Source: MSNBC
When a nuclear power plant shuts down, it still continues to produce heat. Coolants and back-up power supplies come into play to gradually cool down the reactor core. However, in Japan, the external power supply and the back-up diesel generator have failed, said Carnegie’s James Acton in an interview with MSNBC. Examining the range of possibilities that could ensue at Japan’s Fukushima reactors, Acton explained that as the core is not being properly cooled, steam is building up in the reactor containment vessel. Japanese engineers have vented some of the fuel, after evacuating a two mile radius around the plants.
The crucial question that remains is whether the fuel in the core will melt the surrounding metal. If such a meltdown were to occur, it could proceed in a few hours or over the course of multiple days. Acton emphasized that a meltdown is not necessarily imminent; a wide range of potential outcomes exists, from the release of limited radioactive steam to extensive radiation leakage.