A review of HTGR graphite dust transport research

Abstract

Graphite dust is the important contents of source term for safety analysis of high temperature gas-cooled reactors (HTGR). The spherical fuel element circulation in a pebble bed reactor causes many interactions between the fuel elements and other graphite components that inevitably leads to graphite dust production. Micron size graphite particles then move with the helium gas and deposit on various surfaces and in flow dead zones in the primary loop, which complicates equipment maintenance and repair and affects the heat transfer. In addition, the graphite dust is quite porous, so some radioactive fission products will adhere to the dust, which leads to radioactive fission products being distributed on the surfaces of the primary loop. Graphite dust carrying radioactive fission products can also leak into the environment during break accidents leading to radioactive pollution of the environment. Thus, studies are needed for the graphite dust transport in HTGRs. This paper reviews the research on the generation, distribution, radioactivity, deposition, resuspension and coagulation of graphite dust in a pebble bed high temperature reactor. The results show that most of the graphite dust is produced by mechanical wear, while chemical reactions can become an important source during an ingress accident. The graphite dust particles generally have sizes on the order of microns and carry radioactive substances. The graphite dust flows along with the helium in the primary loop and adheres to equipment surfaces. Local turbulent diffusion and large temperature gradients cause the graphite dust to deposit on the surfaces, while gravitational settling has a dominant effect in dead-end zones. In case of accidents or other transients, the dust deposited on the surfaces can become resuspended which will sharply increase the dust concentration, leading to uncertainties about the subsequent operating characteristics. In addition, coagulation and growth of the graphite dust particles due to thermophoresis and electric field forces is also a matter of concern.