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In the news it is often mentioned that some countries are going to enrich uranium to "military grade" (i.e. 80%+) and that it is possible to use it for a nuclear bomb.

1) Is that correct that there are no nuclear warheads in service made of U-235, as plutonium ones are much smaller & much more efficient (i.e. burn most of it's fissile fuel unlike U-235 cannon-type ones)?

2) Is that correct, that the only current military uses for 80%+ U-235 are naval nuclear reactors and in rare occurrences - case and/or X-Ray "reflector" in Teller–Ulam configuration as it is slightly better neutron breeder compared to more commonly used U-238 and allows to slightly reduce mass of "high-tech" plutonium charge at the same yield?

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At very minimum, submarine reactors use HEU almost ubiquitously. Wikipedia gives >93%, but that's because they're out of date. In short, 10 years ago it became extremely politically unpopular in US politics to use HEU for anything and they undertook a systematic campaign of getting it out of civilian facilities. Naturally, the military did studies to reduce their enrichment and they took it down a few percent, but won't tell anyone how much. There's no way they're down to 80%, as much as they would like to be, you just can't technically do it. –  Alan Rominger Feb 27 '13 at 19:37

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Is that correct that there are no nuclear warheads in service made of U-235, as plutonium ones are much smaller & much more efficient

Maybe. Who knows precisely what people use. But publicly available information from US warheads is that they moved away from Uranium to Plutonium fission devices soon after world war II.

Is that correct, that the only current military uses for 80%+ U-235 are naval nuclear reactors and

I would replace 'naval reactors' with 'any small yet powerful reactor`.
In most cases that might come down to the same thing, but it also allows the use of them in space probes etc.

in rare occurrences - case and/or X-Ray "reflector" in Teller–Ulam configuration as it is slightly better neutron breeder compared to more commonly used U-238 and allows to slightly reduce mass of "high-tech" plutonium charge at the same yield?

Then you would replace the cheap, bountiful U238 with a more efficient but very expensive to separate U-235? And with a lot of U-235, which would be sensitive to external neutron flux, thus forcing you to add extra shielding (e.g with an extra layer of Boron).

I am not a bomb design expert, but thus sounds uneconomical. Even if you save some Plutonium.

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About the last part - yeah, I've seen references that this is what was actually done in some cases. –  BarsMonster Feb 27 '13 at 19:23
    
Interesting. By whom? Countries without access to Pu? Or for a smaller weapon with a equal yield? How much was added and how close did their temper approach the critical mass? –  Hennes Feb 27 '13 at 19:29
    
By US, unless this is deliberate disinformation: en.wikipedia.org/wiki/… –  BarsMonster Feb 27 '13 at 21:42
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I guess it makes sense in the W88. Weight and space are at a premium for a ballistic missile payload. And the water above the sub will help block neutrons. –  Hennes Feb 27 '13 at 21:55

1) It is believed that some nations use uranium warheads in their nuclear weapons.

http://www.fas.org/nuke/guide/pakistan/nuke/

The Natural Resources Defense Council (NRDC) estimates that Pakistan has built 24-48 HEU-based nuclear warheads, and Carnegie reports that they have produced 585-800 kg of HEU, enough for 30-55 weapons. Pakistan's nuclear warheads are based on an implosion design that uses a solid core of highly enriched uranium and requires an estimated 15-20 kg of material per warhead. According to Carnegie, Pakistan has also produced a small but unknown quantity of weapons grade plutonium, which is sufficient for an estimated 3-5 nuclear weapons.

Uranium weapons are easier to construct but have additional safety risks to the user. A nuclear weapon must remain servicable for many years and not accidentally detonate. For this reason and for available of certain materials, plutonium is preferred depsite the weapons being more complex to manufacture.

2) The majority of marine (I say marine not military, as civilian nuclear powered ships exist) nuclear reactors use uranium. Many nations use them.

Much of the information you want regarding nuclear weapons and submarines is not open source. If anyone tells you they are giving you "secret" information, it is more than likely they are lying: they would be arrested if not. Furthermore the various defence agencies run deliberate disinformation campaigns to "confuse the enemy". Hence, you probably not be able to answer this question definitively.

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Yeah, I understand that not much information is public. I had an impression that implosion design with U-235 is not possible, due to lack of same convenient phase transition that Plutonium has (delta-alpha transition) –  BarsMonster Feb 27 '13 at 21:45
    
If you trust public sources, then en.wikipedia.org/wiki/Nuclear_weapon_design says "The implosion method can use either uranium or plutonium as fuel. The gun method only uses uranium. Plutonium is considered impractical for the gun method because of early triggering due to Pu-240 contamination and due to its time constant for prompt critical fission being much shorter than that of U-235." –  gecko Feb 28 '13 at 2:53
    
Plenty of classified information is public. The US government advised people that downloading Wikileaks data still violates their classified document policy. They have no qualms about closing the farm door after all the animals have gotten out. –  Alan Rominger Feb 28 '13 at 17:35
    
AlanSE: True but there's different levels of classification. On this particular subject (nuclear weapon design, and submarines) there isn't much that's verifiable, and there's many deliberate attempts to spread false information. –  gecko Feb 28 '13 at 18:47

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