Nuclear Experts on DU! What does this mean?

[Canuckistanian]

http://news.yahoo.com/s/ap/20060411/ap_on_re_mi_ea/iran_nuclear

So apparently, Iran has "enriched uranium". It looks like they've got about 170 tons of UF6 - Uranium Hexafluoride gas. And they've got the gas centrifuges up and working at a production pace (or so they claim).

Do we know what isotope has been produced? Can this isotope be used for weapons?

And how much has been produced? I think that even if they managed to make a microgram they can say they "enriched uranium". But how much is realistically needed for a bomb?

And how much work do they have ahead of them to make a bomb?

Any guesses?

[endarkenment]

1. I guess that they are allowed to do this under the NPT. nt.

[LSK]

2. I claim BULLSHIT on this

Its the same SCIENCE FICTION writers who came up with Iraq's WMD stories.

[frankly_fedup2]

3. So do you think the yellow-cake uranium they were showing and

moving around was really some flour with yellow dye in it?

[LSK]

6. havnt we all been through this recently???

When are we going to learn??

Can you tell yellow-cake uranium from somewhere else??? How do you know that is actually Iran??? How do you know when the footage was shot???

[greenman3610]

4. Hans Blix says 5 years

US intel, per Keith Olbermann, says 8 to 10..also per
Keith, Israeli intel says a year or 2.

somewhere in there is the truth.

[leftchick]

5. I don't believe him

Remember saddam's bluffs? and there are weapons inspecters there. Perhaps they can verify this.

[whopis01]

7. kick n/t

[Alcibiades]

8. Enriching Uranium

Is the hard part. There are a couple of ways to make a bomb using either Plutonium or highly enriched uranium, which contains at least 90% Uranium 235. Assuming they are going the HEU route, the mechanics they need to make a bomb are simple. If they were already working on this, they many have a bomb very soon, especially with Russian or Pakistani help.

[Career Prole]

9. Enriching uranium is one thing. Have they ever professed an

intention to build a bomb? Last I heard they intended it for power stations.

[Canuckistanian]

12. There are reactors that require enriched uranium

This is not necessarily strictly for bomb making.

[Tesha]

21. Esentially all current designs except CANDU require some degree...

Esentially all current designs except your very own
CANDU design require some degree of enrichment.

> Posted by Canuckistanian

But I'll bet you knew that! ;-)

Tesha

[Canuckistanian]

24. Yeah, they use "raw" uranium, don't they?

But I don't think CANDU reactors are being sold anywhere any more.

Too bad, they were once a source of great pride in our country.

[Tesha]

27. Actually, Canada sold at least one series overseas.

> Yeah, they use "raw" uranium, don't they?

Yes. Because of their heavy water moderator, they are
able to burn natural uranium. They also have an inovative
fueling system that allows new fuel to be continually
shoved in at one end of the reactor and spent fuel to
be extracted at the other end, thus eliminating the
wasteful "complete shutdown" period required to refuel
most light water power reactors.

http://en.wikipedia.org/wiki/CANDU_reactor :

* Canada - 16 (+2 refurbishing, +6 decommissioned)
* South Korea - 4
* China - 2
* India - 2
* Argentina - 1
* Romania - 1
* Pakistan - 1

But you're right; fission power reactors are out of style
most places in the world today, and fusion power reactors
haven't quite been invented yet ;-).

Tesha

[mike_c]

10. it means they have have a working gas diffusion enrichment system....

U²³⁸ is the most common uranium isotope but it is not easily fissionable. U²³⁵ is much less common in nature, but it is less stable and therefore more easily fissioned, e.g. more likely to sustain a chain reaction. Gas diffusion works because U²³⁸ is slightly heavier than U²³⁵, so when uranium hexaflouride gas is spun in a high speed industrial centrifuge, the U²³⁸ is more likely to spin out through the aluminum walls of the centrifuge container than the lighter isotope, so the gas remaining in the tube has a higher proportion of U²³⁵ than it started out with. The U²³⁵ enrichment increases as more and more U²³⁸ is spun out. The gas form uranium hexaflouride must then be converted back to the metallic solid.

This gets Iran somewhat closer to a fission bomb in the sense that enriched uranium is one of the fissionable components of a bomb (depending on bomb design), but it actually takes a long time to accomplish the enrichment-- the centrifuging has to be done over and over and only a slight enrichment is produced each cycle-- and that doesn't even begin to address the engineering issues needed to actually construct a working high yield device. On the other hand, slightly enriched uranium pellets are immediately usable in light water reactors for power generation (and are a necessary component of reactors).

