Nuclear Fusion?

Nuclear fusion, if it can be made to work at all, won’t as clean as they claim. Here’s why.

Who am I, to be writing this?

See Nuclear Engineering – a bit of personal history.

Background: nuclear fission

Current nuclear reactors are fuelled with uranium. The energy is produced by the splitting (fission) of uranium (235U) nuclei. When a 235U nucleus splits, several neutrons are released. These neutrons can then hit other 235U nuclei, causing them to split, producing further neutrons, and so on, in a chain reaction.

(Some of the neutrons are absorbed by 238U nuclei, which converts them into 239U, which decay rapidly, in two steps, to plutonium (239Pu). This is also fissionable, just as 235U is. Some of it then gets split in the chain reaction, and some remains unsplit, just as the 235U does.)

In an atom bomb, the chain reaction is allowed to expand rapidly, with as many neutrons as possible going on to split more uranium (or plutonium) nuclei, until all the uranium is used up – or until the energy produced has blown the whole thing apart with some uranium (and plutonium) nuclei still unsplit. In a reactor, most of the neutrons are mopped up by neutron absorbers, leaving, on average, only one from each fission to go on and cause another fission – slightly more if you’re turning the power level up, or slightly less when you’re turning it down. These small variations are achieved by moving control rods (made of strongly neutron-absorbing material) in and out of the reactor.

When uranium (or plutonium) nuclei split, apart from the neutrons, two smaller nuclei are produced in each fission. These fragments are called fission products. They’re not always the same nuclei – there’s a huge range of possibilities, and all the possible fission products are produced in different quantities in a reactor. The exact proportions of each depend on the conditions in the reactor.

Most of these fission products are radioactive – most of them much more radioactive than the original uranium. This is where the bulk of the radioactive waste produced by the nuclear industry comes from. Initially it’s billions of times more radioactive than the original uranium, but the extremely radioactive atoms decay away very quickly, and after a few years it’s only millions of times more radioactive than the original uranium. Nonetheless, it’ll be a couple of thousand years before the level of radioactivity gets down to that of the original uranium.

In addition to the fission products, some radioactive waste is produced by activation of other parts of the reactor, such as structural components, the cooling system, control rod mechanisms etc. Activation occurs when atoms of these other parts absorb neutrons, changing them into different kinds of atoms, some of which are radioactive. In a fission reactor, activation products are heavily outweighed in long-term significance by fission products.

Radioactive waste is nuclear power’s Achilles heel – or one of them, much the biggest.

Nuclear fusion

An alternative kind of nuclear reactor would be a fusion reactor, in which small nuclei get stuck together (fused) into bigger nuclei, rather than big nuclei being smashed into smaller ones as in a fission reactor. The Sun is a big fusion reactor, and fusion bombs have been successfully made (more’s the pity). Controlled fusion reactions for power generation are much harder to achieve – they’ve been trying for over fifty years, and they’ve not succeeded yet. Billions of dollars are still being spent trying to make it work.

A claim often made by fusion enthusiasts is that nuclear fusion will produce much less radioactive waste than nuclear fission does. They say that the only radioactive waste will be activation products. This is true of the fusion reactor itself, but it’s dangerously misleading.

A fusion reactor needs a supply of fuel – deuterium and tritium. Deuterium (1 proton, 1 neutron) exists aplenty in seawater; tritium (1 proton, 2 neutrons) does not. With a halflife of only 12.33 years, it doesn’t occur in significant quantities naturally anywhere on Earth. It has to be manufactured.

Neutrons are required to manufacture the tritium. There are two possible sources of neutrons in sufficient numbers: the fusion reactor itself, or fission reactors (some enthusiasts suggest a third possible source, see below). The neutrons are used in reactions with lithium. The lithium can be either in a blanket around the fusion or fission reactor, or in tubes passing through the fission reactor.

The fusion of one tritium nucleus with one deuterium nucleus produces one Helium-4 nucleus (2 protons, 2 neutrons) and one free neutron. That one neutron can produce one tritium nucleus, which can then be separated from the unreacted lithium for later use in the fusion reactor. (There are complications here, but I’ll deal with them later.) The neutrons produced in your fusion reactor are all needed in the manufacture of tritium – you can’t afford to waste any.

It’s an extremely difficult engineering problem just making a working fusion reactor – without considering that somehow you’ve got to make sure that every neutron produced ends up making new tritium. Any neutrons that are absorbed in the reactor structure, or the superconducting magnets, or any part of the system other than in producing tritium, are wasted. Somehow you’ve got to make up the deficit – which means fission reactors.

You’d probably be doing well to produce half your tritium using neutrons from your fusion reactor – which would mean you’d need to produce the rest in fission reactors. For a given power output, fission reactors produce far fewer neutrons than fusion reactors. The fission reactors would produce several times as much energy as the fusion reactor – and of course generate correspondingly large quantities of radioactive waste.

Overall, the fusion reactor and its fission reactor fuel suppliers will produce less radioactive waste than a fission only system – but not very much less.

In principle, there’s an alternative that needs no tritium, see below.

Economics and renewables

Why isn’t the same amount of effort and money being put into developing solar power and energy storage (for when the sun isn’t shining and the wind isn’t blowing)? The technical challenges are far smaller, and the potential rewards far greater.

