Friday, December 02, 2005

A Nuclear Future?

Nuclear power, now usually dubbed "nuclear" looks set to return to the fold, having been the black sheep of the family for a good three decades. Its protagonists claim that during the lifetime of a nuclear power plant (NPP), only around 20 - 40% of the CO2 generated by an equivalent typical gas fired plant is released, and so nuclear is a more environmentally benign supply of energy. This figure should be taken as representing the CO2 output during the whole lifetime of an NPP, which might be somewhere up to 100 years. The initial carbon debt incurred in constructing and fuelling an NPP takes about 10 years to be paid off. The aspect of fuel requires further consideration. Most NPP's use uranium as their fuel. As mined, uranium comes mainly in two kinds, uranium-235 and uranium-238. Only uranium-235 is "fissile", i.e. can be "burned" in a fission nuclear reactor. "Fissile" means that the atomic nucleus can split into two on absorbing a neutron with the release of a huge amount of energy. To envisage just how much energy is released, a million-fold factor is sometimes quoted, meaning that one gram of uranium-235 releases as much energy as burning one million grams, i.e. one tonne, of coal or oil - roughly, since their energy output is not exactly the same.
In most NPP's it is necessary to enrich the uranium fuel in uranium-235. This is done by depleting it in uranium-238 (which is non-fissile and hence useless as a fuel), which then provides "depleted uranium" on a large scale for use in armaments and explosive shells. It is not widely recognised that uranium is a finite resource just as oil and gas are. Estimates vary, but there are probably reserves containing about 3 million tonnes of uranium available to supply a current demand of around 68,000 tonnes annually. This suggests that the present number of NPP's could be run for another 40 years or so. But what then? Sir David King (the Government's Chief Scientific Advisor) is quoted as saying that since "global warming [is a] greater threat than terrorism", the use of nuclear should be expanded to supplant fossil fuel fired power stations and hence reduce the overall CO2 emissions attendant to power production. This is a view also recently espoused by no less than Professor James Lovelock, of Gaia fame, though Lovelock has never been entirely anti-nuclear. However, since the world generates around one sixth of its electricity from nuclear, if we could replace all the fossil fuelled plants by NPP's overnight, the forty year fuel enforced limit on fission powered NPP's would be reduced to about 7 years, and I have seen estimates as low as 3 - 4 years for this scenario. So, why are we still even talking about it?
There is one alternative, which still uses fission, but can burn the majority (99%) uranium-238, rather than the conventional reactor which is highly wasteful since it uses only <1% of natural uranium, in the form of uranium-235. A special reactor is needed to do this, which is called a "Fast Breeder". In a fast breeder, natural uranium is irradiated with neutrons, some of which are absorbed by uranium-238 nuclei and convert them to plutonium-239. Plutonium-239 is fissile and so can be used as a nuclear fuel. As the reactor runs, producing energy by the fission of plutonium-239, the uranium-238 present as an initial 80:20 mix of uranium:plutonium is conveted to more plutonium-239, hence the term "breeder" because the reactor "breeds" more fuel than it consumes. A blanket of uranium encases the reactor to absorb more of the available neutrons, and can be reprocessed to extract more plutonium fuel, making the overall breeding more efficient still. It is a "fast" breeder, because it is mostly highly energetic ("fast") neutrons that are absorbed by uranium-238.
Overall, the breeder technology is around 60 times more efficient in its use of uranium than a standard fission reactor is, and such an approach could extend the viable lifetime of nuclear to several hundred years, even allowing for the proliferation that King and Lovelock and others are promulgating. However, we would surely be replacing one form of pollution by another. If we cut back on CO2 emissions, which may be a good thing, although there are still some who remain unconvinced as to the link between anthopogenic CO2 production and global warming, further pressure is impressed upon our already heady dilemma of what to do with the existing burden of nuclear waste.
Unmentioned explicitly too in arguments favouring nuclear is the "P-word", "Plutonium", which would be manufactured on an unprecedented scale, presumably in politically dodgy regions of the planet, if it is a global effort that will be made, and ther is every reason to believe that terrorists or megalomaniacal governments could take advantage of the situation, either by getting hold of the material directly and fashioning it into dirty bombs, or by blowing-up the breeder reactor in-situ. Surely, more could be done in terms of energy efficiency and sustainable energy production, unless there is another agenda that requires plutonium for other purposes, such as a revamping of the nuclear weapons programme.


Anonymous said...

Well now that there seems to be an extended solar minimum we may now be able to compare more directly how CO2 heats the globe.

Professor Chris Rhodes said...

That will indeed be interesting as the whole matter of CO2/global warming/climate change is far from resolved in all details!

If it is down to solar activity (more than CO2) as some claim, then we might expect a cooling phase. There was a book published about a year or so ago which made the claim that there is a roughly 1,500 year cycle of solar-activity, which it's authors think correlates with the warm medieval period, the little ice-age etc. and that we can expect to enter the cooling phase of the cycle shortly.

I keep an open mind about anticipating the behaviour of such a thoroughly complex system as the combined "Earth systems" provide.