Uranium-235

Uranium-235 is an isotope of uranium that differs from the element's other common isotope, uranium-238, by its ability to cause a rapidly expanding fission chain reaction. It absorbs a thermal neutron and splits into two lighter substances by way of thermonuclear fission. It either releases 2 or 3 neutrons and continues the reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons without decaying. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.

One atom of U-235 generates 200 MeV = 3.2 × 10^-11 J, i.e. 18 TJ/mole = 90 TJ/kg.

Because only around .72% of all natural uranium is uranium-235, enrichment must take place if any appreciable amount of uranium-235 is to be obtained. Uranium-235 has a half-life of 700 million years [1] (http://www.epa.gov/radiation/radionuclides/uranium.htm).