NUCLEAR FISSION AND NUCLEAR FUSION

1. Nuclear fission

Certain nuclides can react with slow neutrons (meaning those with kinetic energies less than 1 electron-volt = 1.6 x 10-19J), and undergo NUCLEAR FISSION. For example, uranium-235 can capture a slow neutron, and be temporarily converted to the very unstable isotope, uranium-236. This atom breaks up violently in a very short time to form smaller atoms, the so-called FISSION PRODUCTS, as well as an average of 2-3 neutrons that are produced for every uranium atom which undergoes fission. These neutrons cause the fission of more uranium atoms, and so on. This is a NUCLEAR CHAIN REACTION:

A vast amount of energy is released as heat. Atomic bombs (fission type) make use of uncontrolled fission reactions, where the energy is released in a huge explosion, while nuclear power plants harvest the energy by controlling the reaction in nuclear reactors.

The fission products are radioactive, and can constitute highly dangerous FALL-OUT in nuclear explosions, seriously harming those who survive the nuclear blast. In the case of nuclear power plants, the radioactive fission products constitute a waste that contaminates the nuclear reactors. The safe removal and storage of these materials cause very serious ecological problems because some of the radioisotopes produced remain dangerously radioactive for many thousands of years. South Africa stores its nuclear waste in a special facility at Vaalputs in Namaqualand, a remote. geologically stable locality.

The United States dropped a uranium-235 fisson bomb on the Japanese city of Hiroshima on 6 August 1945, causing the instant death of about 70 000 inhabitants. This was followed by a plutonium-239 fission bomb on Nagasaki on 9 August 1945. See the original footage of these events


How safe are nuclear power plants?

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2. Nuclear fusion

At very high temperatures (above 107 million Kelvin), hydrogen nuclei can undergo NUCLEAR FUSION to form helium. (The reaction below takes place in steps):

An enormous amount of energy is liberated. Fusion processes are the source of heat and light in the sun, and indeed, in all medium-sized stars. In stars, fusion processes also lead to the formation of elements up to iron, and are known as NUCLEOSYNTHESIS.

Numerous attempts at harnessing this type of process in order to generate energy are under way. The nuclear reaction which is the most studied is

where tritium, T (an isotope of hydrogen), fuses with deuterium, D (another isotope of hydrogen) to form helium.

A hydrogen bomb explosion, March 1954. Photo in the public domain.

While no peaceful applications of man-made fusion have as yet reached a practical stage (), the so-called HYDROGEN BOMBS (also known euphemistically as THERMONUCLEAR DEVICES) are designed to exploit fusion reactions. The first such hydrogen bomb was detonated in 1952, and was about 450 times more powerful than the first uranium fission bomb that destroyed the Japanese city of Hiroshima in 1945. (See how even top scientists can get it wrong!)


3. Additional questions


Nuclear fusion in the sun

We have to introduce here two elementary particles that we have not come across, which play no part in chemical processes, but which are nevertheless of great importance in nuclear reactions.

The POSITRON is a particle having the same mass as the electron, but carrying a positive charge of +1.60 x 10-19 C. It is the antimatter form of the electron, frequently abbreviated as β+.

The NEUTRINO is an uncharged particle with a extremely small mass, not yet (in 2010) accurately determined. It was postulated by Pauli in 1930, and first determined experimentally by Cowan, Reines and others in 1956. It is usually given the symbol ν.

The fusion of protons in the sun is typical of the process that takes place in stars that are similar in mass to the sun, and takes place at temperatures between 10-14 MK (megakelvin).

Step 1

Two protons collide to form the isotope of hydrogen, deuterium (symbol D). In the process, a positron and a neutrino are formed. This step is very slow, on average, a proton has to "wait" about a billion years before it takes part in this step. If it were not so, the sun would have burnt out long ago! The positron and an electron "annihilate" to form 2 γ-photons.

Step 2

A proton and a deuteron fuse to form helium-3.

Step 3

Two helium-3 atoms fuse to form helium-4 and two protons. We see that the overall process consumes 4 protons to form 1 helium atom. A huge amount of energy is evolved (24.7 MeV per helium atom formed).


Doubts about the existence of the atomic bomb

"Little Boy", the atomic bomb that was dropped on the city of Hiroshima on 5 August 1945, instantly killing some 70 000 people. (Photo in the public domain)

"It's got nothing to do with atoms...All I can suggest is that some dilettante in America who knows very little about it bluffed them into saying: 'Drop this, it has the equivalent of twenty thousand tons of high explosives', and in reality it does not work at all. I don't believe a word of the whole thing."

The above were the views (in a conversation with colleagues which was secretely recorded on 6 August 1945) of Werner Heisenberg, 1932 Nobel Prize winner for Physics, on hearing that an atomic bomb had been dropped on Hiroshima the previous day. During World War 2, Heisenberg had been Head of Nazi Germany's nuclear weapons development efforts, which, fortunately, came to nothing.