As I understand it, the basic principle behind a nuclear weapon is to wrap precisely shaped charges around fissile material, then detonate the charges to compress the radioactive stuff into an uncontrolled supercritical mass.
What would happen if instead of plutonium, you did the same thing with plain, ordinary high explosives? Would a bomb with X pounds of TNT imploding a core of Y pounds of TNT produce a blast of X+Y pounds, or would there be some multiplier effect?
There’s no multiplier effect … although there may be more efficient configurations … however X pounds of TNT plus Y pounds of TNT will produce a maximum of X + Y pounds of boom … and no more …
You can get more efficiency if you design your explosive so that all the reagents react in the explosion. If half of the chemicals don’t react then that’s a huge waste.
But the reason you want that supercritical mass is because of how nuclear reactions work. Nuclear reactions release energy, which can cause further nuclear reactions. But that energy will disperse the nuclear material, which stops further nuclear reactions. That’s the earth-shattering kaboom nuclear bombs are so famous for.
So for a nuclear bomb, you need to get as many nuclear reactions done in the first fractions of a second, because the energy released is going to spray your uranium or plutonium or whatever all over the landscape, and those dispersed particles won’t be critical anymore.
The way it works is that if you’ve got Uranium 235, it sometimes spontaneously breaks down and releases photons and neutrons. But if a U-235 nucleus gets hit by a neutron, that will also cause it to break apart, which releases more neutrons. So if you’ve got a lump of Uranium, if the average neutron release by spontaneous decay flies off into the world without hitting another nucleus, then you’ve got a sub-critical mass. But if the average neutron created by spontaneous decay hits a nucleus splits it apart which creates another neutron which on average hits another nucleus, which creates another neutron and so on, then you’ve got a critical mass.
Your uranium heats up. This is how nuclear reactors work. But if the uranium gets really hot it’s not just a lump of hot uranium metal anymore, it becomes liquid or gas or plasma. That’s really energetic. But that knocks the critical mass of uranium apart, which means it’s not critical anymore.
So the trick with a fission bomb is to take a bunch of lumps of sub-critical uranium or whatever, and bring them together very quickly so that you have a super-critical mass. And the faster you bring them together the more energy you get out because if you do it slowly they’ll just knock the supercritical mass apart before much happens.
There will be no multiplier effect; it’s just a bomb containing X+Y pounds of conventional high explosive. As noted upthread, the amazing energy released in a nuclear detonation comes from the splitting of heavy atoms brought into a supercritical mass by the shell of conventional explosives. If you replace the “pit” of nuclear material with a ball of conventional high explosive, there will be no nuclear reaction to provide the aforementioned amazing energy: the only energy the “pit” can provide is that of conventional high explosives, which is going to be far less than the original nuke.
Counter-intuitively, some seemingly benign and mundane interactions with a chunk of plutonium can have powerful reactions. Much less exotic than shaped-charged explosives.
Like dropping the last tungsten carbide brick in a neutron-reflector assembly on top of the subcritical core, generating enough reflectivity that the normal neutron emissions from the plutonium reflected back into the core, just pushing it into criticality. Not a boom, but an instant flash of intense radiation that persisted until the scientist knocked the reflector assembly apart.
A mundane, trivial accident. Dropping a harmless chunk of tungsten carbide! It killed Harry Daghlian from severe acute radiation poisoning, 25 days later.
The same chunk of plutonium killed another scientist a couple of years later, in essentially the same way: Lou Slotin was manually manipulating a spherical neutron reflector assembly made of two hemispheres of beryllium. As long as he held the hemispheres apart, the resulting neutron reflect-back flux wasn’t enough for criticality.
But his hand slipped, the hemispheres closed, the core went critical, and poured out a deadly pulse of hard radiation and neutrons. Dr. Slotin received a lethal dose and died 9 days later.
A slip of the hand! Completely mundane! So deadly.
The chunk of plutonium came to be called the Demon Core and Los Alamos stopped doing any kind of hands-on criticality experiments on that chunk of plutonium or any other. The core itself was eventually melted down and the plutonium recycled for other weapon cores.
Slotin’s death was, in part, caused by the mind-bogglingly poorly designed apparatus he was using. Instead of raising one half of the core to close the gap with the other half, the setup had the bottom half of the core fixed in place, and the top half was lowered down onto it. All it took was for his screwdriver to slip and the cores came into contact, resulting in a criticality excursion. OSHA would never have approved, and in fact, Los Alamos changed their methodology after that accident.
Furthermore, even if perfectly efficient in exploding the explosive, the X+Y bomb will in general do less damage than two bombs, of sizes X and Y, exploded separately. Damage effects scale worse than linear. So doubling the size of a bomb doesn’t, for instance, double the area it’s capable of devastating.
(In attacking a distributed target like a city, you’re thus better off using multiple warheads, rather than the same megatonnage all in a single big one.)
Note that shaped-charge weapons do concuss embedded material as well as a design principle. But the bang for the buck from outer X material to inner Y material is again different from OP, in that the energy is “formulated” differently, into a spatially focused release, as opposed to 360 degree boom.
Shorty from 1998 at the BEEF at Lawrence Livermore Lab, nice summary/pix; Wiki has more.
Would it be fair to say that doubling the explosive mass doubles the volume of the sphere (or hemisphere, for a ground-level detonation) at which a given blast pressure is observed? If that’s true, then doubling the explosive mass would increase the effective blast radius by a factor of 2[sup]1/3[/sup], or 1.26, thereby increasing the blast area by a factor of only 1.59.
The fun thing about radiation poisoning is that there’s hardly any symptoms – at first. So you get a couple days to look forward to your imminently slow, painful, drawn-out, agonizing death.