Nuclear enthusiasts complain that solar power isn’t economic – and today, at the current state of development and at current prices, that’s still true. How much more true is it of nuclear fusion? What is more, it’s still true of nuclear fission, even after fifty-odd years of commercial nuclear power. Fifty years ago, they were promising "electricity too cheap to meter" – and it’s still so uneconomic that no private company would take it on without government subsidy or guarantees covering future liabilities.

Nuclear enthusiasts also complain that there’s no economic solution to the energy storage problem. Funny how there was a solution to it when the nuclear industry needed one. Nobody even thought about whether it was economic, or ever would be. The pumped storage system at Dinorwig in North Wales was built to store energy from nuclear power stations during times of low demand, because nuclear power stations want to run at full power all the time. This is because they have very high capital costs, which have to be paid whether they’re producing electricity or not – whereas a fossil fuel power station’s costs are mostly fuel, and are dramatically reduced when the power output is reduced.

One tritium per neutron? Well, it’s not quite that simple.

Fusion produces high energy neutrons. If a high energy neutron hits a Lithium-7 nucleus (Li-7), it can produce a tritium nucleus (H-3) and another, low energy, neutron. This low energy neutron can then go on and hit an Li-6 and produce a second H-3. On the face of it, this means that you can get two H-3s for each original, high energy, neutron. However, regardless of the ratio of Li-6 to Li-7, only a small proportion of the neutrons will actually do this. Most will simply bounce off either Li-6s or Li-7s a few times, losing some energy each time, until they’re no longer high energy neutrons.

The other complication is something else that a high energy neutron sometimes does to either an Li-6 or an Li-7. Instead of splitting it and producing an H-3, it can knock two neutrons out. Each of these can then produce an H-3 from an Li-6. Again, however, only a small proportion of neutrons will do this.

Far more neutrons will be lost in other processes that produce no H-3 at all than will produce two H-3s by either of these processes.

For a diagram of all the relevant reactions, see Neutron-reactions-in-a-Lithium-blanket

The third possible source of neutrons: Particle accelerators

You can use a particle accelerator to fire protons at heavy nuclei such as lead, which results in fission (or spallation, a different process but with similar consequences; the difference is of no consequence for this argument) and the production of neutrons. This costs energy (rather than producing it, as in a normal fission reactor) to produce the neutrons, and still produces radioactive fission products. It has two advantages over a fission reactor: it doesn’t produce transuranic elements such as plutonium, and it produces fewer fission fragments per available neutron. Apart from the high energy cost, it has one of the same problems as a fission reactor: the decay heat of the fission products means that it requires active cooling even when it’s not operating – the failure of which was the cause of the disasters at both Chernobyl and Fukushima.

Existing particle accelerators operate on a minuscule scale compared with what would be required. This would be an entirely new and more demanding kind of engineering project, and at best would be only slightly less polluting than using fission reactors.

Fusion without tritium?

At a much higher temperature (400 million °K as opposed to 45 million °K), deuterium will fuse with deuterium, no tritium needed. Containing a plasma at that much higher temperature would be far more difficult than containing the deuterium-tritium plasma – itself so difficult it’s not been done on any useful scale yet, with nearly sixty years of work.

Deuterium-deuterium fission also produces copious neutrons – far more than fission, for a given power output. This would inevitably produce significant quantities of radioactive waste – and degrade the reactor hardware – at a rate proportional to the power output.

Unlike D-T fission, D-D fission also has numerous side reactions. This complicates matters considerably, but a full discussion of this is beyond the scope of this article.

Fusion without neutrons?

At higher temperatures still, deuterium will fuse with the light isotope of helium (He-3). This has the virtue of not producing neutrons, but inevitably D-D reactions will also occur, which do – in fact, these reactions will vastly outnumber the D-He3 reactions. But at yet higher temperatures, two He-3 nuclei can fuse, which again produces no neutrons. With no deuterium present, there wouldn’t be any side reactions.

However, apart from the extreme temperatures required, there’s a problem with the supply of He-3, which is extremely rare. There’s more to be said about this ludicrous proposition, but that’ll be another essay.

Scientific American agrees!

I published an earlier version of this article on deviantart on October 13th 2009 (coshipi.deviantart.com/art/Nuclear-fusion-No-thanks-140178059). In March 2010 Scientific American published an article saying exactly the same thing. I’ll quote one short passage from the Scientific American article here:

“In this chain reaction, you cannot lose a single neutron, otherwise the reaction stops,” says Michael Dittmar, a particle physicist at the Swiss Federal Institute in Zurich. “The first thing one should do [before building a reactor] is to show that the tritium production can function. It is pretty obvious that this is completely out of the question.”

The Scientific American article is Fusion’s False Dawn – in the March 2010 issue. Sadly you can’t read it online without an on-line subscription.

However, there’s a long discussion with the author of the article online: www.scientificamerican.com/podcast/episode/wheres-my-fusion-reactor.

Further reading

There’s an interesting discussion on tritium supply after this piece in the American Association for the Advancement of Science’s journal, Science: Fusion megaproject confirms 5-year delay, trims costs.

See Nirex report: nuclear waste for more information about how radioactive waste is nuclear power’s Achilles’ heel.