UK, reactors are operational that use graphite as moderator, and resp. water and gas as coolant. As the operational temperature and pressure of these reactors is dictated by engineering and safety constraints, both are limited. Thus, the temperatures and pressures that can be delivered to the steam turbine are also limited. Typical water temperatures of a modern Pressurized water reactor are around , with pressures of around 85 bar. Compared to for example modern coal fired steam circuits, where main steam temperatures in excess of are obtained, this is low, leading to a relatively low thermal efficiency. In a modern PWR, around 30-33 % of the nuclear heat is converted into electricity. A third drawback is that when a (any) nuclear reactor is shut down after operation, the fuel in the reactor no longer undergoes fission processes. However, there is an inventory present of highly radioactive elements, some of which generate substantial amounts of heat. If the fuel elements were to be exposed (i.e. there is no water to cool the elements), this heat is no longer removed. The fuel will then start to heat up, and temperatures can then exceed the melting temperature of the zircaloy cladding. When this occur the fuel elements melt, and a meltdown occurs, such as the multiple meltdowns that occurred in the Fukushima disaster. When the reactor is in shutdown mode, the temperature and pressure are slowly reduced to atmospheric, and thus water will boil at . This relatively low temperature, combined with the thickness of the steel vessels used, could lead to problems in keeping the fuel cool, as was shown by the Fukushima accident. Lastly, the fission of uranium and plutonium in a thermal spectrum yields a smaller number of neutrons than in the fast spectrum, so in a fast reactor, more losses are acceptable. The proposed fast reactors solve all of these problems (next to the fundamental fission properties, where for example plutonium 239 is more likely to fission after absorbing a fast neutron, than a slow one.) Fast fission and breeding Although and have a lower capture cross section with higher-energy neutrons, they still remain reactive well into the MeV range. If the density of or is sufficient, a threshold will be reached where there are enough fissile atoms in the fuel to maintain a chain reaction with fast neutrons. In fact, in the fast spectrum, when captures a fast