Experience in Detection of Fuel Washout from Leaking Fuel Rods During Operation of WWER Power Units

Fuel failures still occur during operation of nuclear power plants (NPPs). 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 evaluations 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 paper discusses experience in application of a recent technique for detection of fuel washout during operation of WWER power units. The proposed technique is capable of distinguishing whether the increase in activity of short-lived radionuclides is due to fission products released from the existing fuel deposits in the core or it is due to washout of fuel particles from leaking fuel. (author)