Credit: Justin Randall

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THERE ARE MANY REASONS to move away from coal as our primary source of electricity generation, but it's not an easy task. The list of required attributes for an ideal power generation technology looks intimidating.

First of all, it should offer abundant power.

It also needs to be clean, safe and renewable as well as consistent. And ultimately, it needs to be economical.

Solar power contains much promise as a clean and practically infinite renewable power source. But photovoltaics, the most common form of solar electricity generation, are still a very expensive form of electricity, and lack the consistency to be suitable as a primary source of power - to provide the 'baseload' that is, the kind of power you can rely on to be there to keep everyone's refrigerators humming all day and night.

Wind has seen application in specialised wind farms, both onshore and offshore, especially in Europe where solar power is less efficient than in sunnier climes such as Australia's. Germany alone accounts for around 40 per cent of the total wind power generated worldwide.

Wind is an effective and clean form of power, but it too has its drawbacks. First, it is uncommon for a wind generator to be operating at more than 35 per cent of capacity, and 25 per cent is more common. This means it's idle and not generating power for 65 to 75 per cent of the time. Wind power is relatively cheap, with a cost per kilowatt-hour similar to that of coal in some places, although the volume of wind power is limited and often the best locations for wind turbines are far from the populous areas where electricity is needed. Environmentally, wind power poses a minor threat to birdlife, as well as being considered an eyesore in some communities.

While solar power is relatively expensive, and wind is limited in its implementation, both have a highly important role in renewable electricity generation. Unfortunately, even granting considerable advances in technology and efficiency of both technologies, neither has the potential to become a primary source of electricity because of their intermittent nature: neither could ever be relied upon to meet baseload supply.

IN THE 1950s, nuclear power generation, or the so-called 'peaceful atom', promised to unshackle us from fossil fuels and provide our society with limitless clean power that was going to be "too cheap to meter". Like many utopian visions, the truth was considerably less appealing. While nuclear power has for the most part provided bountiful energy without significant environmental impact, what everyone remembers are the accidents: the Windscale fire at Sellafield in 1957, the meltdowns at Three Mile Island in 1979 and Chernobyl in 1986. At a time when the public psyche was reeling from the fear of global nuclear war, the threats from nuclear power plants were suddenly seen in a similar light.

Another issue that caused growing public concern was the disposal of high-level nuclear waste. Some of the by-products of nuclear power include spent fuel rods: mostly byproducts of nuclear fission, including some highly radioactive actinides with half-lives of many thousands of years - which means they remain lethally toxic for millennia. They have to be housed in waste dumps isolated from all possible contact with the environment for up to 10,000 years. This means building a structure that will survive for twice as long as the Great Pyramid of Egypt has to date.

Needless to say, the engineering difficulties involved in building facilities that can safely contain such waste for 100 centuries, are immense - as are the costs.

Then there are nuclear weapons. Some waste can be reprocessed into weapons-grade plutonium. In particular, the processing of plutonium for re-use as fuel for reactors is difficult and, as such, much of the waste is left to build in weapons-grade stockpiles that could pose a serious security threat were some to fall into the wrong hands.

All three of these issues result from the nuclear fuel cycle in conventional reactors.

The typical nuclear fuel cycle kicks off with a quantity of refined uranium ore. This ore is primarily composed of uranium-238 (U-238), the most common, weakly radioactive isotope that has a very long half-life and is not fissile.

This means U-238 doesn't easily undergo fission, the process in which the nucleus of the atom splits, releasing tremendous quantities of energy.

Usually, a very small percentage of the ore will be U-235. Unlike U-238, U-235 is fissile, and makes up the primary fuel for most nuclear reactors. It is also, incidentally, the uranium isotope that can be used to make nuclear weapons.

This is because when a U-235 atom splits, it releases a spread of high-energy neutrons.

If one of these neutrons then collides with another U-235 atom, it can cause the atom to split, releasing more neutrons in the process.

This runaway chain reaction is responsible for the fantastic explosive power of an atom bomb - and for the meltdowns at Chernobyl and Three Mile Island.

However, there is too little U-235 in mined uranium ore to maintain enough fission for a nuclear reactor or a bomb. The ore needs to be 'enriched', boosting the proportion of U-235 in the ore. Nuclear reactors require around 3 per cent to 5 per cent of U-235, while nuclear weapons often require 85 per cent or more. One of the most popular methods of enriching uranium is a gas centrifuge, where the uranium in the ore is converted into uranium hexafluoride gas and rapidly spun, forcing the heavier U-238 gas to the extremities for separation.

Once a sufficient proportion of U-235 is achieved, the ore can be made into fuel suitable for a reactor. Also, while U-235 is busily destroying itself in the reactor, the U-238 in the fuel is not sitting idly by. This is because U-238 is 'fertile', which means it can transmute into other, fissile elements in a process called 'breeding'. In this process, if an atom of U-238 absorbs a neutron, such as one thrown out by a nearby splitting U-235 atom, it can transmute into the short-lived U-239. This then rapidly decays into neptunium-239, which itself quickly decays into plutonium-239 (Pu-239). Pu-239 is another possible fuel for nuclear reactors because, like U-235, it is actively fissile and can maintain a chain reaction. The problem is that many reactors are not optimised for burning plutonium, and as a consequence large quantities of Pu-239 remain as a waste by-product in spent fuel rods.

Pu-239 can be reprocessed from spent fuel rods and turned into a compound called MOX (Mixed Oxide) fuel. This can then be reused in some nuclear reactors in the place of conventional enriched uranium. However, it is Pu-239 that also represents the greatest weapons proliferation threat. So reprocessing plutonium becomes a very costly and a politically sensitive business. This means it is less likely to be used as a nuclear fuel for a civilian power plant and is less likely to be reprocessed.

Nuclear physics is a complex and messy business, especially when dealing with large unstable elements such as uranium. When the U-235 in nuclear fuel burns down to around 0.3 per cent concentration, it's no longer of use in a reactor. At this point, the proportion of U-238, along with other fission by-products, including some very radioactive isotopes of americium, technetium and iodine, is too high. Many of these elements are called 'neutron poisons' because they absorb neutrons that would otherwise be happily colliding with other U-235 nuclei to spark off more fission.

This spent fuel can be reprocessed - but this is a much more difficult job than basic enrichment because of the high number of fission by-products in the spent fuel. This means that a great deal of spent fuel - highly radioactive as it is - becomes waste that needs to be stored. For a very long time.