[en] Loose TRISO coated fuel particles and compacts fabricated under VHTR design were irradiated by the sweep gas capsules and the in-pile release behavior of gaseous fission products was studied. By the release from the intact fuel the activation energies of the release and the diffusion coefficients of krypton in the pyrocarbon layer were estimated. The relation between release and uranium contamination, and the release mechanism depending on the release level were made clear in the releases from both the intact and the failed fuels. Failure fraction of the loose coated fuel particles was estimated and the retention of the fission gas in the matrix was recognized by analysis of the release from the fuels containing the failed coated fuel particles