<p>To evaluate the fast release of the radionuclides from spent nuclear fuel (SNF), two leaching test campaigns were carried out with SNF samples originating from the Belgian pressurized water reactor Tihange 1 (50.4 GWd/tHM), in contact with a high pH solution and a bicarbonate solution. Using autoclaves, reducing conditions were imposed by the presence of hydrogen and the release of fission gases and radionuclides was monitored. A clear influence of the hydrogen on the dissolution of the fuel matrix was observed. The release of other radionuclides does not depend on the atmosphere in a systematic manner. Cs release looks enhanced in anoxic conditions, but for iodine, strontium and technetium this is not the case. Carbonate seems to have some effect in reducing conditions, especially for Sr and Tc, but with only one test, the evidence is weak.</p> Graphical abstract <p></p>