A multi-scale and multi-physics simulation methodology with the state-of-the-art tools for safety analysis in Light Water Reactors applied to a Turbine Trip scenario (Part II)

The development of the computer technology, as well as the research in the different science fields governing the core behavior of a Light Water Reactor, allows implementing all the known physics and consider detailed scales of analysis. Conversely to conservative approaches, the Best Estimate approach applies the available science by means of models and correlations that are applied in different scales using simulation tools. With this approach, the critical elements of the core can be evaluated with realistic predictions that can adjust the operation conditions and core design to more cost-efficient values without compromising the safety of the Nuclear Power Plant. The authors of this paper present the second part of a multi-scale and multi-physics methodology for the evaluation of fast transients in Light Water Reactors. In this part, the results obtained from the coupled Neutron Kinetics and Thermal-Hydraulics channel-by-channel core model are used for a detailed thermal-hydraulic pin-by-pin analysis and thermomechanics pin model. The aim of this work is to evaluate the safety analysis of the critical fuel rod in Turbine Trip scenario. For that purpose, the critical fuel rod is located using the minimum Critical Power Ratio. This safety variable is predicted in a thermal-hydraulic pin-by-pin model using CTF-UPVIS code. Afterwards, the conditions of the critical rod are loaded in a pin model for a simulation with FRAPCON/FRAPTRAN. Moreover, this paper proves the Best Estimate capability of the presented methodology by means of comparing the results with equivalent simulations that are more conservative, or consist of more limited simulation scales. On the one hand, the Best Estimate prediction is compared against the envelope of the minimum Critical Power Ratio along the axial nodal distribution of the simulated fuel rod. In addition, another comparison is made against assuming constant fuel-cladding gas conductance, showing the enhancement added by considering the axial distribution of this parameter, provided by FRAPCON/FRAPTRAN. On the other hand, the results of this methodology are compared against the limitation of accounting only the bundle radial average value of the minimum Critical Power Ratio. Furthermore, the Best Estimate results are complemented with an Uncertainty and Sensitivity analysis that will define the statistical boundaries of the prediction according to the 95/95 criterion.