Various organizations are developing small modular reactors and microreactors to realize a decarbonized society. Toshiba Energy Systems & Solutions Corporation has developed a heat pipe–cooled microreactor, MoveluX.TM The MoveluX aims to realize unmanned operation at remote places; hence, it has a passive reactivity control device, The In-Gd expansion module (IGEM), has been proposed to assure passive safety. The IGEM uses liquid indium alloy as the neutron absorber. This liquid neutron absorber is stored in a reservoir and its neutron absorber is inserted into the narrow channel by thermal expansion with increasing temperature.

A previous study confirmed that IGEM had enough reactivity worth by numerical simulation. By contrast, the dynamic behavior of the IGEM has not been confirmed experimentally. During reactor operation, the free surface of the In-Gd alloy may oscillate within the IGEM, and some adhesions of the In-Gd alloy may occur on the inner surface of the IGEM. Moreover, the presence of adhered In-Gd alloy may influence the core reactivity.

Thus, the present paper visualizes and measures the dynamic behavior of the IGEM and the adhered In-Gd alloy thickness through neutron radiography under varying temperatures, materials, and surface conditions. Furthermore, the impact of the adhered In-Gd alloy on the core reactivity is also evaluated. As a result, an inhomogeneous liquid surface level was observed during the insertion and extraction into and from the narrow channel, which was likely dependent on the surface condition, including the adhered indium. The thickness of adhered indium on the inner surface of the narrow channel was estimated at approximately 25 μm. This thickness was reflected in the core neutronics calculation, and the analyses showed that the adhered In-Gd alloy layer exhibited 250 pcm of reactivity worth.

This reactivity worth is significant and should be minimized to maintain IGEM performance throughout reactor operation. On the other hand, the long-term adhesion behavior of indium and/or the In-Gd alloy is not examined in the present paper. Therefore, the impact of the immersion time on the adhered indium thickness will be examined in future work.