A low energy nuclear reaction, maybe. Such things have occurred in labs, resulting in fatal exposures to workers-—but it was an exothermic ‘reaction’, not an explosion. An explosion would be the result of the ignition of the hydrogen, not the uranium.
So, how many possibilities are there ~
So I read...
Hydrogen burns in a flash, doesn’t itself explode but the heat generation could certainly cause water in the pool to flash to steam causing further reactions if the fuel assemblies are agitated.
Unit 3 was known to be using plutonium (MOX) in its mix for some of its fuel rods. Fuel ponds contain new and used fuel rods, some ‘spent’ waiting to be transferred to long term storage others ‘new’ waiting to be rotated into service inside the reactor. Plutonium is also a byproduct created inside the spent fuel rods to some small degree.
Why don’t they remove the fuel rods to another location? Because they must be kept underwater to be cooled at all time. Continuing residual heat from the used fuel rods in the assemblies is sufficient to cause them to heat up and catch fire and release their poisons in about 15 minutes, if exposed to ambient air. New/unused fuel rods are cold and could almost be handled by hand since they haven’t been exposed to a reaction yet (neutrons activity).
With Russia and the US agreeing to dismantle war heads, there was a sudden over supply of plutonium to either store or use. Selling it to nuclear power plant operators was one way to get rid of it.
The Savannah River Site is constructing a plant for reprocessing plutonium from warheads to sold as MOX nuclear fuel. After Fukushima, no government wants the stuff because it is highly unstable.
Witnesses reported hearing Unit 3’s explosions ten of miles away from it. Whether the long distance telephoto lens used to record the event had a stereo mike pickup is a mute point, there were explosions and sounds to go along with them.
Strontium in the fallout from Unit 3’s explosion indicates nuclear in nature, among the other types of fallout measured.