Application of DYN3D/ATHLET and DYN3D/RELAP5 coupled codes to simulations of RUTA-70 reactor with CERMET fuel

Computer models of the RUTA-70 reactor facility for simulations with the coupled code systems DYN3D/ATHLET and DYN3D/RELAP5 have been developed. The main specific features of the RUTA reactor concept are low pressure in the primary circuit and natural circulation of the primary coolant at reactor power levels less then 30% of the rated power. According to the RUTA facility design requirements the saturated boiling of primary coolant must be excluded for all regimes of normal reactor operation. Only a small degree of subcooled boiling is allowed in the reactor core. However, the steam generation in the primary circuit can occur during accidents, for example, as a result of the pump failure event. Transition from the single-phase to the two-phase flow in natural circulation systems can lead to unstable system behavior, and this possibility should be properly analysed. For this reason, the thermal-hydraulic model of RELAP5 code was validated against the CIRCUS experiments conducted under low pressure conditions. The comparison between calculated and measured data proved the ability of RELAP5 to model the flashing-induced instability in natural circulation systems. The RELAP5 code predicts measured frequencies and amplitudes of oscillations with a high degree of accuracy.
The transient with a positive reactivity insertion and activation of SCRAM was simulated for the RUTA facility using ATHLET and RELAP5 codes. These calculations were aimed at verification of the RUTA thermal-hydraulic model and were performed with the fixed profile of power distribution in the core and the core power history calculated with the point neutron kinetics model (the same for both compared models). Under these assumptions the ATHLET and RELAP5 models of RUTA give close results for the rated steady-state parameters and transient behavior of the reactor. A BDB accident scenario for the RUTA reactor was simulated using the coupled codes DYN3D/ATHLET and DYN3D/RELAP5 to verify the code systems under conditions of the two-phase flow and possible unstable reactor behavior. The failure of all primary circulation pumps (initial event) and the failure of reactor SCRAM were postulated. The predictions for initial and final reactor states given by the codes are in a good agreement. However, the process of transition between these two states shows a qualitative difference. The DYN3D/RELAP5 code predicts unstable transient behavior of the reactor, while in the DYN3D/ATHLET simulation a smooth change of reactor parameters is observed during the whole accident. It was found that due to differences between the subcooled boiling models of ATHLET and RELAP5 there is a delay in transition to the two-phase flow in the core in the DYN3D/RELAP5 simulation compared to the DYN3D/ATHLET results. This fact leads to a less decrease of the reactor power due to the core coolant density feedback during the initial phase of the DYN3D/RELAP5 calculation. As a result, a higher energy is transferred to the core coolant during first seconds of the accident. Then a sharp transition from the single-phase to the two-phase flow in the core occurs, with a significantly higher volume of steam generated in the most heated fuel assemblies compared to the DYN3D/ATHLET prediction. A strong perturbation of the core coolant density transfers the reactor to unstable state in the case of the DYN3D/RELAP5 simulation.
Despite the different performance of RUTA in the DYN3D/ATHLET and DYN3D/RELAP5 simulations of the BDB accident the obtained results confirm a high intrinsic safety level for this reactor concept. In both compared calculations the allowed safety margins have not been reached.