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Red Team, Part 1: Uranium Enrichment for Dummies

by FMJ Mon May 15th, 2006 at 12:25:42 AM EST

There are many issues surrounding the IC's various assessments of the Iraqi tubes' intended end-use over 2001 and 2002. To understand these, some basic information about nuclear weapons, uranium and gas centrifuge uranium enrichment is required. The following section will cover the necessary details.

The simplest nuclear weapons are fuelled by an isotope of uranium called uranium-235. Uranium-235 is fissile material, which means that when there is enough of it (i.e. when the isotope is at `critical mass'), it can support a nuclear fission chain reaction. The resulting violent release of tremendous energy is commonly known as a nuclear explosion.

When natural uranium (called `yellowcake') is first mined from the earth, it is only about 0.7 percent uranium-235; the rest is a different uranium isotope - uranium-238. Uranium-238 is not fissile material: no matter what its quantity, it will not sustain a fission chain reaction. Uranium-238 cannot be used in a nuclear weapon. To begin building a nuclear weapon from yellowcake, the two isotopes must first be separated.

The process of separating uranium isotopes is called uranium `enrichment'. When a quantity of uranium is over ninety percent uranium-235 it is termed `highly enriched uranium' (HEU). HEU's critical mass, the quantity required for a nuclear weapon, is only about fifty kilograms. However, enriching more than gram amounts of HEU is an extremely difficult process.

More than seven metric tonnes of yellowcake must be enriched to provide enough HEU for one nuclear weapon. Most enrichment methods exploit the difference in weight between uranium isotopes. Atoms of uranium-235 are lighter than atoms of uranium-238. However, as if separating atoms wasn't difficult enough, the difference between the isotopes' weights is also very small - only about 1.27 percent. Enrichment requires a lot of highly-technical and energy-intensive equipment, such as a gas centrifuge.

A gas centrifuge enriches uranium by means of centrifugal force. First, the natural uranium has to be turned into a gas (this is usually done by combining it with gaseous fluorine to create uranium hexafluoride). The uranium gas is then pumped into the centrifuge's `rotor', which is a tube made of high-strength material spinning incredibly fast. To generate the centrifugal force necessary to separate uranium isotopes, the rotor must spin more than 90,000 revolutions per minute (rpm). The rotor's internal diameter must also be as wide as possible. The centrifugal force inside the rotor pushes the heavier uranium-238 molecules to the rotor's wall. The lighter uranium-235 molecules are left to collect in the center.

A lot of the difficulty building a gas centrifuge comes from developing a rotor capable of spinning at the required rpm. Very few materials can withstand such high rotational speeds. The earliest gas centrifuge, the 1940s `Beams-type' centrifuge, named for its inventor Jesse Beams, used rotors made from 2000 series aluminum (`duraluminum'). The Beams centrifuge rotor was supported by an oil bearing, which is why the Beams design is sometimes called an `oil centrifuge'. Duraluminum however, proved to be too weak. The wall thickness required by duraluminum rotors to survive 90,000 rpm made the rotors too heavy. By the 1950s, the Beams centrifuge had been replaced by the `Zippe-type' centrifuge design. The Zippe centrifuge used powerful magnets to support the rotor instead of an oil bearing and could use thinner-walled rotors made from the more resilient 7000 series aluminum. Today, centrifuge rotors are manufactured from even stronger material, such as carbon fiber or maraging steel.

A single gas centrifuge will never enrich more than a nominal quantity of uranium, no matter what its rotor's material or internal diameter. Enriching enough uranium for a nuclear weapon requires thousands of centrifuges running simultaneously, side-by-side, in what's called a `centrifuge cascade'. The centrifuges in a cascade are interconnected by a complex system of pipes made from a material resistant to the corrosive effects of uranium hexafluoride gas, such as steel. The pipes transfer gas rich in uranium-235 from a rotor's center to the next centrifuge in the cascade so the gas can be enriched further. Meanwhile, the pipes return the uranium-238 gas forced to the rotor's wall back to the cascade's start.

The time a cascade takes to enrich fifty kilograms of HEU depends on its number of centrifuges and the size of each centrifuge rotor's internal diameter. A cascade of six thousand centrifuges, each with a rotor internal diameter of 150mm, will take about a year to enrich enough HEU for a nuclear weapon, provided the cascade runs day and night and nothing goes wrong. If the rotors are half that size, then the cascade will need twelve thousand centrifuges to enrich the same amount of uranium in the same time.

A good discussion. Let me say a bit more, though, to clarify the difference between reactor-grade and weapon-grade enriched uranium.

There is a huge difference between merely supporting a chain reaction and supporting a reaction that burns the fuel quickly. Because a nuclear bomb blows itself apart in microseconds, the chain reaction must  burn much of the fuel very quickly, in microseconds. Nuclear weapons require either highly enriched uranium or a different fuel entirely.

In a nuclear reactor, in contrast, the fuel burns over a time measured in years -- about one trillionth the speed of an explosion. Reactors work best with a lazy chain reaction that takes many seconds to build up or slow down, and they ordinarily use uranium that is only slightly enriched. With careful design (as in the safety-engineered Canadian "Candu" reactor), natural, wholesome, un-enriched uranium can do the job. (And produce just as much nuclear waste, and some plutonium too.)

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Mon May 15th, 2006 at 03:32:43 AM EST
I'm afraid Candu would produce even more nuclear waste, in terms of contaminated volume: since the actual power of the reactor is mostly from the U235 you feed it, the more diluted it is, the more total volume you put through the reactor, per MWh produced. Also, if your core is less dense in U235, you need a lot of thermal neutrons to have a good probability of collision and a chain reaction. I think this is why Candu requires heavy water as a moderator. In any case, having more neutrons in and around the core is not great for workers at the plant.

Candu makes for great savings in enrichment (nuclear power plants are like playstations: they're basically sold at no profit, then when you want to play it you pay for the fuel). But I'm not sure you're not winding up with greater costs downstream, in disposal (which is the other place where Areva makes its cash anyway).

The fact that the whole business model of the nuclear industry is based on billing for fuel & reprocessing twists all investments in nuclear energy: truly "sustainable" nukes (yes, this does exist) like thorium -breeders are totally left aside in favor of business as usual like the EPR.

by Pierre on Mon May 15th, 2006 at 05:55:07 AM EST
[ Parent ]
True about CANDU, the nuclear industry's working and EPR. I am not even optimistic about thorium breeders, but that's for another day to detail.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Mon May 15th, 2006 at 06:45:27 AM EST
[ Parent ]
.. you really know you are helping terrorists to build nucular weapons, don't you?

I hope NSA is phone and wire and e-mail tapping you..

for the safety of our beloved country....

Have to leave.. there is a wonderful horny conversation going on up there in Anchorage....wait when my boss listens to it...


Nice details I was not awared of in the series...

(not snarky)

A pleasure

I therefore claim to show, not how men think in myths, but how myths operate in men's minds without their being aware of the fact. Levi-Strauss, Claude

by kcurie on Mon May 15th, 2006 at 01:31:57 PM EST
Urgh, enriching uranium seems such a bother. No surprise most countries prefer plutonium weapons.

Peak oil is not an energy crisis. It is a liquid fuel crisis.
by Starvid on Tue May 16th, 2006 at 03:39:18 PM EST
Correction: most countries prefer no nuclear weapons.

Correction: most countries use their enriched uranium for power production.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue May 16th, 2006 at 03:43:48 PM EST
[ Parent ]

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