A quick question about Nuclear explosions

[Samuel]

Here's a question for you Skimmer. What would be the result of a nudet in a nuclear plant? Not talking 'powerplants are bombs!', talking straight up 'take regular nuclear power plant. Place bomb inside. Detonate.' Just extra fallout? A potentiated blast thanks to the fuel? Weakened overall effect on the rest of the area thanks to the sturdy structure dissipating a little more of the energy than most det sites before going up in a big smoke?

Nuclear reactions occur because the material inside the bomb is forced into critical mass. I don't think the nuclear fuel inside the building would help at all although it probably would make additional fallout.

[Surlethe]

Wouldn't the bomb automatically detonate upon being placed in a sea of thermal neutrons? Anyway, the extra fuel is probably not enriched enough to provide a lot of oomph, and the blast's shockwave would blow it apart from any semblance of critical density within a fraction of a second. That's actually a major problem in bomb design, as I understand: maximizing the payload is equivalent to maximizing the number of reactions, which in turn depends on amount of time spent at critical density, which is shorter and shorter as the energy yield increases, since the more power you give off the faster you blow the bomb apart*. If you don't design your bomb well, it fizzles, like the DRNK's did a couple of years ago.

*Similar to the Eddington limit for stellar formation: if a star is too large (>200 M_sun), the nuclear reactions in the core will occur so quickly, the radiation pressure will literally blow the star apart. That's why there are no (few?) observed stars above blue giant luminosity.

[Spectre_nz]

You wouldn't manage it with a fission weapon. As mentioned, a lot of effort goes into getting the correct geometry for the implosion to make a fission weapon work at all, and even then, the bulk of your fission material usually doesn't get to react. Your nuclear reactor is just going to end up as dirty debris.

I got to thinking that maybe you could do it with a thermo-nuke; they can be staged so you have fission -> fusion -> fission - this was the idea with that Russian monster, the Tsar Bomba. What it is with the Russians building comically oversized to the point of being useless weapons I don't know. But the point is, at full yield it would have been 50Mt of mostly Fusion, then 50mt of fission from the uranium-238 tamper around the fusion sparkplug. (In the one and only test of such a weapon, they substituted lead and got a lower, mainly fusion yield of around 50Mt, or 57Mt by American estimates)
Comparatively, the largest fission only weapon was the 500kt, and that required an aluminium-boron chain to be inserted inside hollow pit of the fissile material to adsorb neutrons and prevent a partial, premature reaction.

So, there’s enough time to adsorb neutrons from a fusion bomb to get one hell of a fission bang before the blast shockwave fragments your parts. But then I realized that with the tamper in place, you're not going to have neutrons left over to trigger fission in your reactor pile, and without it, you're not going to get the thermo-nuke to work.

So... you could turn the entire reactor into a gigantic 4th stage. Use the reactor walls as another tamper and hopefully the fuel rods would catch sufficient neutrons to fission as well. Being outside the reactor core wouldn't cut it, it's designed to catch neutrons, and you want lots of them. You'd need your bomb inside the reactor core, probably on top of the fuel rods, and you'd probably also need to remove control rods and any moderating water as well. all that effort and the fact that you'd need a megaton scale weapon to get the ball rolling in the first place makes any additional benefit from the nuclear pile seem pointless. Oh yes, and nuclear fuel rods are usually composites of uranium oxide as well as neutron moderating materials, so, that probably isn't going to help the reactor-bomb idea any.

There's always the option of trying a neutron bomb, but I'd say that would suffer all the same drawbacks as putting a fission weapon on top of your fuel rods.

[Sky Captain]

An atomic bomb in nuclear reactor would create much more fallout than detonated alone. Basically all nuclear materials available in reactor would vaporize and become airborne and come down somewhere. A Chernobyl reactor released only small part of it`s total radioactive contents and still created a heavy fallout. A nuclear bomb would vaporize the reactor and disperse all it`s radioactive contents it would be like a giant dirty bomb on steroids.

[Winston Blake]

So, there’s enough time to adsorb neutrons from a fusion bomb to get one hell of a fission bang before the blast shockwave fragments your parts. But then I realized that with the tamper in place, you're not going to have neutrons left over to trigger fission in your reactor pile, and without it, you're not going to get the thermo-nuke to work.

So... you could turn the entire reactor into a gigantic 4th stage. Use the reactor walls as another tamper and hopefully the fuel rods would catch sufficient neutrons to fission as well.

Unless the tamper was made of natural uranium (as in the original Tsar Bomba you mentioned), which is a very good tamper material. Then you would have vastly more neutrons 'left over' than the secondary originally produced. Fast fission actually produces more neutrons per fission than thermal neutron capture. Given that an unenriched uranium tamper can be easily fissioned by a nuke's secondary, I imagine that the resulting very intense flux of fast neutrons could fission the (low-enriched) fuel rods directly, no tamper required.

Ideally you would want the bomb to be as close as possible to the fuel rods of a fast reactor, to maximise intensity due to the inverse square law, and minimise the amount of moderator material present.

[Darth Wong]

As is often the case, the word "shockwave" is being used repeatedly in this thread but the person who asks the original question never stops to ask precisely what a shockwave is. He assumes he knows, because he's seen "shockwaves" in movies. They're like ripples in a pond, only really big and in air, and they knock things over, right?

Well, no. A shockwave is not at all like a ripple in a pond, even though we invariably see things which look like that in sci-fi films and they're called "shockwaves". A ripple in a pond is a conventional wave, like a sound wave in air. In a conventional wave, pressure goes up at the wave front, and then it goes back down afterwards. A pressure wave can "pass over" you.

A shockwave can not "pass over" you, because whatever's behind the shockwave is worse than the shockwave itself. It must be, in order to push the shockwave ahead of itself. In effect, the shockwave is just the front of a wall of high pressure. Pressure does not drop back down behind the front of a shockwave; it actually keeps going up.

Normal waves pass through air at the speed of sound, but it is physically impossible to create a conventional wave in air which travels supersonically. A shockwave is not a conventional wave; it is just the shocked air in front of something which is forcing it to move ahead at supersonic speed. For example, there is a shockwave in front of a supersonic jet; it is forced into that condition by the forward movement of the jet itself. The noise you hear is not the shockwave; it is just a normal pressure wave which is created by the shockwave, as a secondary effect.

In the case of a nuclear fireball, the super-hot ball of gas is expanding so rapidly that it pushes air in front of it, thus creating the shockwave. However, what we normally think of as the "blast wave" is different from the shockwave. The blast wave (ie- the huge wind that you see knocking over houses and trees in those grainy old nuclear test videos) is a subsonic pressure wave that is produced once the fireball expansion has slowed to the point that it is no longer expanding quickly enough to maintain a supersonic shock front at its edge. The shockwave is the edge of the fireball, when it's expanding supersonically. Rather than thinking of a shockwave as a ripple in a pond, you might want to think of it as the grille of a Mack truck. It's just the first part that hits you.

Simple rule: if it's subsonic, it's not a shockwave.