Making a gun-type bomb from enriched uranium isn't that hard. Any shop capable of building an auto engine could do it. Even machining uranium isn't that hard; there's a Union Carbide tech note on that. It requires a respirator and protective gear, but not remote controlled machine tools. That's why anti-proliferation people are so concerned about enriched uranium.
But enriched uranium is hard to make. The old gaseous diffusion process was incredibly inefficient - plants over a mile long to make tens of kilograms. Centrifuges have changed that. Centrifuge plants aren't all that big, about Wal-Mart sized. They're still expensive. Highly enriched uranium is only made for bomb purposes; power reactors don't need it (although some submarine reactors have used it), so it's not made without good reason.
There's some concern about laser enrichment, which apparently works quite well and is seldom discussed. The US has at least one laser enrichment plant.
Plutonium bombs are hard to get right. A gun bomb with plutonium will pre-detonate. It takes an implosion. Making a perfectly symmetrical implosion is hard. The explosive lenses have to be very uniform in density and accurate in dimension, while being made out of a soft plastic explosive. There are tricks to the joints where the lenses meet, some of which are still classified. Then there's the whole detonator thing, although getting the big pulse required is easier than it was 50 years ago.
Plutonium is a by-product of reactor operation. Thus, there's more of it around than is really needed. The US and the former USSR made way too much (tons). But working with plutonium is a huge headache. The dust is radioactive and poisonous. The PUREX chemical process to refine it from reactor fuel rods is difficult; places which did it tend to be toxic waste sites now. Its machining properties are strange; it expands when heated but then doesn't contract fully. Plutonium bombs usually have a neutron generator to get things started, and surrounding shells to perform neutron reflection and tamping. While the general principles are well known, it takes a lot of engineering R&D to get it working right, including many non-nuclear test explosions.
That's why we don't see terrorist groups making bombs from spent fuel rods.
It's even harder than that. Unless you remove the fuel rods quickly, in 1-2 months, you get too much thermally hot plutonium-238, and too much plutonium-240, which is the isotope that prevented using reactor bred plutonium in a gun assembly bomb and required developing the implosion design. Another isotope quickly decays into a fierce gamma ray emitter.
At best, from what I've read, you'd have a bomb that prior to detonation would dissipate 100 kW, requiring serious refrigeration and therefore size, and you'd be lucky to get a yield greater than 1 kT. That's of course quite capable of ruining your whole day, but it's perhaps more accurately viewed as a super dirty bomb, that's likely how you could achieve the most damage with it. Not a city killer, but a contaminator.
> Plutonium is a by-product of reactor operation. Thus, there's more of it around than is really needed. The US and the former USSR made way too much (tons).
I was under the impression that there's not that much plutonium kicking about, unless it's the Russians and former USSR states that have the lion's share:
"So, we depend on plutonium-238, a fuel largely acquired as by-product of making nuclear weapons.
But there’s a problem: We’ve almost run out.
“We’ve got enough to last to the end of this decade. That’s it,” said Steve Johnson, a nuclear chemist at Idaho National Laboratory. And it’s not just the U.S. reserves that are in jeopardy. The entire planet’s stores are nearly depleted. The country’s scientific stockpile has dwindled to around 36 pounds."
There are many isotopes of Plutonium. Pu-238 has a half-life of 88 years and is used in Radioisotope thermoelectric generators. That's the isotope that's running out.
So, of course, we come up with schemes to get rid of them, like blending some in with other nuclear fuel. Such as what we did in Unit 3 at Fukushima Daiichi. And then hydrogen gas caused massive explosions, releasing radioactive material. Oops.
Super carriers have a much bigger space budget than submarines, so I wouldn't be surprised if they used less enriched uranium. On the other hand, refueling is a total pain, so that might put a premium on the total U-235 in the fuel rods (granted, some of the U-238 will breed into plutonium and some of that will burn up, but...).
The US and UK use 90% enriched uranium in naval reactors. China, India, France, and Russia use lower levels of enrichment. The US really ought to have converted over by now. There are people lobbying for this for future nuclear vessels.
But enriched uranium is hard to make. The old gaseous diffusion process was incredibly inefficient - plants over a mile long to make tens of kilograms. Centrifuges have changed that. Centrifuge plants aren't all that big, about Wal-Mart sized. They're still expensive. Highly enriched uranium is only made for bomb purposes; power reactors don't need it (although some submarine reactors have used it), so it's not made without good reason.
There's some concern about laser enrichment, which apparently works quite well and is seldom discussed. The US has at least one laser enrichment plant.
Plutonium bombs are hard to get right. A gun bomb with plutonium will pre-detonate. It takes an implosion. Making a perfectly symmetrical implosion is hard. The explosive lenses have to be very uniform in density and accurate in dimension, while being made out of a soft plastic explosive. There are tricks to the joints where the lenses meet, some of which are still classified. Then there's the whole detonator thing, although getting the big pulse required is easier than it was 50 years ago.
Plutonium is a by-product of reactor operation. Thus, there's more of it around than is really needed. The US and the former USSR made way too much (tons). But working with plutonium is a huge headache. The dust is radioactive and poisonous. The PUREX chemical process to refine it from reactor fuel rods is difficult; places which did it tend to be toxic waste sites now. Its machining properties are strange; it expands when heated but then doesn't contract fully. Plutonium bombs usually have a neutron generator to get things started, and surrounding shells to perform neutron reflection and tamping. While the general principles are well known, it takes a lot of engineering R&D to get it working right, including many non-nuclear test explosions.
That's why we don't see terrorist groups making bombs from spent fuel rods.