The government of India is promoting nuclear energy as a solution to the country’s future energy needs and is embarking on a massive nuclear energy expansion




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Part I: The Basics of Nuclear Power


The basic operation of a nuclear power plant is no different from that of a conventional power plant that burns coal or gas. Both heat water to convert it into pressurised steam, which drives a turbine to generate electricity. The key difference between the two plants lies in the method of heating the water. Conventional power plants burn fossil fuels to heat the water. In a nuclear power plant, this heat is produced by a nuclear fission reaction, wherein energy in the nucleus of an atom is released by splitting the atom.

The Atom


Everything is made of atoms. Any atom found in nature will be one of 92 types of atoms, also known as elements. (Actually, an element is a pure substance made up of only one type of atoms.) Atoms bind together to form molecules. So, a water molecule is made up of two atoms of hydrogen and one atom of oxygen. Every substance on Earth—metal, plastics, hair, clothing, leaves, glass—is made up of combinations of the 92 atoms that are found in nature.

Atoms are made up of three subatomic particles: the positively charged protons, the neutral neutrons and the negatively charged electrons. Protons and neutrons bind together to form the nucleus of the atom, while the electrons surround and orbit the nucleus.

Every element is characterised by its mass number and atomic number. The mass number is the number of protons and neutrons in its nucleus, while the atomic number is the number of protons. The chemical properties of an atom depend upon the number of protons in it, that is, its atomic number. There are atoms whose nuclei have the same number of protons, but different number of neutrons. The chemical properties of these atoms are identical, since they have the same number of protons. Such atoms are called isotopes. An isotope is designated by its element symbol followed by its mass number. For instance, the three isotopes of uranium are designated as U-234, U-235 and U-238.

Nuclear Fission


Fission means splitting. When a nucleus fissions, it splits into several lighter fragments. Nuclear fission can take place in one of two ways: either when a nucleus of a heavy atom captures a neutron, or spontaneously. Two or three neutrons are also emitted. The sum of the masses of these fragments (and emitted neutrons) is less than the original mass. This ‘missing’ mass has been converted into energy, which can be determined by Einstein's famous equation E=mc2 (where E is the energy, m is the mass, c is the speed of light).

Typical fission events release about 200 million eV (electron volts) for each fission event, that is, for the splitting of each atom. In contrast, when a fossil fuel like coal is burnt, it releases only a few eV as energy for each event (that is, for each carbon atom). This is why nuclear fuel contains so much more, millions of times more, energy than fossil fuel: the energy found in one kilogram of uranium is equivalent to the burning of 2000 tons of high-grade coal.

It is this energy released in a nuclear fission reaction that is harnessed to convert water to steam and drive a turbine and generate electricity in a nuclear power plant.

Nuclear Chain Reaction


The nuclear fission reaction is accompanied by the emission of several neutrons. Under suitable conditions, the neutrons released in a fission reaction fission at least one more nucleus. This nucleus in turn emits neutrons, and the process repeats. The fission reaction thus becomes self-sustaining, enabling the energy to be released continuously. This self-sustaining fission reaction is known as nuclear chain reaction.

The average number of neutrons from one fission that cause another fission is known as the multiplication factor, k. Nuclear power plants operate at k=1. If k is greater than 1, then the number of fission reactions increases exponentially, which is what happens in an atomic bomb.

Nuclear Fuel


The isotopes that can sustain a fission chain reaction are called nuclear fuels. The only isotope that can be used as nuclear fuel and also occurs naturally in significant quantity is Uranium-235. Other isotopes used as nuclear fuels are artificially produced, plutonium-239 and uranium-233. (Pu-239 occurs naturally only in traces, while U-233 does not occur naturally.)

We discuss the use of U-235 as nuclear fuel here. Uranium has many isotopes. Two, U-238 primarily, and to a lesser extent, U-235, are commonly found in nature. Both U-235 and U-238 undergo spontaneous radioactive decay, but this takes place over periods of millennia: the half-life of U-238 (half-life is the amount of time taken by half the atoms to decay) is about 4.47 billion years and that of U-235 is 704 million years. (For more on radioactivity and half-life, see Chapter 3, Part I.)

While both U-235 and U-238 are fissionable, that is, both undergo fission on capturing a neutron, there is an important difference in their fission properties. U-238 can only be fissioned by fast moving neutrons, it cannot be fissioned by slow moving neutrons; therefore, it cannot sustain a nuclear chain reaction as the neutrons released during its fission inevitably inelastically scatter to lose their energy. However, U-235 has the property that it can be fissioned by slow moving neutrons too. This is what makes it fissile; in other words, it can sustain a nuclear chain reaction and can be used as nuclear fuel.

The concentration of U-235 in naturally occurring uranium ore is just around 0.71%, the remainder being mostly the non-fissile isotope U-238. For most types of reactors, this concentration is insufficient for sustaining a chain reaction and needs to be increased to about 3-5% in order that it can be used as nuclear fuel. This can be done by separating out some U-238 from the uranium mass. This process is called enrichment, and the resulting uranium is called enriched uranium. [Note that not all nuclear reactors need enriched uranium; for example, Heavy Water Reactors use natural (unenriched) uranium.]

As mentioned above, U-235 also undergoes a small amount of spontaneous fission, which releases a few free neutrons into any sample of nuclear fuel. These neutrons collide with other U-235 nuclei in the vicinity, inducing further fissions, releasing yet more neutrons, thus starting a chain reaction.

If exactly one out of the average of roughly 2.5 neutrons released in the fission reaction is captured by another U-235 nucleus to cause another fission, then the chain reaction proceeds in a controlled manner and a steady flow of energy results. However, if on the average, less than one neutron is captured by another U-235 atom, then the chain reaction gradually dies away. And if more than one neutrons are captured, then an uncontrolled chain reaction results, which can cause the nuclear reactor to meltdown; this is also what happens in an atomic bomb. To control the fission reaction in a nuclear reactor, most reactors use control rods that are made of a strongly neutron-absorbent material such as boron or cadmium.

The neutrons released in a fission reaction travel extremely fast, and therefore the possibility of their being captured by another U-235 nucleus is very low. Therefore they need to be slowed down, or moderated. In a nuclear reactor, the fast neutrons are slowed down using a moderator such as heavy water or ordinary water.
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