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[en] During the past three years the DIDO loop and, more recently, the PLUTO loop have provided information on the fission product release from the particular type of fuel originally chosen for HTR design studies. This is a cold pressed mixture of graphite dust and finely powered uranium which happens to stick together firmly and which is remarkably stable during pile irradiation and heating to temperatures around 4000°C. The atomic ratio of heavy metal to carbon is usually about 1:100, and in the PLUTO loop, which has a fuel element closely resembling a possible HTR fuel rod, the metal content comprises a 10:1 mixture of Th-232 and U-235. The granule sizes of metal and graphite in the fuel have not been closely controlled; the metal is up to 25μ diameter and the graphite up to 25μ diameter, the former having a very wide spread of sizes within this limit. The extent to which the granules retain their form during pressing and subsequent heating is not known, although it is clear that the metals react to form carbides.