atmospheric pressure, and the sodium is very hot, and can be allowed to remain at these temperatures even in shutdown, passive cooling (i.e. no pumping requirements) with air is possible. Accidents such as the Fukushima Daiichi nuclear accident are impossible with such a design. The higher temperature of the liquid metal, and therefore the higher temperature of the steam generated by this liquid metal, allows a considerable increase in the electric generating efficiency (around 40% thermal efficiency, as opposed to 30% ). Such reactors have the potential to significantly reduce the waste streams from nuclear power, while at the same time increasing vastly the fuel utilization. Disadvantages As most fast reactors to date have been either sodium, lead or lead-bismuth cooled, the disadvantages of such systems are described here. As a result of running the reactors on fast neutrons, the reactivity of the core is determined by these neutrons, as opposed to moderated reactors. In the moderated reactors, a significant amount of control of the reactivity is obtained from delayed neutrons, which allow time for operators or computers to adjust reactivity. As delayed neutrons play virtually no role in fast reactors, other mechanisms are required for the very short term reactivity control (e.g within one second) in fast reactors, which are thermal expansion and Doppler broadening. Longer term reactivity is obtained from control rods, which are filled with a neutron absorption material. As the entire reactor is filled with large volumes of molten metal, refuelling is not trivial, as optical tools (cameras, etc.) are of no use. Costly, carefully calibrated and positioned robotic tools are needed for the operation of refueling. Also, completely removing fuel elements from the reactor is not easy. The fact that the entire reactor is filled with a metal that has a melting point much higher than room temperature, all the tubing, heat exchangers, and the entire reactor volume must be heated electrically, before any nuclear operation can take place. However, once the reactor produces heat, this is no longer of any concern. To date most fast reactor types have proven costly to build and operate, and are not very competitive with thermal-neutron reactors unless the price of uranium increased dramatically, or building costs decreased. It is thought that given the perception of problematic nuclear waste disposal, such reactors will be necessary. As moderated reactor construction costs are rising (among other) due to ever more stringent