Nuclear Power

Peter R Wallis

The government has announced that it will be reviewing its energy policy, including the potential for new nuclear power stations, in view of the need to reduce carbon emissions which cause global warming. Most of our existing ones will have reached the end of their life in ten to twenty years. Politicians are reluctant to take decisions on issues several elections away, particularly when they see the issue as being unpopular to the UK public, who associate nuclear power with nuclear weapons and possible nuclear disasters. There are also anti-nuclear pressure groups which claim that:

• Nuclear waste will require safe storage for 10,000 years and no way has been found to do so,
• Nuclear technology involves enrichment of uranium and the manufacture of plutonium, which could be diverted by terrorists to make bombs,
• Before long, the readily available sources of natural uranium will be exhausted and have to be replaced by more expensive supplies, making nuclear power even more non-competitive.

These arguments arise mainly from the type of nuclear power stations in use today.

All commercial power reactors in existence today produce energy by splitting the heavy elements, mainly uranium or elements derived from it, which were formed billions of years ago in the supernovae explosions of stars. Natural uranium comes in two forms. The lighter isotope U235 is only 0.7%, the rest being U238. The U235 is called fissile as it is easily split by when struck by a neutron to form lighter elements, with the release of energy and several more neutrons; this allows a chain reaction [1]. The heavier isotope is U238; this is much more stable and neutron capture is more likely to mutate it into plutonium 239 than to split it. Plutonium 239 is also fissile.

The neutrons released in fission are very fast but in the majority of civil power reactors they are deliberately slowed down by successive collisions with light elements in a moderator, to thermal velocities which are well suited for the establishment of a stable chain reaction. These reactors are called 'thermal reactors'. The most common moderator is ordinary water, used also to carry away the heat to the steam generators and turbines, which drive the alternators to produce electricity. These are called 'light water reactors' and, since the water needs to be kept liquid at high temperatures, it needs to be at very high pressures; they are 'pressurised water reactors', PWRs., as at Sizewell B in the UK and in great numbers elsewhere in the world. The rupture in the reactor's pressure vessel could be disastrous, the water changing to steam. Though this would stop the chain reaction, there would be insufficient cooling of the reactor. In the worst case it might catch fire [2], releasing radioactivity to the environment; strong outer containment is therefore required.

The initial fuel in a PWR is usually enriched significantly in the U235 proportion, usually to 3%, using an array of centrifuges. This process means that at most only about a fifth of the uranium ore is actually used in the reactor, the remaining U238 being rejected. After about 3 years of power generation in the reactor much of the U 235 has gone and more than half the power comes from the fission of the plutonium created by neutron capture in U 238. The spent fuel is then removed. It is interesting to note that only about 5 or 6% of heavy atoms in the original fuel have actually been burnt up. The spent fuel contains the remaining 94% plus the ash of the nuclear fire. In some countries, the spent fuel is reprocessed to recover P 239 for reuse in the reactor. This is not done in the USA, President Carter having ruled in 1977 against such processing to avoid the risk of plutonium, which can be used for bombs, becoming available [3]. Without reprocessing, thermal reactors are pretty wasteful in their use of uranium, in that only one hundredth of the original ore is used!

The ashes, 5% of the spent fuel, are a mixture of lighter elements which remain radioactive for several years but, after a decade, the activity is dominated by caesium 137 and strontium 90. They are both soluble in water and so must be contained very carefully. Their activity decays a thousand-fold in 3 centuries and ceases to be a difficult problem. The largest part of the spent fuel, 94%, is depleted uranium, ie without most of the fissile portion, and can be stored safely in lightly protected facilities. The really troubling part, 1%, consists of transuranic elements, mainly plutonium isotopes and americium. Their half-lives range up to tens of thousands of years and this is the most important challenge to the waste technology. In the US there are plans to store waste in Yucca Mountain for such lengths of time. But is there another way to exploit the hidden energy in uranium?

A possible answer lies in using a fast reactor rather than a thermal one; this is argued in an article by W H Hannum, G E Marsh and G S Stanford in the Scientific American [4]. The fast neutrons released in fission are more likely to induce fission in U238 and the heavier transuranic atoms than thermal neutrons. A fast reactor does not use a moderator, but still has to be cooled. A number of prototypes have been built using liquid sodium metal to carry the heat to the steam generators and turbines. One such was the 'fast breeder reactor' at Dounreay in Scotland, though this was not designed specifically as a civil power reactor and is now decommissioned. France has the Phenix and Super-phenix. One advantage of this technique is that the reactor does not need to operate at the high pressures of a PWR, with the serious problems if the pressure system fails. Design problems remain however as sodium catches fire if exposed to water; maybe lead would be a better coolant.

