Core optimization of a Deep-Burn pebble bed reactor

Achieving a high fuel burnup in the Deep-Burn (DB) pebble bed reactor design, while remaining within safety-imposed prescribed operational limits for fuel temperature, power peaking and temperature reactivity feedback coefficients, 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. This modified spectrum can result in power and temperature peaking in the pebble bed core within regions near the graphite reflectors that experience a softer spectrum than the bulk of the core. Furthermore, the interplay of the neutron absorption resonance cross sections of Pu at low energies (the so-called low-lying resonances) can lead to a positive temperature reactivity coefficient for the graphite moderator under certain operating conditions. A Pu-fueled PBMR-400 design serves as the starting point for a core optimization study. The fuel temperature, power peak, temperature reactivity coefficients, and burnup capabilities of the modified designs are analyzed with the PEBBED code. A code-to-code coupling with the PASTA code allows for the analysis of the TRISO fuel performance for both normal and Loss Of Forced Cooling (LOFC) conditions. An improved core design was sought that maximizes the fuel discharge burnup while retaining negative temperature reactivity feedback coefficients for the entire temperature range and avoiding high fuel temperatures (fuel failure probabilities). The problems identified in the introductory paragraph have been addressed. Recent methods and models developed at INL have allowed for the precise quantification of the problem and for the formulation of innovative solutions that demonstrate the feasibility of plausible and workable mitigation mechanisms.