[Tesha]

15. To expand on the previous post...

Many gas centrifuges are joined together in series
to form what is call "a cascade". Each step in the
process (each run through one centrifuge) concentrates
the U²³⁵-containing UF₆ gas a little more.
After a few steps through the cascade, you have "Low-
enriched Uranium", suitable for burning in a nuclear
power reactor or for use breeding Plutonium (another
fissile material used in "implosion" fission bombs).
After many more steps through the cascade, you have
"Highly-enriched Uranium", suitable for making a
"gun-type" fission fission bomb.

I disagree with the previous poster in one area,
though:

> and that doesn't even begin to address the engineering
> issues needed to actually construct a working high yield
> device.

They really don't need efficiency right now, they just
need to create a device that makes a nuclear boom. And
a gun-type device (using highly-enriched uranium) is
trivially easy to build; if someone gave us fifty
kilograms of HEU and access to the web and a machine
shop, any of us could do it. It wouldn't be pretty
and it would waste a LOT of nuclear fuel, but it
would certainly go off.

By comparison, an "implosion-type" bomb (using plutonium)
is a much more exacting engineering enterprise. Unless
very precisely made, it would fizzle when detonated.
But if you get it right, you can make a much bigger
bang with relatively little fissile material. And
it's relatively an easier task to make a whole lot
of plutonium than a whole lot of highly-enriched
uranium. Extracting plutonium from your power reactors'
spent fuel is just chemistry. (Nasty, radioactive chemistry,
but hey, it's just another way to die for the cause, right?
Radioactive martyrdom.) The alternative is to take a long,
long time and a whole lot of gas centrifuges to make that
HEU.

Or maybe you can just buy a bomb?

Tesha

[longship]

17. Except for the initiator.

One had better get that right otherwise the super-critical mass just blows itself apart before the much of the U235 can fission. The difference is between a bomb and a dirty dud.

[Tesha]

19. Even that's just a question of efficiency (and U234 contamination)

As I understand it, the critical factor is amount of the U²³⁴
contamination. U²³⁴ tends to self-fission much more
readily than U²³⁵ so if you have too much of it, the
chain reaction in your gun type device tends to trigger
before the critical mass is fully assembled, leading to a
fizzle.

But if you minimize U²³⁴ contamination, then you either
wait for a natural decay of U²³⁵ or use a deliberate
initiator.

Fast assembly, of course, tends to mitigate this problem as well.

Remember, "Little Boy", our gun-type device, was dropped on
Hiroshima UNTESTED; we had that much confidence that a
gun-type device would operate. And it did.

Tesha

[longship]

22. But Little Boy's initiator was tested independently

It's actually a simple criteria. The bomb needs a certain number of neutrons in a certain amount of time. The timing is the important piece of information. How long does the super-critical mass maintain its integrity before it blows apart in the absense of triggering neutrons? The initiator must then supply the requisite neutrons within that time frame. The time frame is probably on the order of microseconds.

Without an initiator, the bomb will likely not work by assembly alone.

[Tesha]

28. But the initiator, if used, need not be fancy or electronic.

Here's what Wiki says about initiators (emphasis added):

Neutron trigger / initiator

One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield of the nuclear explosive, sufficient neutrons must be present within the supercritical core at just the right time. If the chain reaction starts too soon, the result will be only a 'fizzle yield', much below the design specification; if it occurs too late, there may be no yield whatsoever. Several ways to produce neutrons at the appropriate moment have been developed.

Early neutron triggers consisted of a highly radioactive isotope of polonium (Po-210), which is a strong alpha emitter combined with beryllium which will absorb alphas and emit neutrons. This isotope of polonium has a half life of almost 140 days. Therefore, a neutron initiator using this material needs to have the polonium replaced frequently. The polonium is produced in a nuclear reactor.

To supply the initiation pulse of neutrons at the right time, the polonium and the beryllium need to be kept apart until the appropriate moment and then thoroughly and rapidly mixed by the implosion of the weapon. This method of neutron initiation is sufficient for weapons utilizing the slower gun combination method, but the timing is not precise enough for an implosion weapon design. The "Fat Man" weapon of World War II used a finely tooled initiator known as an "urchin", made of alternating concentric layers of beryllium and polonium separated with thin gold foils.

Another method of providing source neutrons is through a pulsed neutron emitter, which is a small ion accelerator with a metal hydride target. When the ion source is turned on to create a plasma of deuterium or tritium, a large voltage is applied across the tube which accelerates the ions into tritium rich metal (usually scandium). The ions are accelerated so that there is a high probability of nuclear fusion occurring. The deuterium-tritium fusion reactions emit a short pulse of 14 MeV neutrons which will be sufficient to initiate the fission chain reaction. The timing of the pulse can be precisely controlled, making it better for an implosion weapon design.

