- Following the 9.0 earthquake, each of the reactors was shut down, halting the fission reaction
- Backup power from diesel generators came online to operate coolant pumps to remove decay heat from the reactor until these were wiped out by the incoming tsunami;
- Battery backup power came online to operate these pumps until such power was depleted
- Water inside the reactors began to boil away as it heated up.
- As water boiled away, the fuel rods heated up.
- Water interacted with the extremely hot zirconium cladding, producing free hydrogen (which lead to the hydrogen explosions)
- As the fuel rods heated up, the cladding failed, releasing radioactive fission product gasses. The fuel is also believed to have partially melted when it became uncovered by water, releasing more radioactive materials from the core.
- Cooling was restored by injecting seawater into the reactors to quench the decay heat from the rods and prevent further melting.
- March 11, 2:46 PM: External power lost, emergency diesel generators begin to supply power
- March 11, 2:52 PM: Emergency cooling systems (isolation condenser) started
- March 11, 3:37 PM: All AC power lost
- March 11, around 5:00 PM: Fuel exposed, core melt begins
- March 12, 5:46 A.M.: Begin of freshwater injection from fire extinguishing line
- March 11, 2:47 PM: External power lost, backup diesel generators start up
- March 11, 2:50 PM: Emergency cooling system (Reactor Core Isolation Cooling system - RCIC) starts up
- March 11, 3:11 PM: All AC power lost
- March 14, 1:25 PM: RCIC operation stops
- March 14, around 6:00 PM: Fuel exposed, core melt begins.
- March 14, 7:54 PM: Seawater injection from a fire extinguishing line begins
- March 11, 2:47 PM: Loss of external power, start-up of emergency diesel generators
- March 11, 3:05 PM: Startup of emergency cooling system (RCIC)
- March 11, 3:41 PM: Loss of all AC power
- March 12, 11:36 AM: RCIC stops due to loss of power
- March 12, 12:35 PM: Startup of HPCI (high pressure core injection) system as a backup cooling measure
- March 13, 2:42 AM: Stop of HPCI
- March 13, around 8:00 AM: Fuel exposed, core melt begins
- March 13, 9:25 AM: Startup of freshwater injection into core from a fire extinguisher line
|Fuel stored in the spent fuel pool at Fukushima Dai-ichi Unit 4. (Image credit: IAEA)|
However, the drop in the water level which occurred in Unit 4 is still an unknown (due to the lack of instrument measurements inside the building from the power outage) and difficult to determine; experts are currently attempting to perform back-calculations to estimate this.
|(Left): Standby pipe from Units 3 and 4; (right) Enlarged view of junction (Image credit: IAEA)|
|Map of air dose plume release from Fukushima. Red: 9-91 microSv/hr, Orange: 9.5-19 microSv/hr, Yellow: 3.8-9.5 microSv/hr, Green: 1.9-3.8 microSv/hr, Blue: 1.0-1.9 microSv/hr, Indigo: < 1.0 microSv/hr (Image credit: IAEA)|
|Evacuation areas around Fukushima. (Image credit: IAEA)|
|INES accident classification scale (Image courtesy of IAEA)|
Much of what will occur in the aftermath of Fukushima will be to determine how nuclear operators can plan for and respond better to similar circumstances in the future. Much of the problems encountered at Fukushima came from the total loss of power at the facilities, followed by the destruction of the backup diesel generator units in the tsunami (a condition known as "station blackout," the worst possible circumstance which can occur at a plant).
Thus, many of the IAEA's recommendations fall under these categories. In addition to recommending that operators and plant designers harden facilities against earthquakes and floods, a key area of focus will be in ensuring that reliable backup supplies of power can be maintained in order to operate cooling pumps following an unexpected plant shutdown. This includes hardening backup generators (placing them out of reach of flood waters) and making dedicate contingency plans such as mobile power vehicles to supply power in the case of a generator failure.
