Fukushima - The mechanism of the nuclear catastrophe

Boiling water reactor
Boiling water reactor
The final blast at Fukushima
The final blast at Fukushima

About the reactors The nuclear plants at Fukushima utilize Boiling Water Reactors (BWR) which produce electricity with boiling water and spinning a turbine with the steam generated. The nuclear fuel heats up the water, thus producing steam. The steam drives turbines which in turn generate electricity. The steam is cooled and condensed back to water. This water is recycled for heating by the nuclear fuel. A BWR reactor operates at about 285 °C. The nuclear fuel used in the BWR is uranium oxide. Uranium oxide is a ceramic with a melting point of about 2800 °C. The fuel is in the form of pellets, inserted into a long tube made of Zirc-alloy, an alloy of zirconium. Each tube is called a fuel rod. These fuel rods put together form assemblies that make the reactor core. The first barrier is the solid fuel pellet a strong ceramic oxide matrix that retains many of the radioactive fission products produced by the fission process. The second barrier is the Zircalloy casing that separates the radioactive fuel from the rest of the reactor. The core is then placed in the pressure vessel operating at a pressure of about 7 MPa (~1000 psi), which is designed to withstand high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release. The primary loop of the nuclear reactor consists of a pressure vessel, pipes, and pumps that contain the water coolant all housed in the containment structure. This structure is the fourth barrier to radioactive material release. The containment structure is a hermetically sealed, with a casing of steel and concrete. These barriers serve just one purpose- to contain, indefinitely, a complete core meltdown. An additional concrete structure is poured around the containment structure, the secondary containment further embellishes the reactor. The main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out. With such a fool proof mechanism the world at large still battles the nuclear disaster of Fukushima!

Science vs Nature We are at cross roads to choose how far can we trust the acumen and tech savvy capability of our scientists in this age of science and technology. Can our mega advanced science combat Nature? In the BWR, the uranium fuel generates heat by neutron-induced nuclear fission. Uranium atoms are split into lighter atoms. This process generates heat and more neutrons through a nuclear chain reaction. During normal, full-power operation, the neutron population in a core is stable and the reactor is in a critical state. The nuclear fuel in a reactor can never cause a nuclear explosion like a nuclear bomb. At Chernobyl, the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all structures, propelling molten core material into the environment. The control rods are made of boron which absorbs neutrons. The control rods are also used to shut the reactor down from 100% power to about 7% power which is the residual or decay heat. The residual heat is caused from the radioactive decay of fission products. Radioactive decay is the process by which the fission products stabilize themselves by emitting energy in the form of small particles (alpha, beta, gamma, neutron, etc.). There are  fission products that are produced in a reactor, including cesium and iodine. This residual heat decreases over time after the reactor is shutdown, and must be removed by cooling systems to prevent the fuel rod from overheating and failing as a barrier to radioactive release. Maintaining enough cooling to remove the decay heat in the reactor is the main challenge in the affected reactors in Japan right now. At Fukushima nature displayed its prowess when the earthquake destroyed the external power supply of the nuclear reactor. For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived it flooded the diesel generators, causing them to fail. When the diesel generators failed after the tsunami, the reactor operators switched to emergency battery power. After 8 hours, batteries ran out, and residual heat could not be carried away any more. The possibility of core meltdown was a concern. The priority was to maintain the integrity of the fuel rods by keeping the temperature below 1200°C, as well as to keep the pressure at a manageable level. The operators svented off steam to control the pressure.  Some radioactive gases were released to the environment during venting. During  venting, the water level dropped below the top of the fuel rods. Thus the temperature of the fuel rod cladding exceeded 1200 °C, initiating a reaction between the Zirc-alloy and water. This oxidizing reaction produced hydrogen gas. Since hydrogen gas is extremely combustible, it produced an explosion. The explosion took place outside of the containment, but inside and around the reactor building. An explosion occurred at the Unit 3 reactor. This explosion destroyed the top and some of the sides of the reactor building. The nuclear material itself was still intact, but the surrounding Zirc-alloy shell began to fail. Some of the radioactive fission products (cesium, iodine, etc.) mixed with  water and steam and  a small amount of cesium and iodine were detected in the steam that was released into the atmosphere. Since the reactor’s cooling capability was limited, and the water inventory in the reactor  decreasedg, engineers injected sea water mixed with boric acid – a neutron absorber to ensure that the rods remain covered with water. Following a series of explosions, radioactivity was detected harmful to human health in the atmosphere and a majority of the workers were evacuated. But a skeleton team of around 50 workers, the Fukushima 50 – worked in shifts and rotations in a desperate attempt to stop the catastrophic reactor meltdown. The Fukushima Fifty breathe through uncomfortable respirators or carry heavy oxygen tanks on their backs and at all times. Radiation can penetrate their gear and, at the very least, the workers face an increased risk of cancer and shortened life. Five of their colleagues are dead and more injured. Their task was to douse the reactors with water to prevent a meltdown while desperately trying to restore a cooling system that might stabilise the reactors. The site is now so contaminated that it has become difficult for them to work near the reactors for extended periods of time.  A third explosion struck Fukushima nuclear power plant, when Tokyo Electric Power Company (TEPCO) stated that radiation levels reached 8,217 microsieverts per hour near the plant's front gate, roughly two and a half hours after the blast. The blast damaged an essential steel containment structure, and larger leaks of radioactive material are immiment. Reactor No. 4 was not in operation at the time of the earthquake. The reactor contains spent fuel, not fuel rods. A Hydrogen explosion seems to have taken place with No. 4. Some foreign objects fell into reactor No. 4, which caused problems. The blast at No. 2 reactor came 30 minutes after the incident at No. 4. A hole has been observed in the No. 2 reactor; there is a high possibility of container vessel damage for this reactor. Now that the core is damaged and seawater has been poured over the reactors to cool them, but all the damage is beyond repair—resulting in a permanent loss in Japan's power supply. Nuclear engineers working at the Japanese plant are dealing with two problems at the same time: - they are working to fully stabilize the plant’s reactors, as also trying to control the release of radioactive material. It could take weeks or months to stabilize the reactors. Containment and clean up of the radioactive material could take at least 10 years, at a cost of more than $10 billion. For starters, the tops of two buildings have collapsed, this radioactive debris will have to be cleared besides performing a nuclear cleanup task on this mega scale never done before.

 

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