An initiator is not strictly necessary for an effective gun design, as long as the design uses "target capture" (in essence, ensuring that the two subcritical masses, once fired together, cannot come apart until they explode). Initiators were only added to Little Boy late in its design. The use of an initiator can guarantee precise control (to the millisecond) over the timing of the explosion.

[longship]

30. Good post.

Thanks for this.

Containment of the super-critical mass might be more difficult than the initiator in the gun-type device. In the simplest, two assembly gun devices, the Polonium gizmo is on ome of the sub-critical masses and the beryllium is on the other. At the precise moment when the assemblies achieve maximum critical mass the two parts of the initiator contact one another producing the neutrons.

AFAIK, in Little Boy, one part was on the bullet and the other was inside the barrel of the target. When the bullet reached max penetration of the target mass the two pieces collided which produced the neutrons at precisely the right time.

The initiator in the PU implosion device was called a "sea urchin" because it was a ball with spine-like exterior. Inside the Po and Be were intertwined in a complex way (apparently still secret). When the mass and the tamper were compressed to super-criticality, the urchin was crushed, mixing the two elements and producing the requisite neutron flux to start the chain reaction.

If you are interested in this history, I highly recommend Richard Rhodes' "The Making of the Atomic Bomb" which is a very good read and goes into great detail about the Manhattan project and the people who made the bomb. It won the Pulitzer for non-fiction, richly deserved IMHO.

[Tesha]

31. Thanks (for the reference)! (NT)

[TreasonousBastard]

11. UF6 is basic to most enrichment processes...

since it's a gas and works in centrifuges. The idea is to get as much U235 as possible, 'cause that's the fissionable stuff.

Just because they've managed to get LEU doesn't mean they are anywhere near getting weapons-grade uranium, or even the >20% stuff for research reactors. That's a huge undertaking.


http://www.fas.org/nuke/intro/nuke/uranium.htm

<...>
While low-enriched uranium (LEU) could technically mean uranium with an assay anywhere between slightly greater than natural (0.72 percent) and 20 percent 235 U, it most commonly is used to denote uranium with an assay suitable for use in a light-water nuclear reactor (i.e., an assay of <5 percent). Similarly, the term “highly enriched” uranium (HEU) could be used to describe uranium with an assay >20 percent, but it is commonly used to refer to uranium enriched to 90 percent 235 U or higher (i.e., weapons-grade uranium). The term “oralloy” was used during World War II as a con-traction of “Oak Ridge alloy,” and it denoted uranium enriched to 93.5 percent 235 U.
<...>

[MadHound]

13. Well, since they've only had the centrifuges going for only a month or two

The amount of U235 that they have is negligible. Enriching uranium through this method is very time consuming, and I would imagine that they have, possibly, about on tenth of a gram of the stuff now. Enough to hold up to a light and "Voila! We've enriched uranium" If they continue with this, it's going to be years before they have enough for either fuel or a bomb.

And again, this is what one would expect if they were enriching fuel for a nuclear reactor. All this BS Rummy is spewing is just more fear mongering in order to get the public backing for a pre-emptive nuke strike on Iran. Much the same scenario that we saw in Iraq.

[ThomWV]

26. It Is Wrong To Think Of Them As Centrifuges

It gives you a bad impression of the process. Think of them more as compressors and maybe it will make more sense to you but the point is that having just one or two or even one or two hundred of these 'centrifuges' would not be enough for what one might think of as a bomb production run.

[Strelnikov_]

14. "I'll Tell You What This Means, Norm, . . .

no size restrictions and screw the limit!"

[Junkdrawer]

23. Eloborating...

[longship]

16. Not an easy or fast process.

Although a U-235 gun-type bomb is mechanically simple, in no way are the processes easy to get to the point where one has a workable weapon.

Refinement is an extremely difficult process. It takes a long time to get a little bit of refined U-235.

One of the most challenging technologies is the initiator, the device that supplies neutrons precisely and only at the point of assembly. This is a real problem. Without a proper working initiator, the rapid assembly of the super-critical mass will just blow itself apart leaving much of the fissionable stuff, un-fissioned. In other words, the bomb would be a dud, albeit a very dirty one. Without an initiator the bomb would depend on a cosmic ray, or something else to trigger the chain reaction. The design of this element is amongst the most secret of all atomic secrets.

A Pu implosion bomb is a much more difficult thing. The "explosive lenses", the explosive charges that implode the Pu core with its U tamper are amongst the most difficult of technical challenges. The implosion must be spherically symmetric to a very high degree otherwise the bomb would be a dud. On top of that, the initiator is much more complex.

When the IAEA estimates possibly years before Iran has a working bomb, they are likely dealing with technical challenges of these types. Furthermore, the scale of the refinement processes make it extremely difficult to hide them. It would be straightforward to determine how far along Iran is simply by the scale of their refinement processes. It's a Hobson's choice. Make it small and hide it and take ten years, or do it quickly and let the whole world see what you're doing.

