Topic 3 Neutrons and nuclear fission

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Neutrons are much more difficult to produce and to detect and were not discovered until 1932 – several decades after the electron and proton. Because they are uncharged, neutrons interact less easily with their surroundings. In contrast, beams of electrons (called cathode rays) produced impressive glowing traces even in 19th century scientific apparatus. What's more, free neutrons are not normally produced even by radioactive substances. They were only discovered – by James Chadwick – via a rather complicated experiment. Beryllium was found to produce a mysterious new radiation when exposed to alpha radiation. This new radiation could initially only be detected by shining it onto paraffin to produce beams of protons, which could be detected.

Scientists use beams of neutrons – produced from nuclear reactors – to study or alter the microstructure of various materials. The Australian Government plans to replace the Australian Nuclear Science and Technology Organisation (ANSTO) research reactor at Lucas Heights with a new one – the replacement research reactor – which is capable of supporting advanced neutron beam research.

Nuclear fission

Experiments with neutrons in the 1930s soon revealed a number of amazing phenomena. When bombarded with neutrons, some non-radioactive elements become radioactive. Others transmuted (changed) into different elements altogether, for example oxygen into nitrogen. In 1939 such experiments led to perhaps the most dramatic scientific discovery of the 20th century. Scientists were using neutrons to irradiate a rare isotope of the (then) heaviest known element – Uranium-235 – to try and transmute it into an even heavier, yet unknown element. Instead of transmuting, the uranium atom split into two parts. The process was named nuclear fission, and was soon found to release phenomenal amounts of energy – three million times more than burning the same weight of coal. The fission of just one kilogram of U-235 produces the same energy as 20,000 tonnes of the explosive TNT.

Figure 3.1 The fission of a uranium-235 atom produces two daughter elements: often but not always, krypton and barium. It also produces a large amount of energy plus additional neutrons.

In addition to vast amounts of energy, splitting an atom of U-235 produces two or three more neutrons, which can cause more U-235 atoms to split, producing still more energy and neutrons. Scientists immediately recognised the possibility of setting up a chain reaction, in which nuclear fission would become self sustaining.

Figure 3.2 In a chain reaction, neutrons released from the splitting of one atom cause fission in additional atoms. These atoms in turn release still more neutrons, leading to a growing chain reaction.

The potential for electricity generation was clear, but more pressing and ominous on the eve of World War II was the possibility of a nuclear bomb. Today nuclear fission is used widely for electricity generation and, to a lesser extent, nuclear weapons. In Australia, however, ANSTO's small reactor is used only for research and for manufacturing isotopes for use in medicine, the environment and industry.

Critical mass

In order to develop nuclear weapons, and later on nuclear reactors, scientists and engineers of the 1940s and '50s had to overcome some major technological challenges. The most important of these was that very few isotopes can be split – or are 'fissile' – and these are rare. Naturally occurring uranium contains less than 1% fissile U-235. The rest is non-fissile U-238, which absorbs neutrons without splitting, and therefore slows down or stops the chain reaction.

For a chain reaction to occur, at least one of the neutrons released by each fission must strike other fissile nuclei. This doesn't happen in a small or impure mass of fissile material, because many of the neutrons escape, or are absorbed by non-fissile nuclei. A chain reaction requires a minimum amount of suitably pure fissile material to be gathered in one place – it requires a critical mass.

Nuclear weapons are detonated by combining two subcritical masses of almost pure fissile material to produce a critical mass. The resulting uncontrolled chain reaction takes less than half a microsecond and the release of energy is literally explosive. The shock wave produced flattens buildings within a radius of several kilometres. In a reactor, the fuel is much less pure – about 3% fissile – and so the chain reaction is slower and carefully controlled. A reactor cannot explode like a bomb, though it can experience a 'melt down', potentially with very serious consequences.

The most important control on nuclear weapons proliferation is the extreme difficulty of producing a critical mass of fissile material. Refining uranium from 0.7% to over 90% U-235 requires advanced technology and expertise. However, some nuclear reactors can produce substantial quantities of another fissile isotope, plutonium-239, which can be used in reactors or in nuclear weapons. Because of this, many argue that nuclear reactors could be used to provide a source of nuclear material for weapons.

Plodders and hot bloods

An important aspect of nuclear fission is that many of the daughter elements produced are highly radioactive, very much more so than naturally occurring isotopes. Compared to uranium, which 'plods' with a half-life of 4.5 billion years, these 'hot bloods' may decay within weeks, days or even trillionths of a second, and produce much more intense radiation. Some of these highly active radioisotopes are very useful in industry and medicine, and are produced deliberately in research reactors. However, most of these radioisotopes are waste, and create a major problem for safe storage and disposal. Similarly, nuclear fission weapons produce radioactive fallout which may contaminate a wide area.

Nuclear fusion

Since the 1950s, many nuclear weapons have been based on nuclear fusion rather than fission. Nuclear fusion occurs where two small nuclei – typically hydrogen – combine to form one larger one. This process generates even more energy than fission, and so hydrogen bombs are much more powerful than fission bombs based on uranium or plutonium. Nuclear fusion is what occurs within our sun and other stars to produce the heat and light that sustains life on Earth. Scientists are researching fusion reactors, but have not yet found a way to control this powerful nuclear reaction.

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