|Fig. 1: Photo of the Fukushima Daiichi power plant after the earthquake, tsunami, and explosions. (Source: Wikimedia Commons)|
Chernobyl, Three-Mile Island, and now Fukushima. These names conjure images of mushroom clouds, the yellow and black radioactivity symbol, government clean-up crews in all white bio suits with gas masks on, and many other terrible things. But, what actually happens during a nuclear accident? Are they all the same? What exactly is radiation, how can it hurt me, and how is it "released" during accidents? These are the questions I set out to answer for this report. I will use Fukushima as a case study, but will also attempt to point out what is general to most light water reactors (the type we have in the U.S.) and what might have happened at Fukushima had the worst come to pass.
Note to the reader, this report is an attempt to explain what happens during a nuclear accident using Fukushima as a case study. However, I was also interested in determining what went right during the accident, what went wrong, and what may have been preventable versus what may just be inherent risks with nuclear energy. While there is no definite answer to these questions, keep them in mind as we explore the different explanations (from different groups, with possibly different motivations!) for what happened.
Let's first take a look at what we know happened at Fukushima. On March, 11th 2011 at 2:46pm (JST) what is now referred to as the Great East Japan Earthquake occurred off the coast of Japan. This earthquake was a magnitude of 9.0, the 5th largest recorded earthquake in history with over 1000 recorded aftershocks, 80 over magnitude 6 and with many immediately following the earthquake reaching magnitudes in excess of 7. In addition, tsunami's reaching heights of up to 40.5 meters (133 feet) struck Japan. A terrible disaster, and on top of all of that, the earthquake and tsunami caused an accident at the Fukushima Daiichi Nuclear Power Plant.
The exact chain of events that occurred at Fukushima on March 11th and the following days is not known precisely, but based on reports from different sources, eye witness accounts, and the history of previous nuclear accidents, a probably timeline can be constructed. Obviously, the timeline reported here may not be 100% accurate, but is most likely close to the facts and still useful to determine what the causes of accidents are, what threats they pose, and different ways they can be handled or prevented.
The earthquake caused the loss of AC power to the plant because there was a transformer station that was not earthquake resistant and the backup transmission line failed due to an equipment mismatch.  The emergency diesel generators (EDGs) were powered up and the control rods for reactors 1-4 When the earthquake struck, only Reactors 1 through 3 were operating and were "scrammed" or shut down by inserting the control rods that stop the fission reactions from occurring. In addition to knocking out external power, the Japanese National Diet (Japan's legislature) believe that the earthquake could have also caused a minor coolant leak in Unit 1. 
The tsunami that hit the Fukushima Daiichi plant is estimated to have been between 10 to 14 meters high, although the measuring devices in the area couldn't measure above 5 m. [2,3] The tsunami was so powerful that the impact on Units 2, 3, and 5 exceeded the designed for seismic ground motion standard.
The tsunami destroyed 11 of the 12 operating EDGs, knocking out power to all Units except the non-operating Unit 6. The seawater pumps and motors were also totally destroyed, causing the ultimate heat sink for the reactors to be lost. Some of the backup DC batteries were also flooded while others remained until they drained. [1,3] The tsunami threw trucks, heavy machinery, and other equipment into buildings and scattered debris all over the site, making it extremely difficult for the operators to respond to the disaster.
At this point there was almost no power and the reactors had lost their cooling water. What happens to reactors with no power and no cooling water and what was done to keep the worst from occurring?
A nuclear reactor is basically a bunch of fuel rods made of a mixture of fissile uranium (the fuel) and metals for support. The fuel fissions (splits) and releases neutrons and a lot of energy. The energy ends up as heat transfer to the fuel rods, making them very hot and the neutron goes on to cause another fission (hence, the idea of a nuclear chain reaction). When all of the Fukushima reactors were "scrammed", control rods were inserted into the reactors which absorb all of the neutrons and halt the nuclear chain reaction. [1,2] However, the fuel rods were already very hot and the nuclear decay products (commonly called nuclear waste) continued to decay very rapidly and release more energy, heating the fuel rods up further. This is why nuclear waste has to be put in pools for about 10 years, to make sure they are constantly cooled while the short lived decay products die out.
So, since Fukushima's power was out and had lost access to its cooling water pumps, the plant now had an imminent threat of the fuel rods overheating. Fuel rods can essentially heat up to an unbounded temperature unless they are cooled. So, without any cooling water the fuel rods would start heating up until they melt, after which point they will start to fall to the base of the reactor. At this point, the fuel could cause the remaining water at the bottom of the reactor to rapidly boil, increasing the pressure in the reactor and potentially leading to an explosion. If there is no water or sufficient cooling isn' happening at the base of the reactor, the fuel could continue to heat up and melt through the floor of the reactor into whatever secondary containment the power plant has.
Finally, if the fuel rods become hot enough, it is theorized that the zirconium in the fuel rod can react with the water (H2O) to produce hydrogen and oxygen.  If this happens, it opens up the possibility for a hydrogen explosion. Any of these events (explosions or breach) are ways for radioactive material to leak into the environment, the primary concern of nuclear accidents other than a nuclear explosion (which is highly improbable given the design of a light water reactor).
