![]() ![]() The storage pools at Fukushima Dai-1 are reportedly about 14 meters deep the 4-meter tall spent fuel assemblies sit at the bottom of the pool. The storage pool is above the primary containment vessel because the spent fuel assemblies are removed from the top of the reactor, and transferred via water canals to the pool to keep them cool throughout the process. The water keeps the still-radioactive spent fuel from overheating and melting, and also prevents radiation from reaching the atmosphere. This protective building also houses a storage pool where spent nuclear fuel is kept in cool, circulating water. The primary containment vessel and the torus are in turn encased by the secondary containment building, a large box of steel and concrete. (The torus will be important when we explain what went wrong in Fukushima's No. Nuclear reactor meltdown fukushima series#Ringing the base of that containment vessel is a doughnut-shaped structure called the torus, which serves a safety function: If pressure rises too high in the pressure vessel, operators can vent steam into the torus through a series of relief valves. The pressure chamber is encased in a protective steel shell called the primary containment vessel. Water inside the pressure vessel keeps the fuel rods from overheating, and also creates the steam for the turbines. There are thousands of these fuel rods inside a reactor's innermost chamber, which is called the pressure vessel. Pellets of uranium fuel are contained in long, narrow fuel rods made of an alloy of zirconium. The energy from the fission reaction is used to boil water into steam, which drives turbines to produce electricity. When a large enough quantity of uranium fuel is gathered together it starts a self-sustaining chain reaction, in which emitted neutrons smack into other uranium atoms and cause them to split in turn. In most of Fukushima Dai-1's reactors, the radioactive element uranium is the source of the nuclear fission reaction: when one atom of the uranium isotope U-235 breaks down into smaller parts, it produces both energy and neutrons. Let's start at the heart of a boiling water reactor where the nuclear fuel dwells. At the time of the earthquake, three reactors were active and three were down for routine maintenance. Let's step back from the news cycle for a moment, though, and look at both how the Fukushima Dai-1 plant is supposed to work, and what went wrong following the earthquake on Friday March 11.įukushima Dai-1 uses six boiling water reactors to produce electricity for TEPCO. On April 17, TEPCO finally announced a " roadmap to restoration," which the company said would stabilize the plant and end most radioactive emissions within nine months. (TEPCO) confirmed the obvious, and announced that at least four of the plant's reactors would never go back into service. In the third week, further discoveries of highly radioactive water outside the reactor buildings showed that the contamination was spreading. But an accident that exposed three workers to radioactive water highlighted the continued danger, and made it clear that the process of stabilizing and cleaning up the plant will be a long slog. The second week showed signs of progress, as the plant operators worked to reconnect the plant to the electricity grid and get cooling systems working again. 4, which was shut down at the time of the earthquake, raised a new set of concerns regarding spent nuclear fuel.Īs the first week of the nuclear crisis came to a close, the spent fuel had become the primary safety concern. Fires at the building housing reactor No. 3 in the days following the March 11 earthquake and tsunami, and troubling problems next arose in reactor building No. Explosions first occurred in the buildings housing reactors No. The eyes of the world have been riveted on Japan's Fukushima Dai-1 nuclear power plant and its workers' desperate efforts to stabilize the nuclear reactors. This explainer was last updated on May 13. Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. ![]()
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