Abstract: Background: Reactivity control design is a key part of reactor neutronics design. The control-rods made of B₄C-absorber for reactivity control in pebble bed-fluoride-salt-cooled high-temperature reactor (PB-FHR) are located in the hole of side graphite. The space layout, geometric structure, absorber properties are the key influencing factors to the reactivity control. Purpose: This study aims at the influence of different factors of the control rod in graphite reflection layer on control rod worth, including radial position, effective stroke, absorber length, absorber density, etc. Methods: The 10-MW thorium molten salt reactor-solid fuel (TMSR-SF1) model was taken as the reference reactor, and the calculation was completed based on SCALE6. All above mentioned factors such as radial position, effective stroke, absorber length and absorber density, were taken into account for parameters design. Results: Computational results reveal that radial position of the control rod has obvious impact on control rod worth. The absorber length, the upper limit, lower limit and overflow lower limit of the control rod that lead to the change in worth, should be considered comprehensively. The influence of ¹⁰B atomic density on control rod worth was negligible. Conclusion: This study provides basic theoretical reference for the design of PB-FHR reactivity control.

Key words: PB-FHR, Reactivity control, Control rod layout, Control rod worth