A square with side length $s$ and a circle with radius $r$ share the same center. Let $A_S$ be the area of the square and $A_C$ be the area of the circle. The area of the region inside the circle but outside the square is $X$, and the area of the region inside the square but outside the circle is $Y$. If $s=8$ and $X=Y$, what is the radius of the circle?