Investigation of feedback on neutron kinetics and thermal hydraulics from detailed online fuel behavior modeling during a boron dilution transient in a PWR with the two-way coupled code system DYN3D-TRANSURANUS

• General coupling interface was developed for the fuel performance code TRANSURANUS.
• With this new tool simplified fuel behavior models in codes can be replaced.
• The reactor dynamics code DYN3D was coupled to TRANSURANUS at assembly level.
• The feedback from detailed online fuel behavior modeling is analyzed for reactivity initiated accident (RIA).
• The thermal hydraulics can be affected strongly even in fresh fuel assemblies.

Recently the reactor dynamics code DYN3D (including an internal fuel behavior model) was coupled to the fuel performance code TRANSURANUS at assembly level. The coupled code system applies the new general TRANSURANUS coupling interface, hence it can be used for one-way or two-way coupling. In the coupling, DYN3D provides process time, time-dependent rod power and thermal hydraulics conditions to TRANSURANUS, which in case of the two-way coupling approach replaces completely the internal DYN3D fuel behavior model and transfers parameters like radial fuel temperature distribution and cladding temperature back to DYN3D. For the first time results of the coupled code system are presented for a post-critical-heat-flux heat transfer. The corresponding heat transfer regime is mostly film boiling, where the cladding temperature can rise several hundreds of degrees. The simulated boron dilution transient assumed an injection of a 36 m3 slug of under-borated coolant into a German pressurized water reactor (PWR) core initiated from a sub-critical reactor state (extreme reactivity initiated accident (RIA) conditions). The feedback from detailed fuel behavior modeling was found negligible on the neutron kinetics and thermal hydraulics during the first power rise. In a later phase of the transient, the node injected energy can differ 25 J/g, even still around 20 J/g for nodes without film boiling. Furthermore, the thermal hydraulics can be affected strongly even in fresh fuel assemblies, where film boiling appeared in one node in the two-way approach in spite of no onset of film boiling in the one-way approach. For nodes with film boiling, in both coupling approaches, the two-way approach determined always higher maximum node average fuel enthalpies by about 100 J/g and higher maximum node clad surface temperatures by about 230 °C for the corresponding fresh fuel assemblies. Since the numerical performance of DYN3D-TRANSURANUS was fast and stable for these extreme transient conditions. It is, therefore, concluded that the coupled code system can potentially improve the assessment of safety criteria, at a reasonable computational cost.