Investigation of Feedback on Neutron Kinetics and Thermal Hydraulics from Detailed Online Fuel Behavior Modelling during a Boron Dilution Transient in a PWR with the Two-way Coupled Code System DYN3D-TRANSURANUS

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-crisis heat transfer. The corresponding heat transfer regime is mostly film boiling, where the cladding temperature can rise up to several hundreds of degrees. The simulated boron dilution transient assumed an injection of a 36 m3 slug of under-borated coolant into a German PWR core initiated from a sub-critical reactor state (extreme 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 up to 22 J/kg, even 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 a fresh fuel assembly. 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 improve the assessment of safety criteria, at a reasonable computational cost.JRC.E.3-Materials researc