Monday, October 01, 2012
Fukushima Revisited: Lessons Learned
On Mar. 11, 2011, a huge earthquake and tsunami struck Japan, killing thousands of people outright and flooding large areas of the northeastern coastline of the country. But perhaps the most significant legacy of the disaster will arise from what happened at the Fukushima nuclear plant, which was situated in the direct path of the tsunami.
As we mentioned in a blog two days after the disaster, no nuclear plant in history had been subjected to an 8.9-magnitude earthquake before. But all of the six reactors at the plant may have sustained the shock without serious initial damage. As the earthquake struck, automatic shutdown procedures were followed and after the earthquake, the operating reactors were still under control. The problems came with the tsunami, which flooded the lowest level of the plant.
At this point, we turn to the conclusions of two special commissions charged with investigating the accident. Both issued their conclusions just this last July of 2012. One commission was the first of its kind in the entire sixty-six-year history of Japan’s constitutional government. After interviewing hundreds of witnesses and conducting over a thousand hours of interviews, the commissions had harsh words to say about Tokyo Electric Power Company (TEPCO), government officials, and the sadly lacking state of emergency preparedness showed by those charged with the safety of nuclear power generally in Japan.
One problem that could have been avoided concerned the location of the emergency generators that kept cooling pumps operating during cooldown. Turning off a large nuclear reactor is not like just flipping a switch. They operate by heating large volumes of water, metal, and fuel to many hundreds of degrees, and even if the nuclear reaction is stopped almost instantly by some means such as the insertion of neutron-absorbing control rods, the laws of physics say that all that heat has to go somewhere. And the usual place it goes is into the cooling fluid that is circulated through the reactor to remove the heat to boilers to generate electricity.
In the case of a shutdown, the heat can be simply dissipated in cooling towers or other rapid means, but first it has to be extracted by the cooling fluid flowing through the reactor. In an emergency, this fluid has to be pumped even faster than normal, and only mechanical pumps will do the job in the type of reactor used at Fukushima. With the loss of electric power from outside due to the earthquake and from the plant’s own generators due to the shutdown, the pumps had to be powered by emergency generators that were operating from diesel engines. The big problem was, all these emergency generators were in the basement—where the floodwaters rose and stopped them cold.
From that point on, the situation just got worse. With no cooling fluid flowing, the three reactors operating at the time of the earthquake overheated and produced hydrogen from the reaction of water with hot metal inside, and eventually the hydrogen exploded. This was a chemical, not a nuclear, explosion, but it broke open the plant’s housing enough to release a lot of radioactive trash from the wrecked reactors inside—about a tenth of what was released during the much more serious accident at Chernobyl, Ukraine in 1986. But enough radioactive material was released at Fukushima to affect the lives of those who lived near the plant for many years.
The fact that the emergency generators were in a vulnerable position where floodwaters could stop them is only one of a number of design flaws that contributed to the magnitude of the disaster. Higher dikes around the plant site could have conceivably prevented flooding in the first place. Following a call for increased safety measures at nuclear plants in 2006, TEPCO apparently did little or nothing. According to the National Diet report, the firm relied on its close connections with Japanese regulators to avoid taking any substantial actions to improve safety. The reports also faulted government officials for not planning for evacuations of the scale that turned out to be needed. The Fukushima disaster has also given ammunition for groups agitating for the end of nuclear power altogether, and several countries such as Germany have either slowed or stopped their plans for future nuclear plants.
Admittedly, the earthquake and tsunami that led to the Fukushima disaster were at the outer limits of what any reasonable design would take into account. But clearly, some fairly simple measures that might have made routine operations a little less convenient would have reduced or eliminated altogether the tragic events that led to the death or injury of numerous plant workers, the release of radiation that contaminated land for miles around the plant, the bad publicity that nuclear power received, and the total loss of billions of dollars’ worth of machinery and equipment.
One hopes that every nuclear engineer, in school and out, will make a special study of Fukushima in order to use the lessons learned from what went wrong there. With the release of the disaster reports (and, hopefully, their translation into other languages including English), the nuclear industry has been presented with a treasure trove of mostly bad examples of how not to do it. As engineer and writer Henry Petroski likes to point out, engineers often learn more from failure than from success, and Fukushima has presented us with an abundance of learning opportunities. In view of concerns over climate change, the availability of fossil fuels, and the promise of conservation technologies such as smart-grid approaches to power distribution, it would be a shame if we back away from a form of energy that could provide non-fossil power for many decades to come.
Sources: I relied upon the Wikipedia summaries of the commission reports under the headings of “Fukushima Daiichi nuclear disaster” and “National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission.”