The team is on the hunt for nuclear materials such as Uranium and Plutonium, represented by the Atomic symbols U and Pu respectively. Davis says the body they recovered contained traces of uranium-235, indicating Venezuela is making progress in the nuclear weapons program but is still in the early stages. Uranium-235 is one of the two main primary fissile materials used in nuclear weapons, the other being plutonium-239. U-235 is a rare natural isotope of uranium, making up only 0.72% of natural uranium, it is the only primordial natural isotope capable of sustaining a nuclear chain reaction (fissile) and has an extremely long half-life of 703.8 million years. Since U-235 was the only radioisotope found in the body the team recovered that indicates their program is still in the early stages, U-235 is what was used in the first generation nuclear weapons like the ones used in World War II. First generation thermonuclear weapons use several materials, usually Pu-239, sometimes U-235, and secondary materials like tritium (heavy hydrogen) and modern second generation thermonuclear weapons use Pu-239, depleted U-238 and lithium deuteride.