Indirect evaluation of pellet-cladding contact from visual analysis of fuel rod elongation

Since the implementation of the TVSA-T fuel at Temelín NPP (VVER-1000 owned and operated by ČEZ a.s.) the post irradiation inspection programme has been in place performed jointly by the vendor (JSC TVEL) and ÚJV and Research Centre Řež. Its main goal is to monitor the dimensional stability of the fuel assemblies, however other information are obtained as well. Presented work describes the validation of the fuel performance codes used at ÚJV Řež in support of the Czech NPPs using the outcome of the evaluation of the elongation of the fuel rods. Apart from the straightforward comparison of the measured to predicted elongation, it is also possible to focus on the variation of the length of the neighbouring rods. Due to design features of the TVSA-T assembly the rod growth is possible only upwards, therefore this analysis may be easily performed just by comparing the level of the top end plugs of the rods. Our experience has shown that as long as there is no contact between the fuel pellets and the cladding there is very little variation in the elongation of neighbouring rods, thus the observed onset of the uneven growth may be attributed to the closing of the fuel-cladding gap.Data from the inspections of several twice and three times burned assemblies are analysed for above mentioned indications of the pellet-cladding contact. The outcomes are compared with the predictions of the TRANSURANUS code using a statistical approach, which is taking into account the allowable range of the fuel fabrication characteristics. The goal of the effort is to indirectly validate the models used in the pellet-clad interaction risk assessment analysis for core reloads by comparing the predicted and observed onset of interaction between the fuel column and the cladding. (author)