The fuel used in nuclear reactors isn’t weapons grade but if you were to start off with the same raw nuclear materials and, instead of processing it as nuclear reactor fuel, you processed it to make a hydrogen bomb out of it, about how many megatons of yield could you get with the same raw nuclear materials that went into making the kind of nuclear reactor you find on submarines and aircraft carriers?
If I’m understanding the question correctly, it’s my understanding that in typical civilian-use nuclear reactors, the fuel is uranium-238 enriched to about 4% uranium-235. Material for a nuclear bomb needs to be enriched to over 90%.
On US Navy ships, however, they already use weapons-grade highly enriched uranium to fuel their reactors (it helps to reduce the size of the reactor so it fits better on the ship).
I think that, even on a Navy ship, the fuel still isn’t enriched all the way to weapons-grade levels (though it’s more than for civilian reactors).
Assuming that you can enrich it more to where it’s good enough, though, there’s no limit. Once you have enough of a fission device to start a fusion reaction, you don’t need more: You increase the yield of a hydrogen bomb by adding more hydrogen, not by adding more uranium/plutonium.
You couldn’t make a hydrogen bomb out of it since it’s not hydrogen.
You might be able to make a trigger for a hydrogen bomb but, as Chronos points out, the yield would then depend on the amount of hydrogen rather than the amount of uranium or plutonium.
Do you know a way to translate the potential energy in fissile and/or fusion form, of the radioactive materials such as uranium, plutonium or any other commonly radioactive material which is commonly used to make nuclear bombs, which is found in the most common type of US Navy aircraft carrier or nuclear submarine, into a TNT tonnage yield when detonated in a fissile-fusion thermonuclear reaction such as the B83 warhead.
For non-AI: If I take the glowing stuff from a US submarine or aircraft carrier and find some way, somehow to make it go boom as loud as I can, how many tons of TNT is that worth?
Don’t need answer fast.
I believe you can increase the yield of a hydrogen bomb substantially by encasing it in (non-enriched) uranium, right? Though it will also make the bomb quite dirty.
This is covered in the Nuclear Weapons FAQ. For a low-yield weapon, one critical mass of fissile material will produce about 20 kt of explosion. For example, the infamous “Fat Man” had about 6.2 kg or so of plutonium in its core. On the other hand, using 25 kg of plutonium would result in 100-210 kt. The largest pure fission bomb ever tested yielded 500 kt using 75 kg of highly enriched uranium and a 150 kg natural uranium tamper.
As for fusion, well, you know what Dr Strangelove (& Chronos) had to say-- there’s no limit.
Is that the case? I thought that H bombs got most of their explosive yield from the Uranium damper that was fissioned by the huge numbers of neutrons emitted by the fusion reaction. There is a relatively small explosion from the fission trigger which compressed the Hydrogen until it fuses producing a medium sized explosion which produces a flood of neutrons which produces a large fission explosion in the surrounding Uranium damper. A larger explosion comes mostly from a larger damper. That was why the Czar-bomba was so physically big. And why its explosion wasn’t even larger, most of the damper had been replaced with lead.
At least that is my understanding. I will certainly defer to Cronos on physics questions. 
That’s one hell of an assumption.
Nuclear enrichment (purification) plants are big, complicated, high-tech systems that re tough to build & expensive to operate. (Most of the work in the Manhattan Project was done on manufacturing the materials for the nuclear bombs. The theoretical design work was done well before that. In fact, the scientists were so sure of the Hiroshima bomb design that they didn’t even test it beforehand.)
It is true that many current and past thermonuclear weapons (e.g. W88) contain a lot of uranium in components like the “tamper” and case so that 70-80% of the energy yield comes from fission.
If your stolen uranium were not highly enriched, part of the “processing” would be to enrich it. It is not trivial to make a Hiroshima-type bomb out of 4-5% enriched uranium, which is why civilian use of such is supposed to reduce nuclear proliferation risks. Now, this NATO report states that U.S. submarine reactors contain an average of 200 kg of U-235 at an enrichment level of 97.3%, and Russian submarines use 40-90% enrichment. Apparently it is not considered so easy for bad guys to swipe a nuclear submarine, though OTOH there are documented incidents of insiders stealing naval uranium, at least in Russia.
Finally, making a “hydrogen bomb” yielding “megatons”, like in the original question, will require a bit more than merely swiping a fuel rod intended for a naval reactor.
Yeah, the fuel for a nuclear submarine doesn’t include any hydrogen, but hydrogen (even deuterium and tritium, if that’s what your design needs) is a lot easier to come by than weapons-grade fissionable material.
This should be carefully qualified by defining what “hydrogen bomb is”, because what people normally call “hydrogen bomb” (boosted fission device) definitely does derive most of its yield from fission. The hydrogen stage is fairly small and is there mainly to provide neutrons to be captured in a large U-238 jacket, which gets transmuted to P-239, which then undergoes fission.
In the past, there have been experimental “hydrogen bombs” with a fusion percentage in the high 90’s. These were never operationally viable. Not clear if OP was intending to speak to those types of bombs, or boosted fission weapons. The distinction is important but few people bother to draw it.
Nitpick -
The U-238 fissions directly when struck by a fast neutron. It doesn’t need the intermediate transmutation into PU-239.
That should actually be reiterated, that such weapons aren’t viable. They can be built, sure, but they don’t accomplish any useful mission: A small nuke blows up a city-sized target real good, and a large nuke blows up a city-sized target realer gooder. If you want to increase your actual useful firepower, then you need more bombs, not bigger ones, which means you need a critical mass of fissionable material for each one.
Ignorance fought.
There are 2 distinct truths here. First, the Tsar was just too big to deliver (it may also have been cryogenically cooled; not certain on that). Second, it was beyond the point of diminishing returns. The additional energy just went into lifting atmosphere into space rather than damaging the target area.
As a result both sides decided to go with better targeting and larger numbers of “small” bombs. These are still (AFAIK) all entirely boosted fission weapons, just scaled-down ones.
Now, let’s say you were able to procure other components and assemble them into a thermonuclear bomb, what would be the yield of 200kg of 97-100% uranium?
As I recall one of the descriptions of the top-end H-bombs, the claim was it could start forest fires 50 to 100 miles away if exploded at altitude. That provides a whole level of destruction above and beyond pressure wave effects, but yes, there is a law of diminishing returns.
That’s too much U-235 for one bomb - there’s no practical way to assemble that much mass from sub-to-super critical in a short enough time to avoid a fizzle. So, you would probably want to make 3-4 big bombs, each with 500KT or so yield.