Study on OTTO fueling schemes of the HTR-PM with boron burnable particles

The HTR-PM is a demonstration plant under construction with two 250 MWth modules of pebble bed high temperature reactors (PB-HTR). In this work, the once-through-then-out (OTTO) fueling schemes with boron (10B) burnable particles are introduced into the HTR-PM design, which reduces the complexity of fuel handling system and totally eliminate the necessity of burn-up measurement. The parametric analysis upon two important parameters, i.e. the radius and the volumetric fraction of boron particles, is performed to optimize the OTTO schemes. In the lattice calculations, a boron burnable particle is modeled as a small sphere located at the center of a concentric spherical cell, surrounded by the effective fuel particle zone and fuel-free graphite zone with the same volumetric ratios as in the real fuel pebble. Generally, as the volumetric fraction of boron particles increases, the flattening effect of axial power profile is enhanced and the safety-related features become better, while the fuel enrichment required by the equilibrium core also increases. However, the most optimized schemes exist only within a small range of boron particle radius, i.e. around 120 μm. Within this range, the requirement of margin of maximum fuel temperature after DLOFC accidents confines the boron particle volumetric fraction to be larger than 5.8 × 10−5 and the fuel enrichment larger than 13.5%. The mechanism of boron concentration and particle size influencing the neutronic features is also discussed. It is concluded that the OTTO scheme utilizing boron burnable particles is feasible for OTTO variants of PB-HTRs similar to the multi-pass HTR-PM.