The long red tubes are zirconium-alloy-clad fuel rods being fastened together into large bundles that will form the core of a nuclear reactor. Inside the zirconium cylinders are stacked pellets of uranium oxide, the reactor fuel.
To find out what happens to the zirconium cladding and the fuel rods in a "nuclear meltdown", visit Mechanics of a Meltdown Explained. The article explains the problems faced by the Fukushima power plant after the March 2011 earthquake in Japan.
