The first installment of this series described the four main components of the energy/global warming crisis: increasing demand coupled with decreasing supply, the need to get away from fossil fuels, the need for vast amounts of energy just to keep things running the way they are, and the huge social/political/economic inertia that has to be overcome to address the first three components. While there are a number of possible ways to do all of that, there may be an emerging technology that would be the "Silver Bullet" answer to all of the above - or at least the first three.
It's nuclear energy - but not the kind that's been in use for decades. Fission reactors using uranium, plutonium or other combinations of heavy elements come with too much baggage to be easily adopted as The Answer. Nope, the best candidate for a true Silver Bullet is nuclear energy based on fusion. The problem is, it's much harder to get energy out of fusing little atoms together than it is from splitting great big atoms apart and no one has yet made it work well enough to be practical. But first, some science!
This installment is intended as a primer for some of the basic concepts needed to understand the nuclear physics involved in fission and fusion. The really neat stuff will be in Part 4: The Dynamic Alternative, but for those who may be a little uncertain on some of the fundamental stuff, I'm going to sketch it out here first with lots of links. For those who find it oversimplified or incomplete, my apologies. I'm going to try to put in enough links to supplement what I gloss over. (Lots of wikipedia et. al., in other words.)
And of course, many of you are probably looking at Pennsylvania today anyway instead of reading this, looking for reports of another 'meltdown' - though hopefully not nuclear!
Know Your Particles
Let's start with some basics - what atoms are made of, how they fit together, and how they work! An atom is an assembly of three kinds of particles: electrons, protons, and neutrons. Protons and neutrons are big, massive particles compared to electrons. Protons have a positive charge, electrons have a negative charge, and neutrons (as you might guess) have no charge at all.
Protons and neutrons clump together in what is called a nucleus at the center of the atom; the electrons orbit around the nucleus in a cloud. Left to its own devices, an atom will have exactly as many protons as it has electrons. That's because the opposite electric charges of the electrons and the protons have to be balanced. When atoms have an electric charge (become ionized), it's because they've somehow gained or lost some of their electrons. Lose an electron, and an atom become positively charged. Gain one, and it has a negative charge.
Swapping electrons around is the basis of chemical reactions. Swapping around protons and/or neutrons is what nuclear reactions are all about. It's much harder to do because protons and neutrons are held together pretty tightly in the nucleus (by the strong nuclear force), and they're really massive compared to electrons. (Think bowling balls versus gnats.) On the other hand, the amount of energy that can be obtained from nuclear reactions is orders of magnitude greater than that from chemical reactions. MUCH more bang for the buck. (Sometimes literally- that was the rationale behind the first A bombs after all.)
The number of protons and the associated electrons are what make an atom of Boron (5 protons) different from an atom of Carbon (6 protons). Each element has a specific number of protons (the atomic number), from the simplest - Hydrogen with one - all the way up to big atoms like Uranium with 92 protons. Once the atomic number gets up around a hundred or higher, atoms of that size tend to be very unstable.
The number of neutrons in an atom can vary. A certain amount helps provide packing material between the protons in effect, but some combinations will make a nucleus unstable and it will tend to start shedding particles to adjust. (radioactive decay) It's one of the sources of natural radiation.
The Same but Different
When two atoms have the same number of protons but different numbers of neutrons, they are said to be isotopes. Chemically, isotopes of the same element are pretty much identical - but their different weights (protons plus neutrons = atomic mass) means they can be separated by mechanical means. That's what the fuss is about over Iran and centrifuges. A really special kind of centrifuge can spin down Uranium isotopes to separate out the ones that can be used to make nuclear reactors and/or nuclear weapons.
Uranium 238 has 92 protons and 146 neutrons; Uranium 235 has 92 protons but only 143 neutrons. U235 is the one used for fuel in nuclear fission reactors - and atomic bombs - because it is less stable than U238. Deuterium is the name for a Hydrogen atom that's got a neutron keeping company with its single proton. Tritium is a Hydrogen atom that has 2 neutrons packed in there with that proton. Both isotopes are more massive than a plain vanilla Hydrogen atom (aka Protium) - and that has implications for fusion reactions.
