2011 has been privy to one of the biggest disasters ever to occur in a millennium. On March 11, 2011, an unexpected earthquake with a magnitude of 9 occurred off the coast of Japan. The earthquake released enough energy to light up every light bulb in Los Angeles for one year and it lasted for approximately 6 minutes. According to the data released by the US Geological Survey, the overall destructive force of the earthquake was quite large and a similar earthquake in Japan hadn't happened for the last 1000 years. In fact, the destructive force was so large; that it moved the main island of Japan closer to mainland USA by 18 feet. More interestingly, the whole Earth axis shifted by 25 feet and due to change in the mass distribution of Earth, the days became shorter by 1.8 microseconds.
The corresponding Tsunami that came approximately 45 minutes after the Earthquake was also a shattering event, which in itself caused more destruction as compared to the earthquake. The combined destructive force of the earthquake and the tsunami was 1 million times greater than the overall destructive power of the atom bombs thrown in Hiroshima and Nagasaki in the Second World War. Even though the Japanese are very meticulous people and even though they have the best earthquake detection and emergency management systems in the world; still they were unable to cope with the tragedy successfully. The sheer magnitude of the tragedy caused over 18,000 people to die and 15,000 people to be unaccounted for.
One of the first installations to shut down as soon as the early warning system kicked in was the Nuclear power plant at Fukushima. As soon as the earthquake was detected, approximately 49 seconds before the actual event, the emergency response system at the Fukushima Power Plant activated the emergency scram rods which slammed into place. This emergency action stopped the fission reaction taking place in the Fukushima reactor cores in the order of seconds. In essence, the Fukushima Power Plant is comprised of 6 nuclear reactor cores with each one based on the General Electric Light bulb design that works by having pressurized boiling water in the core itself. As the fission reaction takes place, the heat that is generated is used to boil the water and it is kept under high pressure. The water acts as the moderator and the coolant, and it constantly circulates while passing off its energy to the coolant which eventually turns into steam. This steam then passes through a turbine, creating the electricity that our whole world needs. In essence, even though the fission reaction in the core is what drives the whole process, we can think of the coolant circulation as the most vital part of the nuclear energy production. It is essential that as long as heat generation is taking place, the coolant must constantly circulate, so that the reactor core temperature can stay below a critical limit.
When a reactor stops its operation, the fissile material in the reactor core will continue to generate heat due to decay heat of the isotopes and the byproducts. When the fission reaction stops, this decay heat will also dissipate in approximately 10 days in order to reduce to manageable levels. Hence, it is essential for the coolant to circulate around the core for these 10 days, even though the reaction has already stopped.
Thus, the incident at the Fukushima Power Plant actually happened due to the fact that this coolant circulation had stopped because of the earthquake. The main power grid that supplies the electricity to the pumps went offline and this caused the coolant circulation to cease. Since, the reactor had already stopped; the power plant was also not generating any electricity. The GE designed reactor had secondary backup systems with diesel pumps that kept the circulation going. However, when the tsunami came and reached the Fukushima power plant, the secondary systems also went offline, as the pumps went under water. Thus, the third system of pumps kicked in, but that also failed after a short time again due to the tsunami. The last line of defense was the battery operated pumps. However, they were only designed to function for 8 hours and as a result, they stopped working after 8 hours were up and the circulation again stopped. At this time, it was a frantic race for a solution, as the decay heat started to rise immediately and it came to levels that are too high for the nuclear reactor core. The zirconium cladding is designed to withstand up to 1200 Celsius, so something needed to be done as soon as possible, before a possible meltdown of the fuel rods occurred. Unfortunately, due to the earthquake and the tsunami, all the roads were blocked and emergency fire vehicles could not come and bring the much needed water, as well as portable generators that could be used to power up the coolant circulation.
Hence, inevitably the steam pressure building up became too strong and in order to stop a large steam explosion, some steam was vented into the atmosphere. In essence, this was a controlled venting to stop an even bigger incident from occurring. Unfortunately, during the venting process, some oxygen from outside got mixed in the containment chamber and due to high temperatures; some of the water in the core was electrolyzed into hydrogen. Hence, the high pressures and the high temperatures caused the hydrogen to ignite, which caused the explosion event damaging the exterior containment chamber.
Luckily, the steel dome that surrounds the reactor core remained intact and a major incident was averted. Thus, the chief engineer of the plant gave the only possible order of flooding the reactor core with seawater and circulating it with natural means, as well as with the help of mobile generators which had arrived. The flooding of the reactor core was the correct solution, as sea water caused the reactor to start cooling once again. Unfortunately due to seepage in the ground basin of the reactor core, some fissile material was released into the sea as well.
However, it must be understood that the natural disasters that occurred were unavoidable. Nothing comparable had happened in the last thousand years and the reactor design of Fukushima was robust and sturdy, but it was also old with a design from the 70's. Hence, it could not cope up with the damaging effects of both an earthquake and a tsunami. Moreover, the various redundant coolant systems all went offline, which was something unimaginable for the designers of the reactor system. Luckily, no one has died of radiation and it is estimated that the Fukushima incident will not have an everlasting effect on the environment contrary to the media opinion. The Fukushima incident was a combination of natural disasters with some human error thrown in, as the sea water could have been pumped earlier to stop the incident right away. However, due to concern for the reactor's operation (since seawater will damage the core due to its corrosive properties and the reactor core will become inoperable for over a year); this was not implemented until the chief engineer decided that security concern was a bigger issue.
However, the fact of the matter is that the incident at Fukushima has proved that nuclear reactors are very resistant and sturdy structures and if proper safety procedures are followed, all nuclear incidents can be controlled without any casualties. Of course, this incident will help reactors operators to shape up, so that better and more rigorous safety procedures can be devised and implemented. In time with newer and inherently safe reactor designs, the human factor can be eliminated and our world can have enough electricity through nuclear reactors.
Dr. Guven is an Aerospace Engineer as well as a Nuclear Engineer. He has worked as an academician, researcher and teacher in various academic institutions with over 15 years of experience. Moreover, he has worked in the aerospace and nuclear industry as a consultant for solving technical problems as well as for providing guidance and training to young engineers. His lectures and seminars are downloaded by students, teachers and industry professionals all across the world free of charge. Fukushima Tribulations: What happened
