Preliminary design of control rods in the single-fluid double-zone thorium molten salt reactor (SD-TMSR)

Highlights: • A reliable safety system based on control rods has been introduced in the SD-TMSR. • Three different initial fissile loadings are considered. • Six different absorbing materials have been investigated. • Integral and differential control rod worth, SDM, and shadowing effects at steady-state were calculated. Recent studies on Molten Salt Reactors (MSRs) showed that the excess reactivity at the beginning of the operation is large for many fueling strategies and must be compensated by a reactivity control system. The current work introduces a reliable safety system based on control rods in addition to the online feed system reactivity control in the Single-fluid Double-zone Thorium-based Molten Salt Reactor (SD-TMSR). Three different initial fissile loadings are considered: ${}^{233}$U, reactor-grade Pu, and transuranic (TRU) elements as a startup fuel. We applied six different absorbing materials to investigate the main operational and safety parameters in the SD-TMSR: natural B₄C, enriched B₄C with 90% ${}^{10}$B, HfB₂, HfH_1.62, Eu₂O₃, and Gd₂O₃. The present work focuses on control rod design, integral and differential control rod worth, shutdown margin, and shadowing effects at steady-state. We employed the SERPENT-2 Monte-Carlo code to calculate the reactivity worth and analyze the performance of the reactivity control system. We showed that ${}^{233}$U and reactor-grade Pu startup cores maintain adequate shutdown margin with all considered absorbers. Finally, this paper proposes a design of control rod clusters that compensate the excess reactivity of the SD-TMSR loaded with different initial fissile material.