The idea that cesium is present in metallic form is chemically very dubious.
Cesium is extremely reactive, as is noted. In particular, it will readily reduce U(+4) to U(+3). Nuclear reactor fuel is primarily uranium dioxide, so there is ample material there for this putative metallic cesium to react with. Cesium is the most electropositive element, so it will give electrons to (reduce) almost anything.
The state of cesium in the vapor gap will be relatively volatile cesium compounds, like cesium iodide. The core temperature of a uranium dioxide fuel pellet greatly exceeds the normal boiling point of this salt.
Cesium is extremely reactive, as is noted. In particular, it will readily reduce U(+4) to U(+3). Nuclear reactor fuel is primarily uranium dioxide, so there is ample material there for this putative metallic cesium to react with. Cesium is the most electropositive element, so it will give electrons to (reduce) almost anything.
The state of cesium in the vapor gap will be relatively volatile cesium compounds, like cesium iodide. The core temperature of a uranium dioxide fuel pellet greatly exceeds the normal boiling point of this salt.