Physics and world politics: the neutron dance 6

George W. Bush & Co., the United Nations, and the world’s nuclear powers all have their underwear in knots about Iran’s nuclear enrichment plans. It’s a serious problem, enough that BushCo is implying it will take military action against Iran if Iran does not immediately cease and desist.

For the physics-challenged, let’s set aside political sabre-rattling for a moment and gloss on the term “uranium enrichment,” to get an idea what all this brouhaha is about.

First, some elementary (ha, ha) details about uranium, number 92 on the periodic table of the elements.

The 92 refers to the number of protons in the uranium nucleus; that is its atomic number. Protons have positive electrical charge, so without some “glue” to keep them confined in the nucleus, they would just fly apart. (Y’know, “opposites attract, and likes repel.”)

Joining the protons in the nucleus are a whole lot of neutrons, which help exert an opposing, attractive force, whimsically called the “strong nuclear force.” The strong force keeps all atomic nuclei (and in fact protons and neutrons individually) in one piece.

But the strong force, like Superman, has a weakness. The strong force only works within a range of about 10-15 meters, which is not coincidentally the approximate size of the typical atomic nucleus.

Uranium’s nucleus, with 92 protons and anywhere from 142 to 146 neutrons churning around in that tiny volume, is just a little too big for the strong force to hold things together. The electrical repulsion of the protons overwhelms the strong force, and the uranium nucleus breaks apart, or decays.

Like all other elements, uranium comes in “flavors” called isotopes. Different isotopes have different numbers of neutrons, but the same number of protons and electrons. Isotopes of a single element have identical chemical behaviors, but different nuclear behaviors.

The two most important isotopes of uranium are U-235 and U-238. Both are radioactive, but of the two, U-235 is more dangerous. It is also quite rare compared to its heavier sibling.

Radioactivity is also called nuclear decay. When U-238 decays, it releases alpha particles. It will continue to decay in this way until there are no U-238 nuclei left, which can take billions of years. It’s a fairly tame process.

U-235, on the other hand, tends to split (fission) into two lighter nuclei and in the process release two neutrons. Since U-235 nuclei have fewer neutrons to strong-arm things together, getting hit by a wandering neutron tends to split the U-235 nucleus apart rather abruptly. That split releases two more neutrons, which then split up two more U-235 nuclei. These two release four neutrons, which split four nuclei, releasing eight neutrons, which split eight nuclei, and so on.

That, friends, is called a chain reaction. If you can reduce the number of neutrons flying around, you can keep the reaction under control. Nuclear scientists call this process “moderating the reaction.”

Uncontrolled chain reactions lead to one of two scenarios: reactor meltdown (the “China syndrome“) or a whopping big explosion. The key ingredient is the percentage of U-235 involved.

Mixtures of about 2% to 3% U-235 and the remainder being U-238 make for relatively safe nuclear power plants. The heat from the fission reactions is used to boil water, to turn turbines that drive electric generators.

Pure U-235, on the other hand, is the devil’s own playtoy. With a sufficient amount (the “critical mass“), about 50 kilograms, you can make a pretty nifty atomic bomb. One A-bomb is enough to wipe out in a few seconds an entire city. The Japanese so far are the only people to have had this experience, in 1945.

If you can make an atomic bomb, with the right know-how and materials, you can also make an exponentially more powerful hydrogen bomb. H-bomb make A-bombs look like firecrackers.

So what does all this radioactive detail have to do with uranium enrichment? Well, U-235 is but the merest fraction of all available uranium. To accumulate U-235, you have to separate the U-235 from the other uranium isotopes in — you guessed it — enrichment facilities.

Now, Iran says that it just wants to build enrichment facilities for its nuclear power program. Perhaps its leaders realize that oil cannot last forever, and golly, it sure would be nice to have some other source of electricity to fall back on when the oil runs out.

The US and the other nuclear powers (which include Israel, btw) are not willing to give Iran the benefit of the doubt, however. The existing nuclear powers suspect Iran may just decide to use its supply of U-235 for bombs, as well as for electricity. Or maybe just for bombs. After all, they did.

[Bombarding U-238 with neutrons, incidentally, creates another bomb-grade material, the artificial element plutonium (Pu-239).]

As the US and the former USSR have demonstrated, once a nation has the capability to make bombs, it’s just a short step to develop the capability of shooting or dropping those bombs wherever they damn well please. For those of us old enough to have lived through the Cold War, the spectre of those bombs falling was enough to scare you shitless, if you dwelled on the issue too long, like longer than five minutes.

Radioactive fallout from A-bomb and H-bomb blasts has this nasty tendency to hang around a long, long time. It gets in the water supply and in vegetation. Fish swim in radioactive water, and concentrate the radioactive isotopes in their bodies. Cattle and other livestock eat the vegetation, and concentrate the radioactive isotopes in their bodies. People eat the fish, the beef, the cheese — well, you get the idea. The results are increased rates of leukemia, cancers and birth defects. For a long, long time.

And those are among the survivors of a bomb blast.

