Modeling RIA benchmark cases with FRAPTRAN and SCANAIR: A comparative exercise

The need for defining new RIA safety criteria has demonstrated the importance of performing a rigorous assessment of the transient codes capabilities. The present work is a comparative exercise devoted to identify the origin of the key deviations found between the predictions of FRAPTRAN-1.4 and SCANAIR-7.1 codes. To do so, the results of the calculations submitted by CIEMAT to the OECD/NEA RIA benchmark (CIP0-1, CIP3-1, VA-1 and VA-3 experiments) have been exploited. The advantage of these simulations is that the initial rodlet characterization, the transient power and the user assumptions were defined similarly in the two codes, which is essential to perform a consistent comparison. The comparative assessment has led to relevant insights about the codes performances. As for differences reported in the CIP0-1 clad temperatures, the heat transfer modeling across the oxide layer has been found to be mainly responsible. Likewise, the estimation of the gap closure time is directly related to the systematically higher clad hoop deformations predicted by FRAPTRAN-1.4. The differences between FRAPTRAN-1.4 and SCANAIR-7.1 fission gas release predictions reflect the different strategies adopted by the codes to model the complex processes involved in gas release during the transient.