High-temperature gas-cooled reactors (HTGRs) use graphite structures in the core, and there is concern about the impact of graphite dust on the reactor system. To address this and other problems, the helium gas circulator (HGC) of the high temperature engineering test reactor (HTTR) has a filter to collect dust in the helium coolant. Previously, dust from the sliding parts of the primary helium purification system (PHPS) gas circulator, rather than graphite dust from the core, led to increased filter differential pressure*. Improvements were made to these sliding parts and the filter differential pressure remained within the normal range. However, it increased again during operation in 2021. This study investigates the cause of the phenomenon.
 Filter analysis revealed that the dust adhered to the filter surface contains elemental silicon (Fig. 1). Furthermore, silicone oil from the PHPS gas circulator was found to have ingressed into the core (Fig. 2), indicating that silicone oil is the source of the silicon contributing to the increase in the filter differential pressure. The ingress of silicone oil was attributed to the degradation of the charcoal filter used in the PHPS gas circulator. Consequently, the replacement cycle for this charcoal filter was revised to occur at specific intervals.
 The filter differential pressure is increased by silicone oil, indicating that the prismatic-type HTGRs, such as the HTTR generate very little graphite dust from the core. Based on the past event, the PHPS gas circulator is considered a source of particles contributing to the increase in filter differential pressure. By reviewing the replacement cycle of the charcoal filter, the generation of dust in the PHPS gas circulator is expected to be eliminated.

^＊Inaba, Y. et al., Investigation on Dust Captured by Quintuple Filters Installed Upstream of Primary Gas Circulators in HTTR, Journal of Nuclear Science and Technology, vol.51, issues 11–12, 2014, p.1373–1386.