Multi-physics code system with improved feedback modeling

Fuel temperature (Doppler) feedback modeling in the coupled sub-channel thermal-hydraulic/time dependent neutron transport codes system CTF/TORT-TD was improved by accounting for the burnup dependence of the fuel thermal conductivity. TORT-TD is a three-dimensional (3D) time dependent neutron-kinetics code based on the discrete ordinates (SN) method. CTF is the Reactor Dynamics and Fuel Modeling Group (RDFMG) version of the sub-channel thermal-hydraulics code COBRA-TF (COlant Boiling in Rod Arrays - Two Fluid). A burnup-dependent fuel rod model, which takes into account the degradation of the fuel thermal conductivity at high burnups and the effects of burnable poisons, such as Gadolinium, was implemented in CTF. The model is applicable to UO2 (uranium dioxide) and MOX (mixed oxide) nuclear fuels - it includes the modified Nuclear Fuel Industries (NFI) model for UO2 fuels and the Duriez/Modified NFI model for MOX fuels. The in-pellet fuel temperature distributions predicted by CTF/TORT-TD were compared to reference CTF/TORT-TD/FRAPCON calculations, in which the fuel rods were modeled with the fuel performance code FRAPCON. These comparisons were carried out for a 4 × 4 pressurized water reactor (PWR) pin array at hot full power (HFP) steady state conditions. The CTF/TORT-TD fuel temperature predictions were consistent with the CTF/TORT-TD/FRAPCON results. This fact demonstrated that CTF with the new fuel thermal conductivity model can predict the temperature field within light water reactor (LWR) fuel rods as accurately as FRAPCON. Therefore, CTF/TORT-TD calculations can be carried out in fast scoping studies instead of the computationally expensive CTF/TORT-TD/FRAPCON calculations. The performed statistical analyses indicated an improved accuracy of fuel temperature calculations relative to the CTF/TORT-TD/FRAPCON reference numerical solution. Furthermore, better agreement between CTF/TORT-TD and CTF/TORT-TD/FRAPCON in calculated neutronic reactivity was found when fuel burnup effects were considered in CTF/TORT-TD. Therefore, the improved CTF/TORT-TD can be seen as a high fidelity multi-physics computational tool capable of providing accurate and efficient simulations for practical reactor core design and safety analysis.