The Heritage Foundation offers 10 steps to take to stop Iran's nuclear program: strong sanctions, helping the opposition overtly & covertly, plus upgrading our nuclear arsenal & missile defense. A highly informative New York Times front-pager explains why Iran's uranium enrichment task is getting much easier....
The Gray Lady's article introduces chilling nuclear bomb arithmetic:
Four years ago, Iran began enriching uranium on an industrial scale with centrifuges, machines that spin extraordinarily fast to separate uranium 235 from the more common form of the element, uranium 238. Uranium 235 is a natural rarity that splits easily in two, or fissions, in bursts of atomic energy, either in a reactor or a bomb. Reactor-grade fuel is usually defined as uranium 235 of about 4 or 5 percent, and bomb-grade as 90 percent or higher.
The desert complex, the Natanz nuclear facility, raised the level of uranium 235 to roughly 4 percent from its natural concentration of 0.7 percent. Over time, the facility produced two tons of concentrated material, enough, if further enriched, to make about two atom bombs.
Then, on Sunday, Feb. 7, Iran announced it would begin enriching its stockpiled uranium to 20 percent — ostensibly to make fuel for a research reactor in Tehran. Nuclear experts said that although this might sound like a leap, moving to 20 percent from 4 percent was actually a fairly easy step — not at all as demanding and time consuming as raising the level to 4 percent from 0.7 percent. And the ease of further enriching uranium once it is already enriched made the world take notice.
The key factor that makes uranium enrichment get easier at higher beginning levels is "non-linearity":
In scientific terms, rather than political ones, the issue is “nonlinearity,” a word almost as scary to nonscientists as highly enriched uranium is to the International Atomic Energy Agency. The definition is fairly simple, however. In a nonlinear process, a particular input produces an output that is disproportional in size, either big or small. The weather is conspicuously nonlinear, with small changes in one area producing deadly storms elsewhere. So are earthquakes and explosions. The word describes inequality in cause and effect.
A practical illustration of nonlinearity is that Iran — or any other nuclear hopeful — needs increasingly few centrifuges to make uranium 235 increasingly potent. For instance, one industry blueprint features 3,936 centrifuges for enriching up to 4 percent, 1,312 centrifuges to 20 percent, 546 centrifuges to 60 percent and just 128 centrifuges to 90 percent — the level needed for a bomb.
Here is how the arithmetic works out:
Centrifuges that whirl small amounts of gaseous uranium at high speeds are used to separate the element’s different forms, or isotopes, starting with material that consists of 99.3 percent of the heavier uranium 238. At each step, more of the heavy uranium is removed and the remaining material, now with a higher concentration of the lighter isotope, goes through the centrifuge process again.
Uranium ore has about 140 atoms of the heavy isotope for every light one, and separating the two takes a lot of spinning. By the time the enrichment process has reached 4 percent, it has successfully removed some 115 of the heavy atoms.
To get from there to 20 percent — the enrichment goal of the Iranians — the spinning centrifuges need remove only 20 more of the heavy atoms. And from there it is even easier to jump to 90 percent, bomb grade, by removing four or so additional heavy atoms. That is what worries many countries.
Below is a discussion of the arithmetic in the Gray Lady article, with a little more (but not too much!) detail for context:
Enrichment: Simple Background Facts (skipping messy chemistry!). Uranium is the heaviest naturally-occurring element, one of the fundamental building blocks into which atoms are assembled. Found in the Earth, it can be extracted from a mine. (The ore is transformed into softer chemical forms to make working with it easier, the details being beyond the scope of this posting.)
What comes out of the mine is 99.3 percent U-238, plus 0.7 percent U-235, which means that out of every 140 atoms, one atom of U-235 is mixed with 139 atoms of U-238. U-235 and U-238 are isotopes: they have a slightly different atomic structure but are chemically identical. Each U-235 atom has 92 protons (positively charged particles) & 143 neutrons (neutrally charged particles) inside its nucleus, while each U-238 atom has 92 protons & 146 neutrons inside its nucleus. (The nucleus is a tiny sphere at the center of an atom. Negatively charged electrons orbit the nucleus, in number sufficient to electrically offset an atom's positively charged protons.)
