a light water moderated reactor, the presence of the transuranic elements. Such isotopes are themselves unstable, and undergo Beta decay to create ever heavier elements, such as Americium and Curium. Thus, in moderated reactors, plutonium isotopes in many instances do not fission (and so do not create new neutrons), but instead just absorb the neutrons. Most moderated reactors use low enriched fuel. As power production continues, around 12–18 months of stable operation in light water moderated reactors, the thermal nuclear reactor both consumes more fissionable material than it breeds and accumulated neutron absorbing fission products which make it difficult to sustain the fission process, and the reactor has to be refueled. Drawbacks of light water as the moderator in conventional nuclear reactors The following disadvantages of the use of a moderator have instigated the research and development of fast reactors. Although cheap, readily available and easily purified, light water can absorb a neutron and remove it from the reaction. It does this enough that the concentration of in natural uranium is too low to sustain the chain reaction; the neutrons lost through absorption in the water and , along with those lost to the environment, results in too few left in the fuel. The most common solution to this problem is to concentrate the amount of in the fuel to produce enriched uranium, with the leftover known as depleted uranium. Other thermal neutron designs use different moderators, like heavy water or graphite that are much less likely to absorb neutrons, allowing them to run on natural uranium fuel. See CANDU, X-10 Graphite Reactor. In either case, the reactor's neutron economy is based on thermal neutrons. A second drawback of using water for cooling is that it has a relatively low boiling point. The vast majority of electricity production uses steam turbines. These become more efficient as the pressure (and thus the temperature) of the steam is higher. A water cooled and moderated nuclear reactor therefore needs to operate at high pressures to enable the efficient production of electricity. Thus, such reactors are constructed using very heavy steel vessels, for example 30 cm (12 inch) thick. This high pressure operation adds complexity to reactor design and requires extensive physical safety measures. The vast majority of nuclear reactors in the world are water cooled and moderated with water. Examples include the PWR, the BWR and the CANDU reactors. In Russia and the