It is not skilful engineering or clever reactor design that precludes a nuclear explosion at a nuclear power plant, it is the laws of physics.
The materials in a nuclear reactor core and the uranium enrichment level make impossible a nuclear explosion of the type for which nuclear weapons are designed. Despite this, repeatedly we have heard talk of nuclear explosions in the media from observers, pundits, anti-nuclear activists and journalists.
An explosion at a nuclear reactor is not a nuclear explosion and can never be. Power plant grade uranium contains the fissile form of uranium, the uranium-235 isotope, at just a few percent, the majority is non-fissile U238. Compare that to weapons grade uranium which has to contain 80% or more of U235. Moreover, a nuclear weapon requires that a critical mass of the radioactive material be forced together quickly in a relatively small volume. This cannot happen in a nuclear plant, the low concentration of fissile U235 means it can never reach an explosively critical mass of fissile material but there is also no process that could force it together into a small volume quickly, anyway.
The same applies to the mixed oxide type reactor that use plutonium oxide and uranium oxide. It should be noted that plutonium is generated by the normal operation of a uranium-using power plant, although again it is the wrong ratio of isotopes for a nuclear weapon.
Essentially, the fuel concentration in a power plant is far too low to build the high-energy neutron stream needed to cause an explosive chain reaction. As to nucleear meltdown. The term nuclear meltdown is laden with China Syndrome style drama, but it does not lead to a nuclear explosion; a lot of heat is produced, and the nuclear fuel melts (happens at about 2700 Celsius). That’s it. Moreover, the melting process leads to less fissile material being in close contact when it spreads and melts through the floor of the reactor and the rock beneath. Incidentally, the hypothetical limit on depth the molten fuel could penetrate is about 15-20 metres, not the Earth’s core. Inevitably, the chain reaction that generates the heat slows of its own accord.
None of this is to say that what has happened in Japan is any less of a tragedy, it is just that phrases like nuclear meltdown and nuclear explosion used in this context in no way help, they spread fear, give tabloid headline writers scaremongering fodder, and are being hijacked repeatedly for personal agendas. Conventional explosions do occur, as we’ve learned this week from tragic events in Japan. But, these are not nuclear explosions. There is a risk of particles of radioactive material entering the atmosphere or the ocean, but this does not amount to the impact of an actual nuclear explosion.
New Scientist has a useful summary of what is happening at the Fukushima nuclear power plant in Japan following the Sendai earthquake and subsequent devastating tsunami.