Chapter 37: Nuclear Energy: Safety – Accidents – Fukushima Daiichi – Edited by Dr. Mir F. Ali
The Great East Japan Earthquake of magnitude 9.0, considered to be one of the largest earthquakes in the world, occurred on the east coast of northern Japan at 2.46 pm on Friday, March 11, 2011. It generated a series of huge tsunami which struck the east coast of Japan, the highest being 38.9m at Aneyoshi, Miyako with the duration of 3 minutes. Japan moved a few metres east and the local coastline subsided half a metre. The tsunami inundated about 560 sq km and resulted in a human death toll of over 19,000 and much damage to coastal ports and towns with over a million buildings destroyed or partly collapsed. Electricity, gas and water supplies, telecommunications, and railway service were all severely disrupted and in many cases completely shut down. These disruptions severely affected the Fukushima Daiichi nuclear power plant.
Following the earthquake on Friday afternoon, the nuclear power plants at the Fukushima Daiichi, Fukushima Daini, Higashidori, Onagawa, and Tokai Daini nuclear power stations (NPSs) were affected, and emergency systems were activated. The tsunami caused a loss of all onsite and offsite power at the Fukushima Daiichi NPS, leaving it without any emergency power. The subsequent tsunami caused significant damage to at least four of the six units of the Fukushima Daiichi nuclear power station. The resultant damage to fuel, reactor, and containment caused a release of radioactive materials to the region surrounding the NPS. In addition to triggering the tsunami, this earthquake caused thousands of deaths and economic losses approaching $500 billion (USD). Yet, despite the sheer scale of destruction in northeastern Japan, the accident at the Fukushima Daiichi nuclear power station (NPS) has come to define the tragedy for many and has become a momentous event in nuclear power technology.
Of the more than 400 NPPs currently operating throughout the world, accumulating ~16,000 years of reactor experience, >90 percent are light water reactors (LWRs), which produce heat by controlled nuclear fission and are cooled by water. In the United States, all 104 operating NPPs are LWR NPPs. There are two general LWR designs: boiling water reactors (BWRs) and pressurized water reactors (PWRs). In BWRs, the heat generated by fission turns the water into steam, which directly drives the power-generating turbines and the electrical generator connected to them. In PWRs, the heat generated by fission is transferred to a secondary loop via a heat exchanger (steam generator), where the steam is produced and drives the power-generating turbines. In both BWRs and PWRs, after flowing through the turbines, the steam turns back into water in the condenser. The water required to cool the condenser is taken from and returned to a nearby ocean, river, or water supply.
The Japanese NPPs involved in the Fukushima Daiichi accident were BWR NPPs. The following graph illustrates the configuration of a BWR:
In a BWR NPP, the nuclear reactions take place in the nuclear reactor core, which mainly consists of nuclear fuel and control elements. The nuclear fuel rods (each ~10 mm in diameter and 3.7 m in length) are grouped by the hundred into bundles called fuel assemblies power after a few hours, water must be circulated within the RPV to maintain adequate cooling. This cooling is provided by numerous systems. Some systems operate during normal conditions, and some systems, such as the emergency core cooling systems (ECCSs), respond to off -normal events. Normal reactor cooling systems maintain the RPV and temperature and a proper cooling water level, or if that is not possible, ECCSs directly flood the core with more water.
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