Fusion: Our Saviour

fission
Image courtesy of the BBC

Limitless, clean, affordable power. It has been the dream and ambition of many a politician, philanthropist and crackpot perpetual-motion-machine con artist; it is, however, a reality. At least in principal, anyway. But, Jason, how can this be; I hear you say. Well, it’s simple. At least in principal, anyway. All you have to do is create a star 10 times hotter than the core of our sun here on Earth. Easy! This is the idea behind nuclear fusion, the opposite process to what happens in nuclear fission, which is what currently happens in nuclear reactors across the world. In fission, a big atom of uranium-235 is hit with a neutron resulting in the splitting of that atom. This produces two smaller fission products, normally krypton-92 and barium-141, and two or three other neutrons; these other neutrons are free to go and blast into another atom of uranium-235 and so you have your chain reaction. Now, heat is simply a measure of how fast molecules are moving and, given that the neutrons produced are moving very fast indeed, one of the outcomes of this reaction is an enormous quantity of heat. In a nuclear reactor this heat is harnessed like in any other type of power station to heat water which turns a turbine and so on.

Fusion is the opposite of this where instead of breaking large atoms into smaller ones we try to stick small atoms together to make bigger ones. To go into a little more detail, fusion is when we take two hydrogen nuclei and force them together to form a helium nucleus, but we don’t use your common-to-garden variety of hydrogen. Most elements in the Periodic Table can come in a variety of forms called isotopes. For example, there are three stable isotopes of oxygen; oxygen-16, oxygen-17 and oxygen-18. Oxygen-16 is the isotope that occurs most naturally and makes up over 99% of what we find in the atmosphere. Its nucleus contains 8 protons and 8 neutrons; oxygen-17 has one extra neutron and oxygen-18 contains yet another extra neutron. These are all still oxygen atoms as it is the number of protons in a nucleus that determines what the element is not the total atomic mass. Hydrogen also has a number of isotopes; hydrogen-1, hydrogen-2 and hydrogen-3, also called protium, deuterium and tritium respectively. Protium is just one proton all on its lonesome, in deuterium the proton has a neutron buddy along with it and tritium… you get the picture.

So in fusion we are actually pushing together one atom of deuterium and one of tritium, this produces an atom of helium-4 and one spare neutron that goes whizzing off to make a life of its own. One of the great potential boons of fusion power is that the fuels required, these heavier isotopes of hydrogen, aren’t too hard to come by. Deuterium is found abundantly in the oceans of the world, no problem there. Tritium is quite rare but it is believed that by installing plates of lithium in the walls of a fusion reactor it can produce its own tritium as lithium bombarded with high energy neutrons will do just that. Another massive plus is that there are no radioactive byproducts produced like there is in fission reactors. The by-products are heat, which is what we want; neutrons, which we can use to produce the next round of tritium; and helium, which is great because we’re actually running short of it on earth. The radioactive krypton and barium produced in nuclear fission has a very long half life and can be expected to stay radioactive for approximately 100,000 years. To put that into perspective; 100,000 years ago modern humans were still trying to find their way out of Africa using nothing more than spears and hand axes, or to put it another way, we are going to have to keep today’s radioactive waste safe until the Earth has undergone another three Ice Ages (and I don’t mean the next three instalments of the popular kid’s film). I wonder if the cost of that is taken into account when nuclear energy is claimed to be cheap?

fusion
Image courtesy of ITER

So far so simple; there is a problem, though. As hydrogen atoms comprise of a positively charged proton and electrically neutral neutrons the overall charge of the nucleus is positive and as any 12 year old knows: opposite forces attract and similar ones repel each other so forcing together two positively charged nuclei is what’s known in the trade as a non-trivial problem. Anyone who has tried to push together the two north poles of a pair of bar magnets will know that this is not something they want to do; they repel each other. It was James Clerk Maxwell in the 1860s that produced the equations that show electricity and magnetism are merely two sides of the same coin, hence electromagnetism: the force that stops two similarly charged objects getting too near each other, one of the four fundamental forces of nature. The force that holds the protons and neutrons in an atom together is the strong nuclear force, another one of the four forces; the other two being the weak nuclear force and gravitation. The goal of fusion is to push the repellent atoms close enough such that the strong nuclear force overcomes electromagnetism. Put another way, in a fission reaction we use the kinetic energy of a fast moving neutron to overcome the strong nuclear force of a uranium atom, thereby releasing energy. In fusion we are trying to use kinetic energy to overcome the electromagnetism between protons such that the strong nuclear force can take over and bind hydrogen nuclei together to form a helium nucleus, thereby releasing energy.

The reason that I’m talking about this whole nuclear fusion thing in the first place is that this is how the sun works, it is a giant fusion reactor. It has been doing this for approximately 4.5 billion years and it will carry on doing this for about another 4.5 billion years at which point it will become a red giant and swallow up the earth. If we could harness this process here on the 3rd rock then we would have that clean, limitless energy source that I mentioned earlier. Now, obviously we don’t want to turn the whole planet into a pit of hell fire so we need to do this carefully and in a controlled manner. So what are the conditions we need to achieve to get fusion to work on an industrial scale? In the core of the sun, where fusion is an every day occurrence, the temperature is approximately 15 million degrees Celsius and the pressure is 250 billion bar, 250 billion times the air pressure on earth at sea level. They’re some pretty extraordinary numbers. We can actually heat things to that temperature but achieving that pressure is beyond us; to compensate we need to up the temperature. This is why, in the experimental reactors currently being built and tested, the temperature we’re aiming for is more like 150 million degrees Celsius. That’s a spicy meatball.

FestooningLoops
This picture is here simply because it is awesome. Courtesy of NASA

You won’t be too surprised to hear that there is no substance on Earth, nor anywhere else in the universe, that could hold a cup of plasma that’s 150 million degrees Celsius; it would melt, and so the trick is to create these ridiculously extreme conditions without letting them touch anything. This is done by creating a plasma and then controlling it using superconducting magnets in a torus (doughnut) shape – known as the tokamak model. There are fusion reactors in almost every modernised country in the world, they’ve been there for over 50 years; but they haven’t been able to undergo fusion on the scale that would be required to make it sustainable as an industrial power source.

Our best hope for that lies in the ITER project in southern France (they have a great website if you want more detail and pictures of the enormous construction). This is an international collaboration between China, India, the US, Korea, Japan, Russia and the European Union; more than half of the world’s population is being represented. Its inception was at the 1985 Geneva Convention, its design was approved in 2001 and in 2006 the ITER organisation was officially brought into existence. Construction and initial testing is expected to take well over a decade with the deadline for it to be fully operational being November 2020. Only then will we know for certain if the dream of limitless clean energy is a realistic one on an industrial scale. Unfortunately even ITER is only ever intended to be an experimental proof of concept. If it is successful then the world can get on with making the very first demonstration units through the 2020s into the 2030s. Personally, I don’t see us having the first proper fusion reactor online til the second half of this century. The wait, though, will be worth it; though what we’ll fight our wars over I have no idea.

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