Fission and Fusion

From StoneHome

Admittedly, this one isn't so prevalent at the time of writing. That said, it'll be coming up again Real Soon Now™, given that whether or not cold fusion is possible people will keep trying it and the media will keep hyping it, and given what the media allowed to wash up from the murky depths of their editorial abyss around the time of the sonoluminescense is cold fusion maybe-debacle, I feel the need to stage a pre-emptive strike.

This is generally screwed up by the same people which confuse atomic and nuclear.

This One's Easy

Fission and fusion are opposites. Both are actions of the nuclei of atoms.

Fission, Our Nuclear Friend

It's sort of odd for me to refer to fission as nuclear power, when fusion is also a well-wanted power source and is also a function of the nucleus. However, fusion-as-power remains outside current technology, and is something of a short-term (50 years) grail of the energy production community. Fusion was first achieved by Enrico Fermi at the University of Chicago in the early 1940s.

The basics

What we generally call nuclear power - the stuff driven mostly by uranium, plutonium or thorium as carbides, hexafluorides or oxides - is the result of the transfer of heat from a fission source such as a pebble bed or a nuclear pile to some kind of mechanizer like a turbine or a dynamo. The radioactive material decays, throwing off paired pairs of protons and neutrons through alpha decay. Those paired pairs - alpha particles - proceed to whack into some medium, typically water, which is then heated by the kinetic energy and excited into a steam (pretty much the same mechanism that your microwave uses to cook food with its own moisture.) The steam then drives a turbine in the fashion of an 1830s ironclad.

Fission is the result of atoms splitting, or shedding fragments, depending on (if you'll pardon the pun) the particular isotope in question. Radioactive isotopes have predictable decay characteristics involving both time and output. Radioactive decay is suspected to be purely random; the predictability of decay is an observation of the massive Monte Carlo system which results, where potential decays are so common that the statistics plateau far below our threshhold for giving a damn. Most fissionables shed fragments, often in the form of alpha, beta or gamma decay, though there are other things which get spit out in rarer situations. Less commonly still, certain isotopes outright split, leaving two seperate, frequently similarly sized atoms in the wake. Most splits also involve ancillary decay.

What's really going on?

Fission occurs because of the conflict between the strong nuclear force, which the British insist on calling the "color force" as if physics were a saturday morning cartoon, and the electromagnetic force. The electromagnetic force operates on the magnetic charge inherent in particles: in the case of normal atoms, the positive charge of protons and the negative charge of electrons repel one another, causing the distant orbit of electrons around the nucleus. The strong nuclear force by contrast holds the atom together. Problematically, the electromagnetic force operates over long distances, whereas the strong nuclear force only operates over very short distances. When atoms get over a certain size, they actually go past the complete reach of the strong nuclear force, at which point the electromagnetic force can overwhelm the strong nuclear force and spit something out of the nucleus.

This limit is surprisingly low; if current models of particle physics are correct, Fe⁵⁶ - one of Iron's stable isotopes - should be the stablest nucleus possible, and should be the cutoff point for both fission and fusion.

Upshots

Fission produces a very large amount of energy by chemical reaction standards from a very small amount of fuel. A commonly quoted statistic is that a pound of uranium can provide roughly the same power as three million pounds of coal. However, the waste which results from current fission reactors is fantasically toxic and dangerous, with a safety decay span frequently in the five-digit range of years; furthermore, the waste from some reactor types, particularly the otherwise very desirable breeder reactor can be used as otherwise extremely difficult to attain fuel for thermonuclear weapons. Fission fuels are extremely rare, and only certain isotopes among these fuels are actually useful; even using enriched uranium, no current nuclear reactor design known to the public uses more than 0.8% of its fuel before ejecting waste products. It is estimated that our planet currently relies on fission power for about 20% of its power generation, and that were the world's energy economy converted to solely nuclear power, that currently known fuel sources would run dry in approximately 150 years.

Both of the two nuclear weapons used in war, America's Fat Man and Little Boy, used over Hiroshima and Nagasaki during the end of World War Two, were fission bombs. Little Boy and Fat Man were purely fission devices, in the neighborhood of 20 kilotons, and each carefully placed all but wiped out a city which at the time was the largest in its respective prefecture.

Fusion

Fusion, on the other hand, is far more powerful.

The basics

Whereas fission is the result of atoms splitting, fission is the result of atoms being forcibly combined. The amount of energy released by the collapse of two nuclei into one generally puts the thermal conversion of fissiles to shame relatively quickly. Fusion is the force that drives the sun and modern nuclear armaments.

