|Fig. 1: International nuclear and radiological event scale.  (Source: G. Zerkalov)|
The developing world relies heavily on energy. In 2014 total world energy consumption increased by 0.9% to reach 12928.4 million tons of oil equivalent. The production on nuclear energy dropped by 10% in 2011 and 2012 after the catastrophe in Fukushima on March 11, 2011. Nonetheless, the increased production in South Korea, China and France helped to restore the demand for nuclear energy. Thus, global nuclear output grew by 1.8% in 2014 to reach 574 million tons of oil equivalent, which is 4.4% of total energy consumed in the world in 2014.  The Fukushima nuclear accident was the second largest after the accident in Chernobyl (Soviet Union) on April 26, 1986. The International Atomic Agency developed the International Nuclear and Radiological Event Scale, which uses a logarithmic scale to represent the severity of different nuclear accidents. Both accidents were ranked at 7, which corresponds to the most severe event on the scale (Fig. 1).  The accidents had not only short-term but also severe long-term effects and raised questions regarding the reliability and safety of nuclear plants. This work provides a comparative analysis of the two nuclear disasters.
The Chernobyl disaster occurred in the city of Pripyat on April 25, 1986. During the course of the accident large quantities of fission products were released in the biosphere in addition to the initial heat release, which killed onsite workers. The contamination caused by the emission of the radioactive materials seriously affected the surrounding environment and people who lived in the region. The accident resulted from weaknesses in the reactor design and a number of operator errors that violated the safety procedure of the plant. The reactor had flaws in the design and had to be modernized to avoid serious consequences. The accident happened during an experiment, which should have introduced a new way to generate power sufficient to run the cooling pump during electric blackouts. During the experiment many of the safety signals and valves were turned off. When the experiment failed, there has not been a way to keep the cooling pump running, which caused overheating of the reactant, explosion of its core, emission of large quantities of radioactive materials and ignition of the combustible graphite moderator.  The latter promoted the dispersion of radioactive particles by the means of smoke, which was not confined due to the absence of any containment vessel.
|Fig. 2: Photo of the Fukushima nuclear power plant after the accident. (Source: Wikimedia Commons)|
The Fukushima disaster happened at Fukushima Daiichi Nuclear Power plant on March 11, 2011. The main cause of the accident was the loss of the supply of power to the cooling pumps which caused the meltdown of reactors. The power blackout was caused by a strong earthquake (magnitude 9) followed by a series of tsunamis. Shortly after the earthquake, operating reactors 4, 5, and 6 ceased to operate, thus terminating the production of electricity required for their cooling. The diesel generators started to provide power to the cooling systems and operated until the tsunami hit the rooms where they were located, causing them to fail. The secondary emergency pumps run by backup electric batteries stopped working a day after the tsunami hit. The situation gravely worsened when hydrogen-air chemical explosions occurred. The hydrogen was produced from the zirconium fuel cladding-water reaction. Large amounts of radioactive materials spread around with the help of wind, affecting human and livestock industry in the surrounding regions. As seen in Fig. 2, three units suffered severe damage from the explosions.
Plant commissioning date and years in operation: The Chernobyl plant was commissioned in 1977 and by the day of the accident was operating for only 9 years. The Fukushima plant was commissioned earlier in 1971 and operating for 40 years before the accident.
Plant outputs: The Chernobyl plant was producing 3.7 gigawatts while the Fukushima produced 4.4 gigawatts.
Reactor types, number of reactors and amounts of fuel: The Chernobyl plant was using four RBMK-1000 graphite moderated second generation reactors without containment. Only one of them was involved in the accident and contained approximately 190 tons of nuclear fuel - uranium dioxide.  In contrast, Fukushima used six boiling water reactors with containment vessels, four of which were involved in the accident. The total amount of fuel on the plant was 4277.
Causes of the accidents: In Chernobyl, the main accident cause was attributed to human error and violation of safety procedures. The design of the reactor did not contain adequate cooling system as well as was unstable at low power. Furthermore the absence of containment vessel worsened the situation as after the reactor overheat, steam explosion and core melt, the raging graphite fire started, which facilitated the spread of highly detrimental radioactive materials. In Fukushima, the main issue lied in the faulty design of the plant, particularly, the height of the tsunami preventive wall which was only 10 meters high and could not cope with a 15 meter tsunami after the strong earthquake.  Furthermore, the location of the backup diesel generators was poorly chosen. They were located very close to the sea level and after the earthquake destroyed the power lines, tsunami destroyed the diesel generators. Without power for cooling, the temperatures within the reactors rose until is cause meltdowns and accumulation of hydrogen gas with its consequent explosion. This led to the destruction of reactor containment and large dispersion of radioactive materials.
Area affected and radioactivity release: In Chernobyl disaster there has been 5200 PBq released.  It has been estimated that the area within 310 mi has been contaminated,. This forced about 335,000 people relocate. After the accident more than 6000 thyroid cancer cases in children have been registered, nine of which died.  Direct fatalities included 49 people as of 2005.  In Fukushima there has been approximately 770 PBq released into the atmosphere, but the radiation release continues into the Pacific ocean via groundwater. It is estimated that the area within 30 mi has been affected, causing the relocation of 80000 people. Direct fatalities included three people and long term effects have not yet been knows. 
Current Status: The Chernobyl plant was completely shut down in 2000. The damaged reactor is currently covered by the confinement structure made of steel and concrete called the sarcophagus. It is planned to finish a new safe confinement structure by 2017 and then clean up and decommission the plant. In Fukushima, decommissioning will most likely take 30 to 40 years. Cold shutdown was performed by December 16, 2011 and all fuel rods were removed from damaged reactor.
Despite several detrimental catastrophes that happened in the past on nuclear power plants the world keeps relying on nuclear energy and increases capacities of it production. The comprehensive analysis present in this work addresses causes and consequences of the nuclear accidents in Chernobyl and Fukushima. It is important to thoroughly analyze the lessons learned and implement changes to other currently used power plants to increase their safety and minimize the risk of future catastrophes.
© Georgy Zerkalov. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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