Numerical simulation of the telescope sipping of a leaking VVER fuel assembly

The telescope sipping procedure is applied at Paks NPP to identify leaking fuel assemblies. The activity release during sipping test was simulated using the TSKGO computer code. The code calculates the activity inventory in the free volume of the fuel rod and estimates the release of different gaseous and volatile fission products during steady state operation in the reactor, storage in the spent fuel pool and transient (e.g. sipping test) conditions. The calculated results show that the activity release during sipping tests is driven by the expansion of the gas volume inside the fuel rod and the amount and composition of the released radioactive isotopes highly depend on the defect location.