What's really going on?

The same model of atoms as an interplay between the strong nuclear and the electromagnetic forces is at play here, but their roles are reversed. This time, we overcome the electromagnetic force of the protons' mutual repulsion, and shove two atoms together (think about trying to push opposed magnets together.) Once you get the nuclei of those two atoms close enough together, the strong force which holds normal nuclei together can take over, and the nuclei will fuse, emitting one of the neutrons at high energy and causing a miniature shock wave when the electropotential charges between atoms suddenly shift (as if you had a bucket of magnets and two suddenly melted together; the nearby ones would fall nearer to the melted one, then the ones near those, and so forth.) That high energy neutron releases far more energy than it takes to cause two nuclei to be forced together (roughly 17.59 and 0.11 MeV, respectively;) this tends to lead to chain reactions when the fuel isn't in a position to flee.

This mechanism forms the basis for modern thermonuclear weapons, including the Castle Bravo device which devastated the Bikini Atoll and caused the US to partially evacuate the Marshall Islands (the largest nuclear weapon the US has ever detonated) and the Tsar Bomba device detonated at Novaya Zemlya by the USSR (the largest nuclear weapon ever detonated.) In constrast to the 20 kiloton weapons which as a pair killed more than a million Japanese and annihilated two large, thriving cities, the Castle Bravo was 15 megaton (15,000 kiloton!) and the Tsar Bomba 50 megatons (originally designed for double that.)

The Tsar Bomba's blast was so large that at a hundred kilometers a person would be covered in potentially fatal third degree burns; the mushroom cloud was sixty four kilometers tall. For a sense of scale, Mount Everest is nine kilometers tall; the troposphere, where all weather occurs, is about sixteen kilometers tall. In fact, the mushroom cloud reached just barely above the limit of the stratosphere, generally considered the limit of our classical atmosphere, into the very bottom of the mesosphere, named because it's the middle space between the classical atmosphere and the ionosphere where particles are charged by solar radiation. In fact, the blast was so large that much of its energy was released directly into space; this is one of the primary motivating factors for <acronym title="Multiple Impact Re-entry Vehicle - a missile with many warheads which strikes multiple targets">MIRV</acronym>s, to allow larger blast-effect weapons without the inefficiency of losing payload to the high atmosphere and space.

So why isn't this on the grid yet?

Keeping fuel from fleeing turns out to be spectacularly difficult. One can't use a physical container; the temperatures at which fusion occurs in well-known models like the sun (typically in the neighborhood of 95 million degrees fahrenheit to begin) render physical materials into plasma without a second thought. A number of mechanisms have been attempted for controlled fusion, including laser pulse compression of tiny amounts of fuel, twisting toroidal magnetic bottles, electrically charged plasma, and some seriously strange exotic stuff. There has been limited success at generating controlled fusion, starting with British kilocharged plasma and later British pinch devices in the 1950s; the 1968 Soviet Tokamak is generally seen as the first machine to reliably generate controlled fusion. Unfortunately, that design as well as all current designs require more power to create the situation than they produce; whereas we have functional fusion devices, they currently consume more power than they provide.

Aren't there other mechanisms?

Sure, we've tried a bunch of stuff. We've tried replacing electrons with muons, which orbit at 1% the distance of electrons, making it much easier to induce fusion. Unfortunately, muons decay in .0002 seconds and are difficult to produce in any significant quantity, and using the collapsed state only ends up providing about 20% of chain energy levels, so using them to spark fusion has not proven practical thus far. Another attempt, made briefly famous in the late 80s by a team of U. Utah researchers whose school violated publishing ethics regarding reproducability in effort to garner publicity, is sonoluminescence - the use of soundwaves in an excited tritium-rich water to cause glowing bubbles (at the time nobody was sure why they glowed; small-scale fusion at soundwave interaction points was suspected, discarded, suspected again, discarded again, and is currently suspected yet again. Also, eggs are healthy. Today.) We've tried sending pulsed shock waves into plasma, colliding partial fusion states, and a bunch of other stuff. Some things work better than do others; with time, we'll find one that works well enough.

Notably, muon excitation and sonoluminescence are attempts at a specific subfield of fusion called cold fusion, whose goal is to achieve fusion at reasonable temperatures instead of making a very small star. Science is currently bitterly divided on whether cold fusion providing break-even energy is at all possible; most scientists view sonoluminescence as snake oil, but muon excitation is a known working model for cold fusion which just doesn't provide break-even energy. Some take this as an indicator that there's a possible mechanism which we just haven't found; others suspect this is wishful thinking.

