Core analysis, design and optimization of a deep-burn pebble bed reactor

Achieving a high fuel burnup in the Deep-Burn pebble bed reactor design, while remaining within prescribes safety limits for fuel temperature, power peaking and temperature reactivity feedback, is challenging. The high content of Pu and Minor Actinides in the Deep-Burn fuel significantly impacts the thermal neutron energy spectrum as compared to a 'standard' UO₂ fueled core. Regions in the pebble bed core near the graphite reflectors experience power and temperature peaking that result from the local softer neutron energy spectrum. Furthermore, the interplay of the Pu resonances of the neutron absorption cross sections at low-lying energies can lead to a positive temperature reactivity coefficient for the graphite moderator under certain operating conditions. To investigate the aforementioned effects a new code system based on existing codes has been developed for neutronic, thermal-hydraulic and fuel depletion analysis of Deep-Burn pebble bed reactors. A core analysis of a Deep-Burn Pebble Bed Modular Reactor (400 MW_th) design has been performed for two Deep-Burn fuel types and possible improvements of the design with regard to power peaking and temperature reactivity feedback are identified.