total mass of the products of the reaction (fission products and neutrons) is minutely less than the original mass of the nucleus and impacting neutron, the difference having been converted verted into energy according to Einstein’s famous formula E = mc2. Most of this energy is carried by the fission products in the form of kinetic energy (energy due to their motion). As the fission products collide with nearby atoms they quickly lose most of their kinetic energy, which is converted into heat. In a nuclear power plant this heat is used to generate electricity. When one of the free neutrons released as a result of fission impacts another suitable nucleus,it can cause a further fission, releasing more neutrons and energy. Alternatively, free neutrons may bounce off a nucleus (scattering) , escape from the reactor without interaction (leakage), or be absorbed into a nucleus without causing fission (capture). The fuel and other materials in a nuclear reactor are arranged to produce a self-sustaining chain reaction, where on average just one of the neutrons released by each fission goes on to cause a further fission. At that point the reactor is said to have reached criticality. The critical mass is the minimum amount of fissionable material for a given set of conditions needed to maintain a chain reaction. Neutrons with low kinetic energy are known as thermal neutrons; these are the most efficient in causing fission in uranium and plutonium. Fast neutrons have many millions of times more kinetic energy than thermal neutrons. All free neutrons produced by a fission reaction are initially fast neutrons. In current nuclear power plants, a material known as a moderator (often ordinary water) is used to slow the fast neutrons released during fission to the thermal energies needed for fission.
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