Main Difference
The main difference between Radius and Ulna is that the Radius is a segment in a circle or sphere (from its center to its perimeter or surface) and its length and Ulna is a medial long bone of the forearm

Radius
In classical geometry, a radius of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the Latin radius, meaning ray but also the spoke of a chariot wheel. The plural of radius can be either radii (from the Latin plural) or the conventional English plural radiuses. The typical abbreviation and mathematical variable name for radius is r. By extension, the diameter d is defined as twice the radius:
d
≐
2
r
⇒
r
=
d
2
.
{displaystyle ddoteq 2rquad Rightarrow quad r={frac {d}{2}}.}
If an object does not have a center, the term may refer to its circumradius, the radius of its circumscribed circle or circumscribed sphere. In either case, the radius may be more than half the diameter, which is usually defined as the maximum distance between any two points of the figure. The inradius of a geometric figure is usually the radius of the largest circle or sphere contained in it. The inner radius of a ring, tube or other hollow object is the radius of its cavity.
For regular polygons, the radius is the same as its circumradius. The inradius of a regular polygon is also called apothem. In graph theory, the radius of a graph is the minimum over all vertices u of the maximum distance from u to any other vertex of the graph.The radius of the circle with perimeter (circumference) C is
r
=
C
2
π
.
{displaystyle r={frac {C}{2pi }}.}

Ulna
The ulna is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. It runs parallel to the radius, the other long bone in the forearm. The ulna is usually slightly longer than the radius, but the radius is thicker. Therefore the radius is considered to be the larger of the two.

Radius (noun)
The long bone in the forearm, on the side of the thumb.

Radius (noun)
The lighter bone (or fused portion of bone) in the forelimb of an animal.

Radius (noun)
One of the major veins of the insect wing, between the subcosta and the media

Radius (noun)
A line segment between any point of a circle or sphere and its center.

Radius (noun)
The length of this line segment.

Radius (noun)
Anything resembling a radius, such as the spoke of a wheel, the movable arm of a sextant, or one of the radiating lines of a spider’s web.

Ulna (noun)
The bone of the forearm that extends from the elbow to the wrist on the side opposite to the thumb, corresponding to the fibula of the hind limb. Also, the corresponding bone in the forelimb of any vertebrate.
“elbow bone”