Abstract
In-core neutron detectors were used for noise diagnostic measurement in a commercial PWR NPP. Phase dependence versus frequency between axially placed self powered neutron detectors (SPND) exhibited linear behaviour in some cases. Correlation was found between the existence of linear phase behaviour and the power of the Unit. More thoughtful consideration showed a dependence on local power, and on overheating of the surface of fuel elements. Data extracted from conventional measurement of top of core temperature sensors and in-core SPNDs, using the conventional system for linear power production estimation showed that linear phase takes place where linear power production is over 2 MW/m. One dimensional thermohydraulic calculation pointed out that at those places the surface temperature is near or over the saturation value. Thus the appearance of linear phase versus frequency can be attributed to phenomena of local subcooled boiling and therefore its existence can be used for boiling detection in PWRs. All basic considerations had been checked beforehand in model experiments and are cited here.
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