What makes weapons-grade uranium weapons grade?

glass · Post by **glass** » 2007-02-14 11:32am

The question is in the title.

As a follow up question, is it possible (allbeit wasteful) to use weapons-grade material for anything else? ie in power stations? I was watching an episode of The West Wing the other day, and they were talking possesion of a supply of weapons-grade material for security reasons. They were talking about storing it, basically, for ever.

glass.

General Zod · Post by **General Zod** » 2007-02-14 12:16pm

The biggest difference between the two is that weapon-grade plutonium is not as stable as reactor grade, and can be used to generate bigger explosions. But since it's less stable it's not very ideal for reactor use, which requires a steady, predictable output of power. Click me.

Seggybop · Post by **Seggybop** » 2007-02-14 12:36pm

IIRC, weapons grade/reactor grade refers to the concentration of U235 compared to 238. I think weapons grade has to be at least 20% U235.

Ted C · Post by **Ted C** » 2007-02-14 12:59pm

As Seggybop note, it's the concentration of the U235 isotope. Without enough "fissile" uranium, you can't get the critical mass necessary to cause an explosion.

Ariphaos · Post by **Ariphaos** » 2007-02-14 03:11pm

Seggybop wrote:IIRC, weapons grade/reactor grade refers to the concentration of U235 compared to 238. I think weapons grade has to be at least 20% U235.

That's reactor grade. Weapons grade specifics are fuzzy, but much, much purer. I found one figure for 'over 93.5% U-235' in a Uranium weapon.

irishmick79 · Post by **irishmick79** » 2007-02-14 06:18pm

from what I understand, the absolute minimum required is 90% U-235.

Seggybop · Post by **Seggybop** » 2007-02-14 07:11pm

Nuclear Weapon FAQ wrote:6.2.1.1 U-235

This relatively scarce isotope is the only naturally occurring material suitable for the production of energy through fission. For use in a fission weapon, or for convenient power production, it needs to be concentrated to levels higher than that found in nature. Although civilian power plants require uranium with a concentration of 2-4.5% U-235, weapons require a minimum of 80% U-235, and preferably more than 90%. U.S. weapon grade uranium is about 93.5% U-235, U.S. enrichment plants are capable of producing a 97.65% "top product" (this is used in naval reactors). Uranium enriched to 80% or more is known generically as HEU, "highly enriched uranium" (a cut-off of 20% is also used). U.S. weapons grade uranium is called "oralloy", a wartime code-name derived from "Oak Ridge Alloy" that has remained in use. In 1998 ORNL Isotopes Division was offering weapon grade (93% U-235) for sale at $53/gram. Uranium with enrichments ranging from 40% to 80% U-235 has been used in large amounts in U.S. thermonuclear weapons as a yield-boosting jacketing material for the secondary fusion stage.

SyntaxVorlon · Post by **SyntaxVorlon** » 2007-02-14 08:47pm

Since the answer has been stated and I still feel like posting, the process to create weapons grade uranium is laborious and requires you to melt the uranium and allow the higher-density and lower density atoms separate using a centrifuge. You then have to take the scum of U235 out of that and do the whole thing again, and again, and again, and again.

Post by **Beowulf** » 2007-02-14 10:27pm

The uranium isn't melted. It's processed when it's in hexafloride form.

darthdavid · Post by **darthdavid** » 2007-02-14 11:05pm

Yup, and Uranium-Hexaflouride is very corrosive so the gas diffusion machine needs to be properly lined lest a cloud of radioactive, highly corrosive, highly toxic, very expensive gas leak out and kill people and waste money and stuff.

SyntaxVorlon · Post by **SyntaxVorlon** » 2007-02-15 12:01am

Beowulf wrote:The uranium isn't melted. It's processed when it's in hexafloride form.

Oh whoops. Either I've confused two different methods, or I've misremembered what I was taught in intro-qm.

darthbob88 · Post by **darthbob88** » 2007-02-15 12:16am

darthdavid wrote:Yup, and Uranium-Hexaflouride is very corrosive so the gas diffusion machine needs to be properly lined lest a cloud of radioactive, highly corrosive, highly toxic, very expensive gas leak out and kill people and waste money and stuff.

OT: IIRC, this was the reason polytetrafluoroethylene went into mass production, to coat valves and seals used when dealing with UF6 at Oak Ridge. Linky.

glass · Post by **glass** » 2007-02-15 06:59am

Xeriar wrote:
Seggybop wrote:IIRC, weapons grade/reactor grade refers to the concentration of U235 compared to 238. I think weapons grade has to be at least 20% U235.
That's reactor grade. Weapons grade specifics are fuzzy, but much, much purer. I found one figure for 'over 93.5% U-235' in a Uranium weapon.

