The validity of gap conductance approach in realistic ECCS evaluation methodology has been assessed with fuel burnup change. Studies of sensitivity as well as combined uncertainty of each uncertainty parameter to gap conductance have been performed by utilizing FRAPCON-3.4a and FRAPTRAN-1.4 code. Peak cladding temperature (PCT) evaluation during LOCA transient was also carried out by changing uncertainty range of the gap conductance. For the evaluation of combined uncertainty and PCT, several sets of 124 inputs were produced with the uncertainty parameter combinations. Sensitivity study indicated that, in beginning-of-life (BOL) fuel condition, uncertainty parameters related to the gap width showed a strong impact to the gap conductance. But, as fuel burnup increased to 30 MWd/kgU, fission gas release (FGR) appeared as a dominant factor to the conductance. Combined uncertainty study revealed that, to encompass most of the 124 conductance curves, a different multiplicative factor to the base case has to be set as fuel burnup changed. From this study we noticed that the gap conductance approach seems to be effective to simulate the fuel rod uncertainties in BOL fuel condition. But as the fuel burnup changed to 30 MWd/kgU, when fuel and cladding contact happened, the current gap conductance approach is not enough to represent the overall uncertainties. The reason is discussed in the paper.
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