Saturday, September 22, 2007

Kepler's stuff - my 'greatest hits' from 360

Fusion - The Power Generation Technique of the Future - Part 1

One way to categorize various techniques for generating eletrical power is:

1) Mechanical. This covers hydro power, wind power, and some funky variants like ocean-wave power and ocean-current power. All these use some natural phenomena to turn a permanent magnet in a coil, which generates electrical energy. The source can be continuous (like today's hydro) or intermittant.

2) Chemical. This is IMHO the most environmentally damaging... coal, natural gas, internal combustion engines., etc. In terms of the mass involved, it is a step up from mechanical power. That is, it requires far less mass to generate the same amount of power, as compared to mechanical power generation systems.

3) Nuclear fission. It turns out if we split certain atoms in the right way, energy is released. If that energy is harnessed, we get nice stable electric power. This is a step up from chemical power - meaning it doesn't take much mass at all. If done properly it generates very little waste.

4) Nuclear fusion. It doesn't exist... yet. But Any Day Now, Real Soon it will.

Fusion is slamming the right kinds of atoms together fast enough that the nucleii fuse and form a new kind of atom. (this is how the Sun makes energy) Sometimes (depending on the fusion reaction) energy is released also, and sometimes that energy can be usefully harnessed.

The problem is that atomic nucleii are positively charged - they repel each other, so they don't normally collide. But if they are heated to a sweltering 100 million degrees (Celcius, though at this point it hardly matters!) and held in a confined area, the nucleii will be zipping (technical nuclear physics term) around fast enough they'll sometimes collide.

Cool - so all we need it a container capable of holding a hundred million degrees (ouch!) under high pressure!

It doesn't exist, it *can't* exist. Bummer. [as a side note, this is why there was so much excitement over the bogus 'cold fusion' announcement in the 1980s... if fusion could occur at temps and pressures we consider normal, we'd be home free]

But wait - it doesn't have to be a physical container. Because the nucleii are charged, they can be affected by magnetic fields. So if we make a really really really really really really strong magnetic field we can confine the nucleii... this is actually possible with today's technology, even at a temp of 100 mil. Yeaaa, we're home free!

Well, not quite.

Most of today's experimental fusion research programs are trying to achieve the easiest useful fusion reaction involving the fusion of deuterium (hydrogen with an 'extra' neutron in its nucleus) and tritium (hydrogen + 2 extra neutrons).

D(euterium) is found in seawater and we know how to extract it.

T(ritium) is radioactive with a half-life of 12.33 years. So it's not found naturally, but we can (and do) know how to manufacture it.

The nuclear reaction is:

D+T -> He4 + n

The He4 nucleus is positively charged, so it will stay in our magnetic trap, smashing into other nucleii there & helping to heat them.

The n)eutron isn't charged, it won't even see the magnetic trap & it will zip (nuclear physics term) right out at a high rate of speed (about 1/6 the speed of light!) in some (essentially) random direction and smash into the first thing it sees. It's nice if that isn't you or me! It is commonly a lithium 'blanket' used to capture speeding neutrons. The blanket will probably absorb most of the energy of the speeding neutron, getting really hot. Using a lithium blanket does two useful things:

1) It will usually absorb the neutron, generating some Tritium as a possible result. So a D-T reactor can breed its own fuel.

2) Because it absorbs the speeding neutrons it gets hot. Hot enough we can wrap some steam pipes around it and run them to a turbine, creating power while cooling the blanket.

Sadly, we can't guarantee the blanket will capture all the neutrons. The metal of the reactor is bound to capture some as is the metal of the steam pipes. Depending on how exactly the capture occurs, radoiactive by-products will be formed.

So although a D-T fusion reactor doesn't directly create radioactive by-products, they are formed by the practical realities of building such a reactor. The energy created is on the order of 100 times more than the already-good nuclear fission reactor, and the waste is on the order of 100 times less. A D-T fusion reactor that produced as much energy as it cost (to maintain the magnetic fields) was first demonstrated in 1997 at the European JET Tokamak. It's all good... but it can be even better.

Coming next: Part 2, the D-He3 fusion reactor. Watch me get my geek on!

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