The first and most common is uranium-235. This is the fissile isotope of uranium and it makes up approximately 0.7% of all naturally occurring uranium. Because of the small amount of U that exists, it is considered a non-renewable energy source despite being found in rock formations around the world. Uranium-235 cannot be used as fuel in its base form for energy production; it must undergo a process known as refinement to produce the compound UO 2. The UO 2 is then pressed and formed into ceramic pellets, which can subsequently be placed into fuel rods. This is when UO 2 can be used for nuclear power production.