Won't we just have more oil wars?

Real estimates of hydrogen fusion fuel sources in Earth's oceans alone at current power consumption rates give us an estimated 80 million years of fuel; that's before you consider the atmosphere, or the immense amounts of deuterium and tritium in the Moon's surface. To put that into a more tangible context, at current power consumption rates, the hydrogen from a coffee mug of clean water would be able to power New York City for about four and a half years; therefore, we could power the entire planet from a reactor hooked up to the party drain in a fraternity basement, or the groundwater runoff from watering a single large lawn. Furthermore, the fuel is extremely easy to extract from water through simple electrolysis (you can do it with about twelve dollars of gear from your average Wal-Mart, and you can do it quite well if you also get a thirty dollar earring to use as an anode; yay platinum.) Of course, the second we have that kind of energy, we'll find a way to use it, and then we'll have a power crisis again, but that's for my children's children to figure out.

Is this safer than fission?

The process is difficult, but not terribly dangerous; if everything goes wrong, all that's happened is the release of a small amount of trivially radioactive light gases (less than you'd see in a day's waste from a coal-burning plant.) Moreover, because the ideal fuels are hydrogen and helium, both available in abundance in our atmosphere and ocean, and because the waste products are helium, lithium, beryllium and boron - all relatively benign substances - fusion is a good contender for a long-term clean power source, once we get it working. Some reactors are proposed to use higher-weight fuels, which emit more of their energy as charged particles and less as neutrons during fusion.

And this makes weapons? Like the sun?

The Atomic Archive maintains a fairly horrific estimate scenario for the detonation of a 150-kiloton bomb in Lower Manhattan. Please note that Fat Man was a 22 kiloton bomb, and that Tsar Bomba was a 50,000 kiloton bomb, while reading this scenario.

Why is iron a cutoff point?

The size of Fe⁵⁶ happens to be right at the size where the strong nuclear force and the electromagnetic force are in equilibrium. If a nucleus is larger than that limit, then we can exploit the electromagnetic force to cause the atom to spit out particles. If a nucleus is smaller than that limit, we can smash nuclei together and get power from the resulting particle collapse. The further a nucleus is from that limit, the more powerful a reaction you get. Should you attempt to cross that limit, you actually need to input energy. Fusing atoms larger than iron takes energy; fission of atoms smaller than iron also takes energy. Therefore, Fe⁵⁶ can be seen as the cutoff point for getting energy out of either process.

The color of stars

As you may remember from chemistry, the color of a flame indicates its heat and the composition of the fuel. Stars are no different, since it's actually the light signature of the excited fuel and gasses (rather than the oxidation) which gives us that color signature. (Remember, the sun's not burning, it's fusing. Very different processes.) Consider the case of a large, young star made mostly of hydrogen. As time progresses, that hydrogen is fused into helium. Helium continues to fuse, with slightly less power than the hydrogen had; because it takes up less space per quark than helium does, and because there's less thermal expansion, the star takes far less space at this point, and its color shifts from white to either light blue then blue or to light yellow then yellow, and from either then onto red, depending on their chemical composition. This is the basis of stellar spectral classification.

Simple stellar models generally expect stars to terminate as their masses fuse towards iron. However, the immense gravity can cause trans-iron fusion at the core, which can occasionally lead to internal fission; the chemistries of old stars can be quite complex. Some red and brown dwarves also have spectra indicating molecules rather than just atoms.

Nuclear, Atomic and Hydrogen Bombs

These phrases don't actually mean something specific. However, through common use, they have begun to take the shape that atomic bombs refer to primarily fission devices and hydrogen bombs to primarily fusion devices. It's important to realize that in modern ordinance the line between the two isn't particularly clear; a smaller, more easily initiated fission reaction is usually used to spark a fusion reaction, and fusion reactions are doped with lightweight elements to cause localized fission, which because of things like the Mossbauer effect on local small bodies and crystal resonance to sub-phonon excitation can cause significantly more efficient fission. Nuclear bomb tends to be a catch-all phrase.

To Review

Fusion is pushing two atoms together until their nuclei combine, and those atoms must be smaller than the iron isotope Fe⁵⁶ if one wants to net energy from the reaction. Fission is using or encouraging the natural tendency of superheavy atoms to split, and taking the resultant energies into a medium, generally to be converted to electrical or mechanical energy.