After the tsunami hit, the only power left on site was the EDG powering the offline Reactor 6 and DC batteries powering some instrumentation, controls, and lights for Unit 3. The batteries would only last for 30 hours. There are isolation condensers (ICs) that cool reactors in the event of emergencies, which activated for Reactors 1, 2, and 3. However, the IC for Reactor 1 was manually shut down 11 minutes after it came online. TEPCO claims this was to prevent too rapid of a reactor temperature decrease, while Diet claims on-site operators shut it down to check for a suspected coolant leak because the reactor's pressure was falling rapidly. [1,2] The IC for Reactor 1 was cycled a few times before ultimately failing. The IC for Reactor 2 lasted for 3 days while Reactor 3's IC only lasted about 20 hours. In all cases, the next option to maintain cooling for the reactors is the High Pressure Coolant Injection (HPCI). The HPCI system started for Reactor 3 but failed after about 14 hours. 
Because cooling was lost to Reactor 1, the operators used a fire truck to inject high pressure water into the reactor. Because the reactor had become exceedingly hot, inject water into the reactor quickly lead to a buildup of high pressure steam in the reactor. Therefore, the operators had to vent the reactor. To activate the emergency relief valve, the operators had to use an engine driven air compressor and an engine-generator for AC power. At 2:30pm on 3/12, the pressure was confirmed to be decreased, but there was a hydrogen explosion 1 hour later.  At this point all cooling options ceased until 3.5 hours later a method of injecting seawater was established to contain further thermal runaway of the fuel rods.
Reactor 3 had a similar course of events. Fire pumps were used to inject water into the reactor with the operators using a car's battery as DC power supply and using a nitrogen tank to supply the compressed air needed to open the pressure relief valves. The pressure build-up was vented from the reactor to the containment vessel. At 11:01 am on 3/14, there was a hydrogen explosion in Reactor 3, two days after the first explosion. The explosion in Reactor 3 caused significant damage to the structure and likely a breach in containment as well as temporarily stopping the cooling of Reactor 3. 
The next day, March 15th, there was an explosion in Reactor 4. The IAEA says the hydrogen present in the Reactor 4 building was likely due to a backflow of hydrogen from Reactor 3 through vent lines.  The reason they believe this is that they believe the spent fuel pools appeared to be covered with water, precluding the generation of hydrogen. This is unconfirmed.
It is expected that the majority of fuel in Reactor 1 melted at only 5.3 hours after the tsunami and that there was a central molten pool of fuel after only 14.3 hours. At 15 hours, they believe the fuel had slumped to the bottom of the containment vessel. Reactor 2's fuel was thought to have formed a molten pool at 87 hours after the tsunami, and 1 week later they expected there was still a small molten pool surrounded by melted fuel. At 64 hours, the fuel in Reactor 3 was thought to be a smaller molten pool, but had not ever reached the bottom of the reactor. 
Unfortunately, while not covered in this report, the accident has been deemed a "man-made" accident by the National Diet of Japan. Claiming it was a "result of collusion between the government, the regulators, and TEPCO, and the lack of governance by said parties." While not investigated in this report, it is worth mentioning here and further consideration. 
While the events reported here likely do not 100% reflect the actual day, and the heroic actions of the operators acting in the dark with almost no instrumentation or equipment, in a disaster zone, cannot be praised enough, this examination of what likely happened, what the threats were, and what was done to avert them or not is an interesting story. It is clear that cooling the nuclear waste is of the utmost concern. If not cooled, there is threat of hydrogen generation or explosions and the threat of the fuel melting through the base of the containment vessel into the environment.
Unfortunately, in order to cool the reactors in the emergency conditions at the plant, water had to be injected and the steam vented to prevent further damage. Therefore, radioactive material was likely vented to the atmosphere in this venting process. Additionally, the difficulties of maintaining proper cooling in an environment without power, equipment, lights, or other necessary resources led to insufficient cooling and multiple hydrogen explosions, damaging buildings, breaching containment, and the release of radioactive materials.
The total amount of damage caused to the people and land of Japan are currently still unknown, but it is clear from this report that there were heroic efforts by on-site operators to contain the accident as best as possible, with people using anything available to them to cool the reactors and prevent further releases of radioactive material.
© John Fyffe. 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.
 "The Offical Report of the Fukushima Nuclear Accident Independent Investigation Commission: Executive Summary," National Diet of Japan, 2012.
 "Evaluation of the Situation of Cores and Containment Vessels of Fukushima Daiichi Nuclear Power Station Units-1 to 3 and Examination into Unsolved Issues in the Accident Progression," Tokyo Electric Power Company, Inc., Progress Report No. 1, 13 Dec 13.
 "IAEA International Fact Finding Expert Mission of the Fukushima Dai-ichi NPP Accident Following the Great East Japan Earthquake and Tsunami," International Atomic Energy Agency, 16 Jun 11.