Some observations of the carburization of mixed oxide fuel element cladding in the Dounreay fast reactor

The possibly deleterious effects of carburisation of fuel pin cladding by mixed oxide fuel or the reactor coolant are of concern to fast reactor designers. In addition to the change in mechanical properties there is evidence that intergranular attack by fission products may be catalysed by carbon diffusing along the clad grain boundaries. The first evidence of clad carburisation during irradiation in DFR was grain boundary enhancement during etching. Bulk analysis of clad samples subsequently showed carbon contents increasing to as much as 0.2 wt%. More sophisticated techniques of analysis now enable the distribution of carbon to be studied in detail. The carburisation of fuel pin cladding in DFR has been studied with the Harwell nuclear microprobe. Clad samples were scanned by a focussed beam of 1.3 Hey deuterons and the carbon content of each 5μm surface spot calculated from the protons emitted by the 12C (d,p)13C reaction. Over thirty clad sections from DFR oxide pins were scanned to produce carbon content profiles through the clad wall. Data from a selection of these profiles and a set from one M316 clad pin is given. The fuel and to an even greater extent the DFR coolant have significantly carburised the cladding. The degree of penetration of the cladding clearly increases with temperature, but the surface carbon content does not show such a clear temperature dependence