In cartesian coordinates, The equation of the points on a circle is (x -a)**2 + (y -b)**2 = r**2 where a and b are the coordinates of the center (a, b) and r is the radius. Points within the circle are solutions where x,y result in <= r**2. So this question is given a grid of Horizontal x Vertical discrete points, an x,y "bomb target point" and a radius. The idea is to "color", i.e. fill-in the circle or change all elements to x that are within the circle. The resulting "picture" will look more and more like a smooth circle as the number of elements in the grid increases.
This sort of thing has to happen all the time in video games and I'm sure there are very good algorithms for this. To write my own code, I need to think some more. But, thanks for making the requirement more clear!