[en] The fuel rods in the gas-cooled fast breeder reactor (GCFR) are vented. Thermal flux irradiation tests with vented pressure-equalized rods and fast flux irradiations with simulated vented rods have been or are being performed. These irradiations have used mixed oxide fuel pellets containing 15% to 25% PuO₂, with oxygen/metal (O/M) ratios ranging from 1.95 to 1.99, and clad with 20% cold-worked 316 stainless steel. Both sol-gel-derived and mechanically blended fuels have been or are being tested. The results of PIE gamma spectrometry and destructive examination of GCFR fuel rods indicate the following: (1) Cladding attack and diametral changes were minimal for a rod with a fuel O/M ratio of 1.95. However, for rods with a fuel O/M ratio of 1.98, the attack ranged from 0 to approx. 100 μm and diametral changes were 0.2% to 0.9% ΔD/D. (2) Cs-137 transport is promoted by low O/M fuel. Cs-133 was released and transported principally as its precursor Xe-133. After deposition in the rod and decay to Cs-133, it was permanently retained at that location. Cs-134, an activation product of Cs-133, serves as a monitor of the behavior of the 133 mass chain. However, release of Xe-133 should not be related to the O/M of the fuel