Migration behavior of cesium in mixed oxide fuel pins

Migration behavior of cesium was examined for fast reactor mixed oxide fuel pins. Cesium migrate radially and axially to the cooler region of fuel pins. Axial accumulation of cesium is observed at the interface of fuel/UO2 insulator pellets and pellet to pellet gaps. For high burnup and high power pins, cesium accumulation are sometimes observed within the fuel column. And it appears to increase the irregular redistributions of cesium within the fuel column with increasing the burnup and power. A secondary gray phase is often observed at the fuel/UO2 insulator interface and fuel cladding gaps. Microprobe analysis revealed the presence of uranium, cesium, iodine, tellurium and barium and no plutonium in this reaction zone. Thus the gray phase may be formed by UO2-cesium reactions. These local cesium accumulations sometimes induced the cladding strain at fuel/UO2 insulator interface and within the fuel column. The strain appears to be resulted from the volume increase of UO2-cesium reaction products. The cesium accumulation is influenced by oxygen to metal ratio and temperature gradients. However, these local accumulation of cesium does not seem to limit the fuel performance. 4 figures