So I came across a typically goofy Pravda story about how a UFO prevented an explosion at Chernobyl:
One thing that stopped me, though, was this:
Would it even have been possible for the nuclear material to go critical and explode? I know this isn’t possible in U.S. facilities (where nothing even approaching explosive critical masses of nuclear material are brought together), but the design and operations of Chernobyl were significantly different. Leaving aside the UFO issue, could Chernobyl have suffered an accidental nuclear explosion?
To get a nuclear bomb type explosion, you have to arrange for the nuclear material to go from subcritical to supercritical fast enough for it not to be blown apart or scattered by the heat buildup. Nuclear bombs use precision-times explosives to do this. There’s really no way for a pile of melted uranium and reactor parts to get into a configuration where it would explode with any kid of high yeild. If some part of it approaches supercriticality, it’ll blow itself apart in a smaller explosion before building up to the criticality levels where it could trigger a real explosion.
There were apparantly two explosions at Chernobyl - first a steam explosion that ruptured the reactor vessle, then a explosion of vaporized carbon from the graphite fuel rods. The remais of the core just got really hot and burned for a while. The explosions may have helped scatter the fuel - I hear there were chunks of uranium and control rods scattered around the plant parking lots.
The UFO story just goes to show how Pravda is becoming the Russian equivalent to the Weekly World News.
I’m far from an expert in nuclear explosions and to follow up on AndrewL’s post. I believe that a nuclear explosion from a reactor is unlikely. One of the major design problems of the atomic bombs was that of getting a critical mass together, and holding it together long enough for the explosive chain reaction to proceed to a conclusion. So for a reactor core, it would probably melt down and change its shape to the extent that a high-yield event probably wouldn’t happen.
In the uranium bomb, a sub-critical mass with a hole in it was the main part. Then a mass large enough to make the main part critical was shot into the hole from a “cannon.” In the plutonium bomb the reaction mass was loosely packed and then its density increased to get critical mass by setting off a covering shell of explosive material around it.
Getting a complete reaction and therefore a high-yield explosion is always a problem. The material has to be confined in something that is strong enough to contain the reaction so all the material, or at least most of it, reacts but not so strong as to confine the resulting pressure.
The British “blockbuster” bombs in WWII had real problems in the design phase because the explosive charge was so big that it would blow itself apart too soon and hurl large chunks of unexploded charge all over the place. I don’t know how many detonators they finally wound up with or their placement and timing but I do know that it was hard to get the whole charge to go.
I would say that a nuclear fission explosion of a reactor is more than just unlikely–it is impossible.
In addition to all of the reasons presented already, still another factor is that the enrichment of the fuel is insufficient. Civilian reactor cores contain uranium that is enriched higher than that found naturally, but far less than that of bombs.
The tragedy at Chernobyl is almost the worst that can happen in a nuclear reactor accident. Radioactive fuel, fission by-products, and other radioactive contaminants were spread all over the landscape. I say “almost” only because of the mitigating actions of the heroic fire fighters.