was thus allowed to continue to operate. In the meantime, the neutron alarm setpoints were raised to limit the frequency of the alarms.