Other factors include features such as means to mitigate hydrogen production in circumstances such as this, through the installation of hydrogen "recombiners" (which steadily burn hydrogen rather than allow it to accumulate) and dedicated "blow-out" panels to allow for emergency venting of hydrogen from the outer containment buildings. It was noted that over 300 metric tons of hydrogen was produced in the three reactor units (in other words, over 300,000 kilograms), thus leading to the rather dramatic hydrogen explosions witnessed.
Other recommendations focus upon measures for emergency response, such as procedures for handling nuclear emergencies. A particular complication in the case of Japan was in the widespread devastation beyond the scope of the plants themselves; many workers at the plant were uncertain about the safety of their homes and families, leading to confusion and initial difficulties in maintaining sufficient staff levels to respond to the emergency at the plants.
|Proposed safety systems upgrades (click for larger version; image credit: IAEA)|
Finally, the IAEA recommends regulatory reforms such as instilling a more thorough safety culture and reinforcing the regulatory safety infrastructure by fostering a greater separation of regulators and utilities, which have been frequently criticized for enjoying too cozy of a relationship in Japan. An additional criticism made was in that preservation of economic assets (e.g., the reactors) was potentially prioritized above safety in the early stages of the response, delaying such measures such as seawater injection (an emergency measure to cool the core which almost certainly would mean a total economic loss for the reactor unit).
Nuclear safety is a constantly-evolving process; the events at Fukushima will undoubtedly be studied by engineers and scientists for years to come in order to develop better safety features and response techniques to ensure maximum possible safety to the public. To some degree however, there comes a point where one plans against the essentially unpredictable (for example, a record earthquake and tsunami); thus, not every measure can simply be preventative in nature, but rather in how to respond to and mitigate such events. (Examples of this include hardening backup generators and designing robust containment buildings to prevent radioactive releases.)
It likewise is useful to place events like this in context; while it is clear that Fukushima was an extremely serious event in hindsight (it would now appear to be far more serious than Three Mile Island, although less serious than Chernobyl), the consequences must be compared to the risks encountered by the available alternatives. For example, one consequence of the earthquake was the destruction of a hydroelectric dam, destroying over a thousand homes. Liquefied natural gas facilities were also destroyed in the earthquake, leading to further casualties. By comparison, the only person who has died as a result of the Fukushima disaster was not even killed by a radioactive release, but rather was killed by a falling crane.
Risk is an unfortunate fact of life, and in particular of energy production. This applies to all forms of energy production, not just nuclear - each form of energy production comes with equivalent risks and trade-offs, including economics, pollution (for example, the radioactive releases from coal plants in smokestacks are far higher than that of nuclear plants), land utilization (diffuse energy sources such as solar and wind consume enormous land footprints), etc. There is, alas, no "free lunch" when it comes to energy. The best that we can hope for is to understand and minimize risks by constantly striving for better and safer designs and better ways of responding to accidents.
In particular, it is worth noting that the Fukushima plants were of a prior vintage - built in the 1960's and nearing retirement. Newer plants built at Fukushima Dai-ini did not suffer nearly the same consequences as those at Dai-ichi, in part due to enhanced safety features within the designs. Likewise, plant designs being proposed today take advantage of decades of engineering experience, including advances in technology (particularly in advanced computing - perhaps the most amazing thing about much of the existing nuclear fleet is that it was designed with pencils and slide-rules...). New designs such as small modular reactors promise further enhancements in safety.
Ultimately, safety will be an ongoing challenge, one not entirely technical in nature. After Three Mile Island, one of the most radical changes to the U.S. nuclear industry was a focus on human factors - particularly looking at aspects such as instruments in the control room, operator training, and instilling a safety culture. Similar lessons may ultimately have to be internalized in the case of Japan.
It has often been said, "Experience is the harshest teacher, and her lessons the most expensive." For the Japanese, no doubt this case is little different.
A special thanks to Randy Beatty and the IAEA for both presenting and making these resources available to me.