Simulations of RUTA-70 reactor with CERMET fuel using DYN3D/ATHLET and DYN3D/RELAP5 coupled codes

RUTA-70 is a pool type reactor with the rated power of 70 MW designed as a district heat supplying facility. The basic design principles of this reactor are simplicity of the design and a high safety level due to a low pressure and a large coolant inventory in the primary system. The core design with the CERMET fuel rods also contributes to the reactor safety due to a high thermal conductivity of the fuel matrix and its role as the additional barrier to the fission products release.

RUTA-70 model for simulations with the internally coupled code systems DYN3D/ATHLET and DYN3D/RELAP5 was developed. A 3-D power distribution in the core is calculated by DYN3D with thermal-hydraulic feedback from the system codes. The reactor facility model includes reactor pool, two reactor loops, circulation pumps, primary and the secondary heat exchangers and core channels. The core model consists of 16 fuel assemblies in the 60-degree symmetry sector with different burnable absorber (Gd) concentrations.

The steady-state with the rated reactor parameters and a BDB accident scenario were simulated with the DYN3D/ATHLET and DYN3D/RELAP5 coupled code systems to verify these codes. The simulated accident is initiated by the trip of two operating primary circulation pumps. Additionally to the initial event the SCRAM failure is postulated.

The compared codes give close predictions for the initial and final states of the accident but not for the transition between them. The observed deviations are explained by differences in the subcooled boiling models of the system codes ATHLET and RELAP5. Both simulations confirm a high level of the reactor inherent safety. The allowed safety margins were not reached.