Nuclear reactor

A typical water-cooled nuclear power reactor contains an array of uranium U rods surrounded by water. The uranium in the form of UO2 is sealed into zirconium alloy tubes (see periodic table of elements). The water removes the heat and also slows down the neutrons by elastic collision with hydrogen H nuclei. The slow neutrons have a much higher probability of causing nuclear fission in U-235 than do faster neutrons. The heated wate percolates through a cooling circuit which - through a heat exchanger - transports the heat generated to a turbine. Nuclear energy is released by way of the nuclear fission chain reaction. In this process, neutrons emitted by fissioning nuclei induce fissions in other fissile or fissionable nuclei; the neutrons from these fissions induce fissions in still other fissile or fissionable nuclei and so on, leading to a sustainable nuclear, or chain, reaction (chain reaction (I), Fig. 1). Such a chain reaction can be described quantitatively in terms of the multiplication factor k, defined as the ratio of the number of fissions in one generation to the number of fissions in the preceding generation. When k is greater than 1, the number of fissions increases from generation to generation and the energy released by the chain reaction increases with time and is said to be supercritical. This among other conditions has to be the case in an atomic bomb. However, if k is less than 1, the number of fissions decreases with time, and the chain reaction is subcritical and will eventually stop. In the special situation where k = 1, the chain reaction proceeds at a constant rate, energy is released at a steady level, and the system is said to be critical. To make a reactor critical it is necessary to balance the rate at which neutrons are produced within the reactor with the rate at which neutrons disappear. Neutrons can disappear in two ways: as a result of absorption in some type of nuclear reaction, or by escaping from the surface of the reactor. The absorption devices used to maintain the reactor in a critical state, are the so called control rods which contain boron, cadmium and/or indium which can absorb neutrons.

In a fast reactor, no light nuclei are present in the system and the average neutron velocity is much higher . In such reactors, it is possible to use the excess neutrons to convert U-238 to U-239. Then U-239 undergoes radioactive decay to Pu-239, which is a fissile material capable of sustaining the chain reaction. If more than one Pu-239 atom is provided for each U-235 consumed, the system is said to breed, i.e. more fissable fuel is made than used. In this so called breeder reactor the U-238 becomes the fuel. This process increases the energy yield from uranium deposits by more than a factor of 60 over a typical water-moderated reactor, which employs U-235 as fuel. A majority of power reactors use both the moderator and the coolant. However a limited number of reactors use heavy water instead of light water. The advantage of this system is, that it is possible to use uranium as a fuel so that no uranium enrichment is needed.

DT