What Is a Nuclear Meltdown?

Lifes Little Mysteries www.lifeslittlemysteries.com What is a Nuclear Meltdown? A nuclear plant generates electricity by boiling water into steam. The steam turns turbine generators to produce power. Heat to boil the water is provided by a controlled nuclear fission reaction, but if the reaction goes out of control, a nuclear meltdown can happen. EMITTED PARTICLE RADIATION SUBCRITICAL MASSES OF RADIOACTIVE ELEMENT- RADIOACTIVITY: A radioactive element such as uranium contains unstable atoms, which spontaneously split into smaller atoms. Subatomic particles are emitted that carry away energy. This process is called radioactive decay. RADIATION FISSION: When a sufficient quantity (a critical mass) of uranium is collected together, the emitted particles cause a chain reaction to begin. The particles hit other uranium atoms, causing them to split and emit more particles, in an accelerating cascade called fission. This process releases heat and a distinctive blue glow called Cherenkov radiation (photo, right) GRAPHITE RADIATION CONTROL RODS: To control the reactor, a material such as graphite can be inserted into the mass of radioactive material in order to absorb some of the particles and slow down the reaction. CONTAINMENT BUILDING REACTOR PRESSURE VESSEL STEAM STEAM WATER COVERS CORE -FUEL RODS EXPOSED - URANIUM FUEL RODS WATER LEVEL DANGEROUSLY LOW GRAPHITE CONTROL RODS PRESSURE NORMAL PRESSURE HIGH In normal operation, the uranium fuel rod assem- bly (the core) is kept submerged in water to If the water circulation system stops for any reason, cooling water that is normally below the boiling point reaches saturation and starts to boil. In the sealed containment vessel, pressure rises with no keep it cool and absorb additional emitted particles, helping to control the nuclear fission reaction. Control rods can be inserted into the way to escape. Even with the reactor turned off and no nuclear reaction taking place, radioactive fission products continue to decay and produce heat. core to further moderate the reaction. Meltdown UPPER GRID DAMAGED CAVITY With the reactor shut down and water pumps MELTED URANIUM CORE not operating, the core can continue to build up COATING OF PREVIOUSLY MOLTEN MATERIAL -CRUST boat heat until the fuel rods melt. In this event, radioactive materials could be released into PREVIOUSLY MOLTEN MATERIAL HOLE IN BAFFLE PLATE the reactor vessel and MELTED DEBRIS COLLECTED AT BOTTOM OF PRESSURE VESSEL then into the containment building through coolant leaks. DAMAGED INSTRUMENT- GUIDE RIGHT: DIAGRAM OF MELTED REACTOR CORE AT THREE MILE ISLAND, 1979 In the case of the Chernobyl accident in 1986, the reactor was in an unstable condition during a test. When control rods were inserted, a power surge occurred. A series of failures ensued until an explosion tore through the reactor and its containment building, hurling fuel fragments and hot graphite into the air. SOURCES: TEMITOPE TAIWO, ARGONNE NATIONAL LABORATORY; OAK RIDGE NATIONAL LABORATORY; INTERNATIONAL ATOMIC ENERGY ASSOCIATION; UNITED STATES NUCLEAR REGULATORY COMMISSION; WORLD NUCLEAR ASSOCIATION KARL TATE, LifesLittleMysteries.com