Experimental research on the radioactive dust in the primary loop of HTR-10

Presence of radioactive dust, and also potential for its release, are considered important issues as regards the safety of high temperature gas cooled reactors, especially the pebble bed types. To obtain data and insights into the quantity and characteristics of such dust, a sampling loop (with stainless steel powder filter elements to collect the dust) has been built in the helium purification system of HTR-10 enabling sampling of radioactive dust circulating in the primary loop of HTR-10. Two sampling experiments were conducted during the operation of HTR-10 in 2015. The concentration of the radioactive dust in the primary loop was deduced from the dust deposited on the filters, and was estimated to be 5.57(17) mu g/m(STP)(3) and 1.97(8) mu g/m(STP)(3), respectively. The nuclides present in the dust have been identified by their. spectra. After the removal of dust from filter elements, the particle size distribution was measured with an optical microscope. In the first experiment, the average particle diameters of the radioactive dust were determined as 43.0 mu m and 13.6 mu m captured respectively, by the filter elements of 80 mu m and 50 mu m pore size. In the second experiment, the average particle diameters were 62.9 mu m and 11.3 mu m as captured respectively, by similar filter elements. The data show existence of large particles in the primary loop of HTR-10, generated most likely by abrasion.