Experimental study of dust behavior during depressurization

A small amount of graphite dust is present in the primary cooling system of a graphite, moderated gas-cooled reactor during normal operation and condensable fission products may be absorbed on the dust. Since it is possible that the fission products are released with dust under the accident conditions such as depressurization events, they have a potential hazard of radiation exposure to the environment. In order to obtain liftoff fractions (fractional amount removed) of the graphite dust during the depressurization, an investigation was performed by the blow down test. In the test, graphite powder attached on the sample was blown down under the high speed gas flow to simulate flow condition during the depressurization accident. According to the shear ratio concept, the liftoff fraction can be described by shear ratio, which is defined as the ratio of wall shear stress in the blow down condition to the normal condition. From the test result, it is concluded that the shear ratio concept is not completely enough to describe the dust behavior during depressurization while the liftoff fraction increases with the shear ratio. (author)