The research progress on the radioactive graphite dust in HTR-10

In the high temperature gas-cooled reactors, the dust safety issues have been received wide attentions, especially in the pebble bed reactors. Dust complicated the prediction of fission and activation products transport since it presented a parallel transport path and was thought to be an important source term during the depressurization accident scenario. A modification in the helium purification system of HTR-10 has been finished which built a new experimental system to sample the radioactive graphite dust in the primary loop after the restart of HTR-10. During the modification, some pipes, a valve, and a dust filter of the primary loop were cut off and used for further measurement. Besides, an inspection on the helium circulator of HTR-10 has been implemented. Some cotton balls for sampling the radioactive graphite dust during the inspection have been obtained. The measurement of γ dose rates of all samples indicates the existence of fission and activation products. The inspection on site with endoscope inside the old dust filter shows the deposition morphology of radioactive graphite dust. Co-60 and Cs-137 have been determined from a preliminary γ spectrometry measurement. It can supply important information for determination of the source term, decontamination and decommissioning work, and radiation protection during maintenance, and promote the study on the radioactive graphite dust in the pebble bed reactors. (author)