Design of innovative systems for the optimized control of reactivity in Gen-IV fast neutron reactors

The Generation-IV reactors could benefit from the fast neutron spectrum to maximize the utilization of uranium resources, improve the management of fissile materials, and help the transmutation of nuclear waste. As the absorption cross-sections decrease with incident neutron energy, the fast spectrum challenges its reactivity control.The conventional control rod is a cluster of open pins with boron carbide (B₄C) as the absorber. ¹⁰B enrichment can be adjusted to satisfy the requirements of different cores. However, the operating lifetime of B₄C is limited due to its characteristics under irradiation. Alternative absorbers such as gadolinium oxide (Gd₂O₃), europium oxide (Eu₂O₃) and hafnium diboride (HfB₂) may present some advantages and be used with local addition of moderators to optimize the design of control rods in sodium fast reactors (SFRs).In the conventional fast reactors, the control rod is usually the only reactivity control system, which would lead to fuel melting in control rod withdrawal (CRW) accidents. Therefore, two burnable poison (BP) designs are investigated to reduce core excess reactivity and thus improve the inherent safety performance of reactors. The first BP design load minor actinides in homogenous or hybrid mode. The second BP design combines depleted B₄C and moderators in dedicated assemblies.These designs are investigated using the advanced calculation scheme in APOLLO3 that is developed and validated in this thesis. These designs are applied in a large industrial SFR and a small modular SFR, which proves their excellent flexibility to optimize reactivity control in a wide range of fast reactors. (author)