BushCo may be using the Iranian’s uranium plan as a political football, to boost his sagging ratings at home, but the administration has a legitimate worry. As with certain prescription drugs, uranium enrichment can lead to unwanted side effects among some users, including the tendency to make bombs. Results may vary. Consult your doctor.

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6 thoughts on “Physics and world politics: the neutron dance

  1. Reply eljefe Mar 29,2006 4:49 pm

    Jim —
    Thanks for the corrections. Here we see evidence of how tricky it is to simplify explanations and still get the science right. Or it just could be incipient senility.

    Dave —
    My intent was not to counteract the fearmongering in the MSM, but to try to explain why the nuclear powers are worried by Iran crashing the party. Reading over my own post, though, I agree with you that I could have spent more time explaining the “we just want electricity” side of the story, since I do bring up the issue. My personal feeling is that Iran just wants electrical power, because you can’t modernize a country without it.

    Jim & Dave (as opposed to Sam & Dave) —
    So I am going to rework the piece to reflect your suggestion, in a few days. I need a refresher on nuclear reactor design first.

    On a slightly different issue, I’ve been amused by the eclipse-driven press reports about our former arch-enemy, Libya. Ghaddafi has brought fresh water into the coastal cities, retired his sabre and looks like he wants to be Europe’s best buddy. So who’s to say Iran can’t follow suit?

  2. Reply eljefe Mar 29,2006 11:33 pm

    Beta decay is when a neutron “splits” into a proton, an electron and an anti-neutrino. The proton stays behind, while the electron and the anti-neutrino take off. The electron is the beta particle. The weak nuclear force is responsible for the interaction.

    In the late 19th century, physicists identified three kinds of nuclear radiation. One could be stopped easily by a sheet of paper; they named it alpha, for the first letter of Greek alphabet. The second would pass through paper, but could be stopped by a sheet of metal. It got named beta. The last could only be stopped by a thick chunk of metal, so it got the name gamma (the third Greek letter).

    Eventually, alpha radiation was identified as helium nuclei (2 protons + 2 neutrons). Beta radiation is either an electron or positron (antimatter electron). Gamma rays are high energy photons.

    Originally, beta decay was believed to be neutron –> proton + electron, but the nucleus behaved very strangely. Although it spits out a beta particle at a pretty good speed, the parent nucleus does not recoil. It would be like firing a loaded pistol and not feeling the kick. In 1930, physicist Wolfgang Pauli proposed there was another, unseen particle involved in the decay process, which was “stealing” the missing energy and momentum. It also had to be electrically neutral (to conserve charge), so in 1934 Enrico Fermi, the Italian physicist who later played a key role in the Manhattan Project, called the hypothetical particle the “neutrino” for “little neutral one.”

    Neutrinos are slippery little fellas, and can zip through the earth as if it were made of air. They were not detected until 1959.

  3. Reply Jim Baerg Mar 29,2006 10:54 am

    Correction: The moderator doesn’t slow down the reaction, the moderator slows down the neutrons so the reaction will go using uranium that is mostly U238.

    As neutrons are slowed the chance of a neutron causing a U235 fission increases faster than the chance of the neutron getting absorbed by U238. So to get a nuclear fission reaction you can use highly enriched uranium ie: mostly U235 & no moderator, or you can use a moderator such as water to slow the neutrons & need only a few % U235, or even with an even better moderator, heavy water, use natural uranium 0.7% U235. Look up CANDU for more detail on the option of no enrichment.

  4. Reply Dave Strickland Mar 29,2006 11:15 am

    Nice post, but I’d say it doesn’t do as much as it could to balance out the fear-mongering “nuclear enrichment -> they want nukes” side of the argument that the media dutifully repeats all the time.

    (a) Various European countries get a non-neglible fraction of their electrical power from nuclear power, so it is not unreasonable for Iran to wish for nuclear power as it continues to develop and support the increasingly large population of Iran. And lets not forget the US lobbied hard in the 70’s (back when the Shah ruled undemonocratically with Western support) for Iran to start a nuclear power program. Its not like they suddenly said “lets go nuclear” after all.

    (b) Plutonium production is a better indication of an A-bomb program in the making, and you need to breed that in a reactor by bombarding U-238 with neutrons in a nuclear reactor (which you mention). But that only works well in certain types of reactor. The Iranians are building a pressurized light water reactor, which no-one has ever (?) used to breed Plutonium for weapons – there are much easier ways to breed Pu and everyone whi has made nuclear weapons has used those easier ways. This doesn’t prove the Iranians don’t want weapons (and why shouldn’t they want them, or be allowed to have them? Vastly more dangerous countries have them already), but it adds weight to the argument that they are genuinely interested in nuclear power for electricity.

  5. Reply Chris Mar 29,2006 7:42 pm

    One thing about this account I don’t understand: how can bombarding a nucleus with *neutrons* result in a new element like plutonium? Don’t you have to change the number of *protons* to do that?

  6. Reply Jim Baerg Mar 29,2006 9:31 pm

    Hi Chris:

    What happens is that U238 absorbs a neutron & so becomes U239. However, U239 is unstable & quickly goes through 2 beta decays which convert 2 neutrons to 2 protons & you end up with Pu239.

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