Meltdown can also occur in the pools containing spent fuel rods. “But if that containment is ruptured, then potentially a lot of material could go into the environment,” Ferguson said. That containment is shielded by protective layers of steel and concrete. If the vessel is ruptured, the material could flow into the larger containment building surrounding it. Partial melting can solidify and block cooling channels, leading to more melting and higher temperatures if adequate cooling isn’t present.”Ī full meltdown would involve all of the fuel in that core melting and a mass of molten material falling and settling at the bottom of the reactor vessel. “This can clog and prevent the flow of more coolant,” Ferguson said. And that’s just the beginning of a potentially catastrophic event. Exposed parts of the fuel rods eventually become liquid, sink down into the coolant and solidify. In time, that ultra-hot zirconium metal starts to melt. High temperatures cause the zirconium coating that covers the surface of the fuel rods to blister and balloon. This oxidation process releases additional heat, further increasing the temperature inside the core. But it’s a trade-off, as there’s no way to do this without also releasing some radioactive material.Ī nuclear meltdown is an accident resulting from severe heating and a lack of sufficient cooling at the reactor core, and it occurs in different stages.Īs the core heats, the zirconium metal reacts with steam to become zirconium oxide. In Japan, they’ve been relieving pressure by releasing steam through pressure valves. This superheated core must be cooled with water to prevent overheating and an excessive buildup of steam, which can cause an explosion. In the event of a cooling failure, water gets injected to cool the fuel rods, and pressure builds. It contains boiling water and steam, and as temperature rises, so does pressure, since the steam can’t escape. “Basically, each uranium atom splits into two parts, and you get a whole soup of elements in the middle of the periodic table,” said Arjun Makhijani, a nuclear engineer and president of the Institute for Energy and Environmental Research.Ī reactor is like a pressure cooker. In a working nuclear reactor, water gets pumped into the reactor’s heated core, boils, turns into steam and powers a turbine, generating electricity. These are radioactive fragments, such as barium, iodine and Cesium-137. As uranium atoms split, they produce heat, while creating what’s known as fission products. Under normal circumstances, energy is generated by harnessing the heat produced through an atom-splitting process called nuclear fission. Inside the core of the boiling water reactors at Japan’s Fukushima Dai-ichi facility are thousands of zirconium metal fuel rods, each stacked with ceramic pellets the size of pencil erasers. “But there are different steps along the way.” “This term ‘meltdown’ is being bandied about, and I think people think that you get the fuel hot and things start melting and become liquid,” said Charles Ferguson, physicist and president of the Federation of American Scientists. Which raises the questions: What exactly is a nuclear meltdown? And what is a partial meltdown? After a powerful explosion on Tuesday, Japanese workers are still struggling to regain control of an earthquake and tsunami-damaged nuclear power plant amid worsening fears of a full meltdown.
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