High burnup mixed-oxide fuel irradiation experiments

The European Institute for Transuranium Elements has performed a series of mixed-oxide irradiation experiments in the Dounreay Fast Reactor. The main parameters varied were the O/M ratio of the fuel, radial fuel distribution, and fuel form. Three low burnup experiments with nine pins were terminated in 1969. Eight of the corresponding nine high burnup pins were removed from the reactor subsequently, after several intermediate nondestructive examinations. One pin is further irradiated. Three of the eight pins failed. The Inconel-625 claddings of the sound pins did not change their o.d. at a total neutron dose of 7.3 × 1022 n/cm2.The three pins of the DS-2 experiment, in which fuels with different O/M ratios have been irradiated to a burnup of 7.3 at.%, were examined destructively. A low initial O/M (1.94) led to a negligible attack of the cladding at a maximum temperature of 650°C, whereas fuel of stoichiometric composition corroded severely. No influence of the different O/M on the swelling was detected. The reduction of the O/M ratio to low values seems to increase the risk of central fuel melting.