2.3.2 Chernobyl disaster

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.

60 75 90 105 120 135 150 165

Figure 2.2: Fission product yield for thermal fission of 235U

However, although fast neutrons are less efficient than thermal neutrons in producing fission in certain isotopes, they can be effective in fissioning a wider range of isotopes. A “fast reactor” is one that contains no moderator and is based on fission caused by fast neutrons. Several countries have built and operated prototype and demonstration fast reactors. When the nucleus of an atom captures a neutron and does not fission, it may become less stable and change into another element as a result of radioactive decay. In a nuclear reactor, this results in the creation of isotopes of long-lived artificial elements, including neptunium-237 (237Np) (halflife 2.1 million years), plutonium-239 (239Pu) (24 000 years) and americium-243 (243Am) (7 400 years).All these isotopes are radioactive, and some – particularly plutonium – can be used as nuclear fuel. Because of their long half-lives and toxicity they are another important component of high-level nuclear waste, and are the reason why such waste must be isolated for very long periods.

Nuclear fission is an extremely potent source of energy with a very high energy density, i.e. energy produced per unit mass of fuel. Compared to chemical reactions such as combustion of fossil fuels, fission requires a much smaller volume of fuel material to