\tkzDefPoint(0,0){O} \tkzDefPoint(\a,0){S1} \tkzDefPoint(0,\a){S2}
\tkzDrawPolygon[fill=blue,opacity=0.3,color=blue](O,S1,S2)
\tkzDrawArc[fill=blue,opacity=0.3,ultra thin](O,S1)(S2)
\tkzDrawArc[color=blue,very thick](O,S1)(S2) \tkzDrawSegment[color=blue!30!white](S1,S2)