Detection of fuel washout from leaking fuel rods during operation of WWER power units

Fuel failures still occur during operation of nuclear power plants. One of the possible and most severe consequences of a fuel failure is that fragments of fuel pellets or fuel grains may be washed out from the leaking fuel rod into the coolant. Reliable detection of fuel washout is important for further handling of leaking fuel assemblies. An indication of fuel washout is achievable in the frame of coolant activity evaluation during reactor operation. For this purpose, 134I activity is historically utilized in WWER power units. However, observed 134I activity may increase during operation even in cases when leaking fuel in the core is absent, and fuel deposits are the only source of the fission products release. The present paper discusses a criterion elaborated for detection of fuel washout from leaking fuel rods during operation of WWER power units. Some examples of application of this criterion at operating NPPs are demonstrated.