Breaking Up Is Easy To Do
As I mentioned before, fission reactors are comparatively easy to build, because they use elements like Uranium isotope 235 for fuel; U235 is ready to fission all by itself. It's so unstable atoms of U235 will spontaneously spit out neutrons at random, and that starts a sequence of further break down (chain reaction) that eventually ends up with all of the Uranium turning to Lead - plus a lot of radiation and other forms of energy along the way.
If you put enough U235 atoms together (a critical mass), at any given time there are enough neutrons getting spat out that they start hitting the nuclei of other Uranium atoms that are still waiting their turn, and that sets them off! If they all go at once, you have an atomic bomb. If you have some elaborate machinery to control how many neutrons are shooting around at any given time, you have a nuclear reactor with controlled fission.
If you want a vivid image to picture this, think of a bunch of hand grenades all equipped with timers that will set them off at random intervals. Scattered around, with stuff piled up around them, you get random explosions that don't do much outside the immediate blast area. Start putting them closer together, and every time one grenade goes off, it may set off another one or two or three so you get a constant series of explosions which can be contained and put to work. Pile them all together, the first one to go off will set them all off! Best to be elsewhere when that happens.
But Getting Together is Hard
Fission reactors need elements that are already breaking down on their own to operate. Fusion is something altogether different. It doesn't happen spontaneously; it involves the nuclei of two or more atoms being forced together hard enough to overcome the repulsion of the positive charges on the protons in the nucleus. The larger the nucleus (the more protons), the greater the repulsive force. That's why most fusion work is concentrated on fusing the smallest atoms (Hydrogen in its various isotopic forms) into the next larger atom, Helium. It's possible to use other atoms though, and the end products can be different. The new, bigger atom may be stable as it is, or it in turn may divide into smaller atoms and stray particles, and so on until the final result is an assortment of stable atoms and debris - and energy.
Theoretically, any two atoms of lighter elements can be fused together up to the atomic mass of iron or nickel and there will be some excess mass that gets converted to energy in the process. (Yes, this is the classic Einstein E = mc squared equation.) Fusing atoms heavier than Iron requires more energy than is released. That's why it only happens naturally inside exploding stars. It takes the energy of a supernova to slam atoms together hard enough to make the really big elements.
So, in practice the bigger the atoms, the harder it is to fuse them. But, pick the right combination of atoms and the net result is a new set of atoms along with the release of energy. And - this is where fusion differs from fission - it's theoretically possible to select elements for fusion fuel such that when they combine, the results are not radioactive, or only minimally so. Potentially, fusion reactors have none of the radioactive hazards of fission reactors, don't produce long-lived radioactive wastes, and do not emit any green house gases when they operate. And, the fuel itself doesn't have to be dangerous and can't be used to make weapons.
Theoretically, potentially, practically - weasel words that suggest fusion is still a long ways off. Maybe yes, maybe no. Possibly, a practical fusion power reactor will be built somewhere around 2050. Or, thanks to some fresh ideas and research that is only now being made public, it could be a lot sooner and much easier than anyone could have expected even as recently as 2006. The next installment of this series - The Big Squeeze - will lay out the details of the current state of the art. I'm going to try and get all the installments out this week, usually posting around 5:00pm east coast time, so stay tuned.
Update: If you want a really sobering assessment of where the world energy is headed, this story from NPR's Morning Edition will give you chills. A short item in the afternoon news on oil prices only reinforces the message. Listen to the first one to get the full story - the transcript omits the part where Steve Inskeep gets Shell strategist Jeremy Bentham to acknowledge that they don't expect to see a scenario where the world's political leadership organizes a real response to the energy situation.
Meanwhile, for some appropriate mood music, try this track from Bill Parsons.