Modification and update of FROBA-ROD code and its applications in fuel rod behavior analysis for PWRs

To analyze the behavior of PWRs fuel rods under steady operation conditions, some models such as modified Forsberg-Massih model and TUBRNP model were applied to FROBA-ROD code, which was developed by Xi'an Jiaotong University in 2012. Then more significant fuel rod behaviors could be simulated by FROBA-ROD code, which facilitated the safety analysis of fuel elements especially under accident conditions. To conduct transient thermal calculations and to improve the simulation accuracy, existing thermal models in FROBA were modified with the new fuel rod thermal conductivity model and burnup distribution model. The new models were validated by comparing it with the results of authorized commercial codes FRAPCON-4.0 and ABQUS and by comparing it with IFA-432 Rod 1 experimental benchmark data conducted by Oak Ridge National Laboratory. The relative calculation errors of centerline temperature calculated by modified FROBA-ROD, original FROBA-ROD and FRAPCON were 5.44%, 11.68% and 9.47% separately, which indicated that the modification and update of FROBA-ROD were valid. Modified FROBA-ROD code was applied to perform the behavior analysis of the AP1000 fuel rod. The simulation results indicated that there were enough safety margins for fuel rod behavior of AP1000 at rated operation conditions. The maximum oxidation thickness of cladding appeared at the upper part of the cladding tube in AP1000, which indicated that this area suffered a higher risk of cladding failure. Modified FROBA-ROD code is expected to deal with fuel rod scenarios under accident conditions after some additions of transient models.