Uranium has two Isotopes
Uranium has two Isotopes
Define radioactive decay
Nuclear fission is all about splitting of atom to form smaller parts to release energy.
What is the difference between fission and fusion?
The nuclear reactions that produce energy from uranium fuel are complex, but in principle the generation of electricity is no different from that in fossil fuel power stations.
The fuel (fossil or nuclear) is used to heat water to above its boiling temperature in a boiler, and the resultant high-pressure steam drives turbines that generate electricity. The steam is then condensed and returned as water to the boiler. This is a closed circuit, and the steam never comes in contact with the fuel.
Every atom has a nucleus consisting of positively charged protons and electrically neutral neutrons. Protons and neutrons have virtually identical mass and the total number of protons and neutrons defines the mass number of a particular atom. The number of protons in the nucleus is the atomic number and this quantity is always the same for each particular chemical element.
However, some elements have several isotopes, each with different numbers of neutrons, but with the same number of protons. In full notation an isotope is represented by its chemical symbol preceded by a subscript number showing the element's atomic number (the number of protons in its nucleus), whereas the superscript is the mass number of the isotope (the sum of protons and neutrons in its nucleus).
Uranium has two isotopes of interest:
uranium-235, which has 92 protons and 143 neutrons, written as
uranium-238 which also has 92 protons but 146 neutrons;
i.e. both isotopes have the same atomic number (92) but different mass numbers (235 and 238).
These isotopes have such large nuclei that they are inherently unstable. They spontaneously break down, or decay, by two possible processes:
• radioactive decay, a process that emits alpha particles, which are equivalent to the nuclei of helium atoms, with two protons and two neutrons
• nuclear fission, a much less frequent process, in which the whole nucleus breaks apart, releasing energy.
Natural nuclear fission of uranium nuclei occurs much less frequently than radioactive decay. In its natural state, uranium-238 undergoes one nuclear fission for roughly every million alpha emissions involved in its radioactive decay.
So, for nuclear fission to become a viable energy source, the fission rate must be greatly increased. Before describing how this is done, you need to understand a little more about uranium-235 and what happens to it during the nuclear fission reaction.
The occasional natural fission of uranium atoms releases neutrons. Nuclei of uranium-235 may capture these low-energy or slow neutrons, each capture increasing the mass number of uranium-235 to produce uranium-236:
The uranium-236 nucleus created in this way is highly unstable and breaks down by fission:
In this reaction three neutrons are produced for every atom of uranium-236 that undergoes fission. If these neutrons are captured by more nuclei of uranium-235 the reaction will continue. That produces still more neutrons and more energy in an uncontrolled chain reaction, as in atomic weapons, unless it is controlled in some way, as in a power station.
Such a chain reaction can only begin, however, if sufficient atoms of fissionable uranium-235 are present in a small volume, as in a reactor core. It needs this critical mass for the chain reaction to be self-sustaining.
In a nuclear reactor, uranium atoms are bombarded with neutrons. This increases the rate of fission, thereby releasing energy much more rapidly. In nuclear reactors the chain reaction can be controlled by slowing down neutrons and absorbing any in excess of those neutrons needed to keep the reaction going at the required rate.
The energy released in this reaction is used, after a series of steps, to drive turbines that generate electricity. We will review the different types of nuclear reactors in the next section.