The authors claim that such reactors offer the prospect of generating energy from 99% of the uranium ore instead of less than 1% and that there is already plenty of uranium available in the spent fuel of existing thermal reactors and further mining for uranium may not be needed for centuries.

They say that waste management would be greatly simplified, as most of the transuranics would have been consumed in the reactor itself. Only a few centuries of storage would be needed for the ash. Some reprocessing of the spent fuel would be needed to separate the ash from the heavy metals, a complex mixture of uranium 238, plutonium isotopes and other transuranics, for re-use in the reactor. These metals would however not be useable for bomb-making and there would be no need to separate the dangerous plutonium 239 which would be. Both the US and Russia have advocated techniques of high temperature electro-refining to separate the metal from the ash. It is claimed that the amount of spent fuel would be much smaller than with a thermal reactor and reprocessing could probably be sited at the reactor, reducing the need to transport radioactive material, which is probably the most vulnerable stage to terrorist attack. Both India and China have recently announced their intention to deploy fast reactors to extend their energy resources. This form of nuclear power when fully developed could provide answers to the challenges listed in the first paragraph.

There are still prejudices against nuclear power in the UK, so political acceptance of a government decision to deploy new reactors of whatever type might be slow. The public is however more understanding of the threat of global warming and in many areas are less than happy at windmills covering the country and the power cuts that would occur when there is little wind. They have also noticed the increase in gas prices and doubt the wisdom of relying entirely on overseas sources of gas and oil when our own has gone. It is of interest that not all countries are unhappy at the prospect of more nuclear power. France already has 58 nuclear reactors generating three quarters of all their electricity and President Chirac has recently announced [5] that a third generation is planned for 2012. He said that by 2026 no train in France will be powered by conventional fossil fuels. He called for the ITER project, an experimental fusion reactor that will be based in southern France, to demonstrate "the ability to harness the energy of the sun by the end of the century".

Let us in the UK also show that we can think sensibly about the future and deploy science to solve the world's problems.

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[1] This is well-known to be the basis for the atom bomb dropped on Hiroshima. For civil power generation the process has to be controlled of course. Back
[2] As at Chernobyl. Back
[3] France, Japan, the UK and Russia did not follow his example and continue to reprocess for plutonium, which can be re-used in the reactor. More of the uranium is therefore used. Back
[4] "Smarter Use of Nuclear Waste", Scientific American, December 2005. Back
[5] His New Year Address, 5th January 2006. Back

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IS THE 'BIG BANG' A 'DAMP SQUIB'?

Doug Daniels

When I first became interested in astronomy, way back in the 'dark ages' of the 1950's the favoured theory concerning the creation of the Universe was the Steady State Theory, the chief proponents of which were Herman Bondi, Thomas Gold and Fred Hoyle. Simply stated, this theory proposed that the Universe had always existed and looked much the same everywhere to all observers, in other words it was homogeneous and isotropic. In this respect it conformed to the perfect cosmological principle and it would continue 'ad infinitum'. The expansion of the Universe was accounted for by postulating the continual creation of matter at a rate of just 10-10 nucleons (protons or neutrons) per cubic metre per annum. However the mechanism for this creation of matter was never convincingly described. The Steady State Theory was in many respects an elegant theory that appealed because of its non-violent origin and by virtue of the fact that it allowed the Universe to continue forever – a comforting notion. The theory also added weight to the idea of nucleosynthesis whereby the heavy elements were brewed up in the cores of massive exploding stars, and this part of it still holds today.

However new evidence was accumulating and observations using the 'new technology' of radio astronomy were being made that were soon to cast serious doubt on the veracity of the Steady State Theory. From then on its chief rival, the Big Bang Theory, proposed earlier by Friedman, Lemaitre and Gamov, took precedence.

The 'death knell' of the Steady State Theory was finally sounded with the discovery of the cosmic Microwave Background Radiation (MBR), discovered by Penzias and Wilson in 1964. George Gamov had suggested the possible existence of this radiation as early as 1945 and in 1948, Alpher and Herman went so far as to calculate a predicted temperature for it of about 5degrees K. But its discovery had to wait for the application of radio telescopes, the technology for which had been accelerated by the rapid development of radio physics in the Second World War.

When discovered, the microwave background radiation (MBR) was found to be highly isotropic (the same in all directions) and was described as 'the echo of the Big Bang'. Its presence suggested an explosive, violent creation of the Universe at a singular point in time and space. From then on the rival Big Bang theory was generally accepted as more likely to be correct.