I hope this helps.

[Canuckistanian]

18. Thanks, very informative

Isn't the hardest part detemining the critical mass?

I hear that the proportions and size are the hardest things to figure out and the most guarded secrets of nuclear countries. Or are there formulae for these calculations?

[Tesha]

20. There are formulae.

There are formulae. And you can determine some of this
stuff empirically through experimentation with sub-
critical hunks of fissile material and a neutron laboratory
source.

You can also hedge your bet; a gun type device need not
have the critical mass assembled from just two sub-critical
masses, although that certainly leads to the easiest timing
(one single explosive detonation and Bob's your former uncle).
If you assembled the core from more than two pieces, you have
improved odds that the fully-assembled core will be super-
critical.

Tesha

[longship]

25. Yes, it depends on many factors.

Critical mass is easy to determine and is probably available in any CRC physics manual. However, you're correct, that isn't the only parameter. Geometry of the sub-assemblies. the introduction of triggering neutrons, the use of a tamper to contain the super-critical assembly longer, etc. The end result is a very complex thing based on simple principles.

That's why not every country has an A-Bomb.

From what I understand, the initiator is one of the key elements and is kept in the highest secrecy.

[originalpckelly]

29. Okay, an explanation of what I understand...

In all naturally occurring uranium there are two isotopes, U-235 (the stuff used in a nuke) and U-238 (pretty much useless.) Enrichment is really a misnomer, what really goes on is the separation of gasified uranium (uranium hexafluoride) into U-238, which spins to the outside of the tube because it is slightly heavier, and U-235, which collects near the center of the tube. The centrifuges are usually made of aluminum (though this unlikely in Iran's case) or Maraging steel tubes (hence the 2003 state of the union) that spin at an extremely high 90,000 RPM (hence the reason the thin tubes mentioned in the state of the union weren't for enrichment) in a vacuum (chamber) to minimize air resistance. The hexafluoride is put through a succession of separations until it becomes pure enough to use in a nuclear reactor or weapon. Right now if the Iranians don't "enrich" their uranium any further it cannot be used in a nuclear weapon. If it is low-grade, however, it can still be used in most types of nuclear reactors. If they continue to enrich it to high grade, there would be no other reason than to build a nuclear weapon.

For further reference:
Zippe-type centrifuge (the one A.Q. Khan taught Iran how to make):
http://en.wikipedia.org/wiki/Zippe-type_centrifuge

In general, enriched uranium:
http://en.wikipedia.org/wiki/Enriched_uranium

Note: UF6 is merely a chemical formula, and it does not give any clue other than the fact they have gasified uranium, which was already know, A.Q. Khan smuggled it to them through the Port of Dubai. The isotope is dependent upon the number of neutrons in the nucleus of each uranium atom.

[Tesha]

32. Excellent, except preferably, not aluminum tubes, and...

Excellent, except preferably, not aluminum tubes...

See:

http://en.wikipedia.org/wiki/Maraging_steel


...and there's also a smidgen of U²³⁴ mixed into
the natural uranium as well. That's an undesireable
contaminant as it makes it harder to avoid fizzles
(see my post above).

Tesha

[originalpckelly]

34. It is much easier to get a pure aluminum tube...

...than to get a complex alloy like Maraging steel. I uncertain as to whether the Iranians could get it for their centrifuges, but I do know their plans came from Urenco, and I am fairly sure they would use Maraging steel. I have significant doubt as to whether they could substitute aluminum and still use the plans. I should have made that clear.

[longship]

33. UF6 is nasty, nasty stuff

The main problem with UF6 is that it is *highly* corrosive. The gas diffusion membranes would degrade quickly, requiring frequent replacement. The same would be true of the aluminum in the centrifuge. Whatever way one does this, the gaseous form of U is not nice stuff to handle. It's nasty, deadly stuff. The facilities needed to handle this stuff is not simple, not small, and not hide-able.

This is why we can make a good approximation of whether Iran is actually making a bomb, or is just wanting to build reactors. And, we can know approximately when the product of this enrichment will be ready.

I am presuming that ChimpCo will overstate the danger here, by playing up the fact that Iran is doing enrichment, totally ignoring the conclusions about the character of the product they are producing and when it might be complete. The general populace is not going to understand the devil in the details here and will inevitably overreact. Nukes are scary things. ChimpCo will again scare the populace with moonshine instead of facts.

[originalpckelly]

35. So long as they don't enrich any further...

...the uranium can only be used in a nuclear reactor. It takes a highly enrichment amount of uranium to make a nuclear weapon. Four to five times the usual percentage of U-235 for a nuclear reactor is needed to form a crude nuclear weapon.