Burnable absorbers in pebble-bed high-temperature reactor designs

Pebble-bed high-temperature reactors (HTRs) that operate with an OTTO (Once-Through-Then-Out) fuel cycle face challenges such as strong initial reactivity excess and axial power peaking, which can compromise safety margins. This study evaluates the integration of burnable absorbers (BAs) directly into TRISO particle coatings as a strategy for controlling reactivity and flattening power distribution. A comprehensive neutronic analysis was performed using the Serpent 2 Monte Carlo code, which explicitly modeled the double heterogeneity of pebble fuel. All elements with natural abundance were screened and categorized based on their required mass loading and depletion behavior. At the pebble level, boron, indium, and gold provided significant initial reactivity suppression with stable burnup characteristics, while lithium and europium were effective for long-term reactivity control. At the core level, erbium, boron, iridium, mercury, and protactinium were successful in reducing axial and pebble-level power peaking, effectively shifting the axial maximum downward. The results demonstrate that optimized configurations of burnable absorbers can both control reactivity and improve power distribution in OTTO-cycle HTRs, offering a practical approach to designing safer and more efficient reactors.