Why can no nuclear power plant (NPP) EVER be a nuclear bomb?
Well the simple technical answer is there is 'insufficient neutron flux' - due to:
- The wrong fuel.
- Incorrect 'arrangement' of fission materials.
So to create a fission bomb we need a very high proportion of U235, better than 90%, that is high enrichment, getting U235 and isolating it from the rest of the Uranium isotopes is not cheap or trivial, but it's also not needed for a nuclear power plant. Most NPPs operate with something like a 5% (or less) level of U235, some operate with effectively no enrichment, 'Uranium dug from the ground' kind of proportions of U235 under 1%.
Once sufficient U235 exists to generate a fission weapon we need all of the atoms to be in very specific arrangements. Some U235 atoms are going to be triggered into fission by some induced neutrons (from a so called neutron spark plug - something else missing in a NPP) the U235 needs to very specifically paced such that as many of the liberated neutrons as possible collide with subsequent U235 atoms and so on. If you don't have this special, planned arrangement most of your neutrons will sail into vacant space and nothing much is going to happen.
When atoms fission, HUGE quantities of of energy are released, in the order of MeV per fission. Standard chemical explosions occur due to energy releases in eV per reaction (millionths of the energy released from a fission). When a million times the energy per 'reaction' is released what you get is a truly epic rate of temperature increase. The fission fuel goes from a 'warm' solid, to hot solid, to exceedingly hot solid, to liquid and then to gas/plasma in microseconds, if I had to guess, probably two of them, maybe less. Now remember what I said about that special orientation we need to keep our fission fuel in, that's achievable in a solid, but impossible in a liquid and even more impossible (yes, in this case, that is a thing) in a plasma state. The induced 'uncontrolled' fission is all over by the (incredibly short) time the fuel turns to a liquid.
That means that in every photo, in every video you've likely ever seen of a nuclear explosion the fission is well and truly stopped and all your looking at are photons if the flash is still there, gas plasma and pressure waves.
So for a nuclear power plant to become a nuclear bomb (and Khomyuk aka 'the conglomerated scientist' and Legarsov both referenced Chernobyl yielding 'megaton' sized explosions which are strictly the domain of thermonuclear ie 'hydrogen bomb'
devices) they were clearly implying that the fuel could somehow concentrate itself from less than 5% - 3% for RBMK IIRC, to better than 90%, then arrange itself into a desirable shape, self trigger and all within a couple of microseconds. The fuel was already 'melted'...
Consider it for yourself, consider 100x 0.5g dark chocolate chips evenly distributed in 950g of white 'chocolate' such that you have a block of chocolate 1000g in mass with a 5% concentration of dark chocolate. What are the chances of those chocolate chips migrating to conglomerate such that they all come together in a single location within the block to create a spot where all 50g of dark chips are encompassed in a mass that is less than 10% white 'chocolate'? - Sufficiently close to zero.
If you think that's not zero, then suppose the perfect structure for a chocolate 'nuclear' bomb is a ball of 1000g mass. Without using an appropriately shaped container (NPPs are not likely to be the right shape for a nuclear weapon) form, or otherwise maintain (preferably by accidental means) that shape whilst the chocolate is in a melted state. - It's ridiculous.
The first explosion at Chernobyl was thermal, nothing more than a gas being heated to the point where it generates a force capable of breaching the sealed environment containing it. See the numerous 'documentaries on YouTube of idiots putting dry ice into Coke bottles - that's all the first explosion was.
The second explosion was probably not the hydrogen explosion that is claimed, it was I think most likely the largish amount of either very hot solid, or more likely even hotter, molten Zirconium hitting water, like sodium or potassium hitting water. Zirconium burns with a very pretty yellow/green sparkly flame, it's quite vigorous, I bet it being really hot and hitting water would yield quite a reaction.
When I say I have a problem not so much with the 'story' but with the physics in HBO's Chernobyl, this is largely why.
They used a character playing a leading physicist/nuclear scientist and a character representing a conglomeration of physicists, nuclear scientists and engineers to perpetuate a myth of danger that just didn't/couldn't exist - there or in any NPP anywhere.