Detailed examination of the MBR had to wait until space probes could be built because the Earth's atmosphere hindered observations in the MBR's millimetric wavelengths. COBE, the cosmic background explorer satellite launched by NASA in 1989, demonstrated that the MBR's spectrum conformed to a perfect black body radiation curve for a temperature at 2.735degrees +/- 0.06degrees K. (just 2.735degrees above absolute zero). It also revealed slight temperature fluctuations in the order of one part in 100,000. These fluctuations were very important for an understanding of how large-scale structures like galaxies could possibly form in the early Universe. More recently, the Wilkinson Microwave Anisotropy Probe (WMAP) has revealed even more detailed temperature fluctuations down to 1 millionth of a degree. The results from WMAP have caused cosmologists to re-examine the timings of events in the early Universe and have concluded an age for the Universe of 13.7 billion years.

The Big Bang theory has held sway now for over 40 years but there are many problems associated with it. Not least is the question as to whether or not the Universe is 'open' closed' or 'flat'. The resolution to this is dependent on the mean density of matter and energy in the Universe and how close this value approaches the 'critical density'. Observations from WMAP suggest that the Universe is 'flat' in other words its expansion is equal to its escape velocity. Over time, the expansion rate of the Universe ought to slow down constrained by the gravity of matter, yet in fact the expansion rate appears to be increasing with time. In order to accommodate this phenomenon it has become necessary to 'invent' a new force in the form of 'Dark Energy', a kind of anti-gravity repulsive force which nobody can yet explain but which nevertheless appears to comprise 73% of the mass of the Universe!

Another big problem with the Big Bang is concerned with the formation of galaxies in the early Universe. A recent image from the Hubble Space Telescope – the Deep Field Image, is so far our furthest view of the early Universe. It shows countless galaxies fully formed around 13 billion years ago, when the Universe was just 5% of its present age. To achieve this speed of galaxy formation it has become necessary to postulate the existence of large quantities (23%) of 'Dark Matter', an invisible substance, the existence for which we have, as yet, no direct observational evidence. What we can actually observe is just the 4% of baryonic matter that comprises everything that we can actually see in the Universe. In point of fact, we have, as yet no convincing model describing how galaxies are formed and how they are maintained. Recent observations indicate that many spiral galaxies (perhaps all such galaxies) have super-massive black holes at their centres. These appear to be orbited by swarms of massive hot blue stars, which in the light of present understanding should not be able to exist in such a region.

Finally, to return to the Cosmic Background Radiation, the Big Bang theory does not adequately predict the observed uniformity in this radiation. This is only achieved by postulating 'Inflation' – a process of sudden ultra-rapid expansion, possibly as much as x10⁵⁰, that mysteriously took place just 10^-35 seconds after the initial Big Bang explosion and ended before 10^-30 seconds. It is during this brief period of change that the electromagnetic and strong nuclear forces are deemed to have separated to different values and in so doing, released the energy to power this ultra-rapid expansion. During this period of 'symmetry breaking' certain defects in 'spacetime' would have been created. These have been described as 'monopoles' and 'domain walls' Monopoles are extremely massive single units of magnetic charge that do not exist in the present Universe. Domain walls are two-dimensional defects in spacetime that similarly and conveniently could not exist in the present Universe.

There is also the vexed question as to what exactly lit the Big Bang's fuse in the first place? The answer to this we are told is that it can never be known because the physical laws that govern the behaviour of matter/energy only came into existence at the moment of creation and what occurred before that is pure conjecture. Perhaps someone said: "Let there be light"?

The test for a good theory is in the predictions which it makes and which can be supported by observation and experiment. This is after all, how the Steady State Theory met its demise. At the moment it would seem that the Big Bang theory is also failing these criteria.

We have seen in the past that when a 'grand theory' begins to fail, those who favour it do all in their power to prop it up – it's human nature after all. Before Kepler, astronomers added epicycle upon epicycle in an attempt to force the planets into circular orbits. Fred Hoyle tried very hard to prove that the fossil Archaeopteryx in the Natural History Museum was a fake because its age did not fit with the timescale required by the Steady State Theory, and even the great Einstein added a fake term in his equations because he didn't like the idea of an expanding Universe.

It occurs to me that if we are to construct theories concerning the formation and evolution of the Universe based on observations of a mere 4% of it, then we must not be too surprised if the best theories which we can come up with are apt to be found wanting from time to time.

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