I am sure that all my readers will know that an atom bomb explosion near a populated area would have a very horrible effect. It would lead to countless deaths as a result of heat and blast.
It is important to understand that the reactors in Japan can not explode like an atom bomb with a nuclear detonation.
To create a nuclear detonation requires that the bomb design must satisfy a series of demands, the two most important ones here are.
1. The system must go from being subcritical to super prompt critical
Right now the reactors are subcritical and no fission reaction is occurring inside them. The addition of boric acid and the control rods should keep the reactors subcritical.
2. This change must occur very quickly, faster than the time between the spontaneous appearance of a neutron in the fuel
It would be impossible to remove the control rods at the speed which would be required. Also the removal of the boric acid would be very hard to do. Also the rate of neutron generation inside used power reactor fuel (either uranium dioxide or MOX) will be quite high. This fuel is close to the total opposite of a material which lend it’s self to building an atom bomb.
The plutonium isotope signature is likely to be very bad for bomb making. The Pu-240 level in used MOX, used uranium dioxide fuel and reactor grade plutonium will be high. The Pu-240 is one of the key isotopes which makes it harder to build bombs out of plutonium than it is to make a uranium-235 based bomb.
So I think that a large explosion like one of the atom bombs dropped by the USAF on Japan at the end of world war two will not occur in the damaged reactors or in the spent fuel store in Japan.