Dust is to be limited in the primary circuit of the high temperature gas-cooled reactors (HTGRs) with regard to the reactor operational stability and structural integrity of the heat exchanger because the dust in the coolant adheres to the heat transfer pipe surface, and it is lowered with the performance of the heat exchanger. Furthermore, the dust, including the fission products (FPs) which adhered to the piping, must be reduced in order to be discharged in the depressurization accident with coolant helium.

In High Temperature Engineering Test Reactor (HTTR), the rise of filter differential pressure of primary helium gas circulator (HGC) showed that the dust was arising in the coolant. The increase of differential pressure at the filter of the HGC for the primary pressurized water cooler (PPWC) in HTTR was observed from the beginning of the HGC operation. The differential pressure rose with the increase in operating time of the HGC, and the filter had to be exchanged the filter approximately in about every 8400 hours.

Therefore, the source of the dust should be investigated using The Scanning Electron Microscope (SEM) and The X-ray Fluorescence (XRF) analyzer. SEM was used to observe the appearance of the captured dust in detail and to identify its shape and its size. This result clearly indicated that the dusts mainly composed of carbon. XRF was used to estimate the sources of the carbonaceous dust particles. It was clarified that the source of dust is the slide member of the compressor which is not of a primary cooling system.

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