So reactor grade has a certain percentage of U235 and weapons grade has a much higher percentage. More or less what I thought.

Is there any particular reason why you could mix a bit of weapons-grade with a whole bunch of sub-reactor grade to make it reactor grade?

glass.

General Zod · Post by **General Zod** » 2007-02-15 09:28am

glass wrote:
Xeriar wrote:
Seggybop wrote:IIRC, weapons grade/reactor grade refers to the concentration of U235 compared to 238. I think weapons grade has to be at least 20% U235.
That's reactor grade. Weapons grade specifics are fuzzy, but much, much purer. I found one figure for 'over 93.5% U-235' in a Uranium weapon.
So reactor grade has a certain percentage of U235 and weapons grade has a much higher percentage. More or less what I thought.

Is there any particular reason why you could mix a bit of weapons-grade with a whole bunch of sub-reactor grade to make it reactor grade?

glass.

Why would you want to? Weapons grade isn't stable enough to really be desirable for reactors, and technically you could make a nuke using reactor grade for a primitive device. It just wouldn't pack as much "instant" power.

glass · Post by **glass** » 2007-02-15 11:07am

General Zod wrote:
Is there any particular reason why you could mix a bit of weapons-grade with a whole bunch of sub-reactor grade to make it reactor grade?
Why would you want to?

The question was prompted by an episode of The West Wing, which I watched on DVD the other day, where the administration were offered a bunch of weapons-grade uranium that a former soviet republic had and couldn't afford to look after properly. I was just wondering if there was any way to make use of it, rather than having to guard it for ever.

Obviously, you wouldn't deliberately make weapons grade stuff and then make it non-weapons grade again, but if you've got the stuff anyway, and it is basically a liability... Two birds, one stone, kinda thing.

General Zod wrote:Weapons grade isn't stable enough to really be desirable for reactors

But if you mix it with lesser material so the U235 count was closer to 20% than 90, it wouldn't be weapons grade any more, would it? Or is that too simplistic? Am I missing something?

glass.

General Zod · Post by **General Zod** » 2007-02-15 11:11am

glass wrote: The question was prompted by an episode of The West Wing, which I watched on DVD the other day, where the administration were offered a bunch of weapons-grade uranium that a former soviet republic had and couldn't afford to look after properly. I was just wondering if there was any way to make use of it, rather than having to guard it for ever.

Not short of making it into nuclear weapons.

But if you mix it with lesser material so the U235 count was closer to 20% than 90, it wouldn't be weapons grade any more, would it? Or is that too simplistic? Am I missing something?

Uhm, no. To my knowledge there is no way of "downgrading" the purity of refined uranium to magically make it more stable. Methinks you need to do some actual research on the subject yourself.

Wyrm · Post by **Wyrm** » 2007-02-15 03:07pm

General Zod wrote:Uhm, no. To my knowledge there is no way of "downgrading" the purity of refined uranium to magically make it more stable. Methinks you need to do some actual research on the subject yourself.

When you make enriched uranium, the U-238 you took out to make the uranium enriched has to go somewhere, and that's into a much larger quantity of so-called "depleated uranium". Conceptually, you can blend this depleated uranium back in to recover the original feed stock. (You can also dilute the HEU with natural uranium.)

Un-enrichment of uranium fuel is done in the Megatons to Megawatts program in a process called "downblending."

drachefly · Post by **drachefly** » 2007-02-15 03:34pm

General Zod wrote:Why would you want to? Weapons grade isn't stable enough to really be desirable for reactors, and technically you could make a nuke using reactor grade for a primitive device. It just wouldn't pack as much "instant" power.

... yeah, like it would set nearby objects on fire instead of destroying downtown.

Wyrm is totally right that you can just mix it with depleted uranium to get lesser purity. What made you think not?

General Zod · Post by **General Zod** » 2007-02-15 03:55pm

drachefly wrote:
General Zod wrote:Why would you want to? Weapons grade isn't stable enough to really be desirable for reactors, and technically you could make a nuke using reactor grade for a primitive device. It just wouldn't pack as much "instant" power.
... yeah, like it would set nearby objects on fire instead of destroying downtown.

Wyrm is totally right that you can just mix it with depleted uranium to get lesser purity. What made you think not?

The article I found seemed to indicate it wasn't. Though admittedly some of it wasn't as clear as I would have liked.