The funny thing about plutonium is that it is highly magnetic. Although some of the late stage fission products are not magnetic, plutonium and uranium, which would make up most of the radioactive fallout, and some of the other early-stage fission products are. Given the circumstances of the blast, and the fact that the initial blast would have probably "released" an even stronger magnetic field than the one we saw them experiencing, it's perhaps not that implausible to consider that the magnetic field may have "imploded" the nuclear explosion, and prevented the radioactive contamination of the entire island. That could also explain why they filled it with concrete, if the radioactive materials were all still stuck to the exotic matter at the bottom of the hole. The concrete is not for the magnetic properties of the area, but for shielding the radioactivity.

So when Jack and Sayid discussed the fact that a whole area was filled with concrete... "like Chernobyl", it was truly Jack and Sayid that made that necessary by detonating the nuke there. Perhaps they saved the world, though, just like Desmond did when he turned the failsafe key. Maybe the magnetism would have grown until it crushed the world into a black hole if they hadn't nuked it.

Wouldn't two opposite yet equally strong magnetic fields cancel each other out? Instead of the bomb being the incident (as has been suggested), maybe it really did prevent the incident...

That plutonium trigger bomb is not big enough to produce an explosion large enough to even shatter the windows of Dharmaville five miles away (well, maybe I'm exagerating a little). But it definitely isn't large enough to destroy the buildings in Dharmaville, let alone devastate the entire island. Additionallly, the trigger bomb doesn't have enough fissionable material in it to produce a huge amount of radiaoactive debris - it's just not that large. However, your point is very interesting - maybe even the small amount of radiation will be sucked into the shaft.

Wouldn't two opposite yet equally strong magnetic fields cancel each other out? Instead of the bomb being the incident (as has been suggested), maybe it really did prevent the incident...

I should have worded that better. By "highly magnetic" I meant highly attracted to magnetic fields... like iron or steel.

That plutonium trigger bomb is not big enough to produce an explosion large enough to even shatter the windows of Dharmaville five miles away (well, maybe I'm exagerating a little). But it definitely isn't large enough to destroy the buildings in Dharmaville, let alone devastate the entire island. Additionallly, the trigger bomb doesn't have enough fissionable material in it to produce a huge amount of radiaoactive debris - it's just not that large. However, your point is very interesting - maybe even the small amount of radiation will be sucked into the shaft.

I think the primary of an H-bomb would have to be at least as power as the bombs dropped on Hiroshima and Nagasaki. You're collapsing a critical mass of plutonium with explosives. Of course, what Jack was carrying around wasn't it. The plutonium core would be small and easy to carry around, if you don't mind a little radiation, but there's no way to "detonate" it without several tons of explosives and shock lenses in a huge sphere surrounding it. But I guess that's creative license. It was odd they made it cylindrical. Either an H-bomb secondary or a gun-type uranium bomb could be cylindrical, but of course they'd both be bigger and heavier too.

I think the primary of an H-bomb would have to be at least as power as the bombs dropped on Hiroshima and Nagasaki. You're collapsing a plutonium core into a critical mass with explosives. Of course, what Jack was carrying around wasn't it. The plutonium core would be small and easy to carry around, if you don't mind a little radiation, but there's no way to "detonate" it without several tons of explosives and shock lenses in a huge sphere surrounding it. But I guess that's creative license. It was odd they made it cylindrical. Either an H-bomb secondary or a gun-type uranium bomb could be cylindrical, but of course they'd both be bigger and heavier too.
Believe it or not, the small trigger bomb that Jack took from Jughead was very accurate looking.

Here's how a hydrogen bomb works: it fuses two hydrogen atoms into a single helium atom, releasing (in the process) an INCREDIBLE amount of energy, which we see in the form of a gigantic explosion, massive sonic shock wave, and intense infrared heat, as well as a lot of stuff we don't see (radiation, gamma and x-rays, etc).

However, in order to get hydrogen to start the process of fusion, it must be first exposed to incredible heat. In space this is done when an incredibly large amount of hydrogen gas coallesces through gravitational forces together into a very dense cloud, which at the core becomes very hot, and this is what sparks off a fusion reaction that ignites stars and makes them light up.

However, in a hydrogen bomb, the temperatures needed to start up a fusion reaction occur by touching off a small plutonium/fission bomb. The plutonium bomb explodes, and it creates the temperature necessary to start the fusion reaction to begin. The plutonium bomb doesn't need to be large at all - in fact, it would only add uneccessary weight to the overall bomb. So, the ideal plutonium trigger bomb would be as small as possible to obtain the necessary temperatures needed for the fusion reaction to begin.

Thus, the bomb we see Jack carrying (the fusion/plutonium trigger) is just the right size. It has a cylindrical shape, but that is also fine. The outside shell would be made up of conventional highly explosive chemical bomb material, and the inner core would have the plutonium.

So, it is detonated by a spark going simultaneously to all the charges on the outside shell, which we can see with the wires connected to electrodes placed in various spots on the cylinder's shell. This explosion would be focused in, and would jar loose the unstable electrons circling the plutonium atoms in the core, which would cause it to begin its nuclear fission reaction, as the plutonium turned into more inert material.

Here's the basic difference between fusion and fission: fusion fuses two atoms into one, fission fissiles out extra electrons, making an atom with a lot of electrons orbiting it into an atom with much fewer electrons. These extra electrons that are sloughed off are what creates the explosion and the radiation.

I see absolutely nothing that would indicate this was anything other than the plutonium/fission trigger bomb that Jack removed. It was precisely what he said he was removing, too. No mistakes here, people - the production crew got it right.