The objective of uranium enrichment is to separate U-235 atoms, which are highly fissile--they split easily when bombarded by other neutrons--from U-238, which is merely fissionable--can be split but not nearly as readily as U-235. A critical mass of slightly enriched U-235 (Low-Enriched Uranium, LEU) can fuel a commercial nuclear reactor via a slow, continuous & controlled chain reaction (check the drawing in the Wiki entry) as neutrons split the nucleus ("fission"). But U-235, in concentrated form, allows for the rapid enough splitting of nuclei (plural of nucleus) to release the vast uncontrolled energy that makes for a nuclear bomb. Enriching the fuel to 93 percent U-235 (highly enriched uranium, or HEU) enables assembly of the fuel into a super-critical mass needed for a nuclear explosion. (Another way to make a bomb, using plutonium, is not covered in this posting.)
The Simple Arithmetic of Uranium Enrichment. Mind you, it is hard to actually perform the industrial engineering tasks to enrich uranium; indeed, it is VERY hard. It is hard because you are separating isotopes that are infinitesimally different in weight, with U-235 all of 3 neutrons lighter than U-238; with neutrons fitting inside an atom they weigh even less than an atom, which is, needless to say, not much.) But while the isotope separation process is arduous the arithmetic is elementary school level, and is well worth showing the lay reader, so as to explain fully why Iran's course is increasingly dangerous.
You start, as noted above, with uranium taken from the mine that is 99.3% U-238 and 0.7% U-235. For every 140 uranium atoms in what is taken from the mine, 139 atoms are the heavier isotope U-238 and 1 atom is the lighter isotope U-235.
To enrich to the 3.5 percent U-235 level, 1 atom out of 28 uranium atoms must be U-235 and 27 atoms are still U-238. So, out of the 139 U-238 atoms you started with you must filter out 112. This creates minimum commercial reactor-grade U-235 fuel. This step separates out 80 percent of the U-238 atoms.
To enrich to the 20 percent U-235 level, 1 out of every 5 uranium atoms must be U-235 and the other 4 are U-238. This means that 135 atoms of the original 139 U-238 atoms have been filtered out. This creates medical-grade research U-235 reactor fuel. (Medical grade is actually 19-75% enriched U-235, but we are simplifying.) This step separates 23 more U-238 atoms, equal to 17 percent of the original 139 U-238 atoms. Having now filtered 135 of the 139 U-238 atoms you started with, you have now done 97 percent of the total separation work.
To get to 100 percent U-235 every atom left after filtering must be U-235. All 139 U-236 atoms will then have been filtered out. In the real world, 93% U-235 makes for a top-quality US-standard weapon. (Thus, in every 14,000-atom group an extra remaining U-238 atom would be found in 7 of the 100 140-atom groups; this would mean that after 13,900 atoms have been separated out, what would remain are 93 of the U-235 atoms + 7 of the U-238 atoms.)
This last enrichment step creates weapons-grade U-235 bomb fuel. In this highly concentrated form a bomb is small enough to fit as a warhead shielded inside a compact missile nose cone of the size Iran has on its missiles. (It actually is possible to get a nuclear detonation below 20% enrichment, well below what is regarded as weapons-grade, but the explosive yield will be far lower and the bomb will be far larger; it would take either a monster missile--larger than anything Iran has in its arsenal--or a bomber aircraft to carry the weapon, or a terrorist could carry one in a truck.)
In sum, 80 percent of the arduous uranium enrichment work takes you from what comes out of the mine to what powers a commercial reactor; 97 percent of the enrichment work takes you to medical research use. What remains is only 3 percent of the total enrichment work needed to fuel a bomb.
At full tilt Iran could take 20 percent enriched uranium to full 93 percent weapons grade within a month. There are 20 uranium enrichment plants worldwide, with most of these plants NOT making weapons-grade uranium.
Bottom Line. Iran marches on, the UN diddles, Russia & China obstruct, America flails, and Israel watches the sorry spectacle--but for how long?
Letter from the Capitol, LFTC, 9/11, National Security, Terrorism, Homeland Security, Nuclear Proliferation, Arms Control, WMD, Foreign Policy, UN
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