Following explosions in at least two reactor cores at Japan's Fukushima nuclear power plant - and as the developing crisis is rated at level 6 of seven levels on the International Nuclear Events Scale (previously being 3, or below 3 Mile Island status, to keep the panic level low)- attention is turning to just what is happening inside the 40-year-old power plant in north-east Japan. Al Jazeera's Dan Nolan explains how a meltdown would happen.