A Radioactive Situation

Japan's earthquake could shake public trust in the safety of nuclear power.

BY CHARLES D. FERGUSON

Is nuclear power too risky in earthquake-prone countries such as Japan? On March 11, a massive 8.9-magnitude earthquake shook Japan and caused widespread damage especially in the northeastern region of Honshu, the largest Japanese island. Nuclear power plants throughout that region automatically shut down when the plants' seismometers registered ground accelerations above safety thresholds.

But all the shutdowns did not go perfectly. Reactor unit 1 at the Fukushima Daiichi Nuclear Power Station experienced a mechanical failure in the emergency safety system. In response, officials ordered the evacuation of residents who live within two miles of the plant. Also, people living between two to 10 miles were ordered to stay indoors. The Japanese government described this order as a precautionary measure.

A worst-case accident would release substantial amounts of radioactive materials into the environment. This is unlikely to happen, but is still possible. Modern commercial nuclear power plants like the Fukushima plant use defense-in-depth safety measures. The first line of defense is fuel cladding that provides a barrier to release of highly radioactive fission products. Because these materials generate a substantial amount of heat, coolant is essential. Thus, the next lines of defense are to ensure that enough cooling water is available. The reactor coolant pumps are designed to keep water flowing through the hot core. But loss of electric power to the pumps will stop this flow. Backup electric power sources such as off-site power and on-site emergency diesel generators offer another layer of defense.

Unfortunately, these emergency power sources were knocked out about one hour after the plant shut down. Although it is unclear from the reporting to date, this power outage appears to have occurred at about the same time that a huge tsunami, triggered by the earthquake, hit that part of Japan.

Sustained loss of electric power could result in the core overheating and the fuel melting. However, three other backup systems provide additional layers of defense. First, the plant has batteries to supply power for about four hours. Second, the emergency core cooling system can inject water into the core. Finally, the containment structure, made of strong reinforced concrete, surrounds the reactor and can under even the most severe conditions prevent radioactive materials from entering the environment.

But the earthquake -- the largest in the 140 years of recorded history of Japanese earthquakes -- might have caused some damage to the containment structure. Japanese authorities announced that they will vent some steam from the containment structure to reduce the pressure buildup. This action may release small amounts of radioactive gas. The authorities do not expect any threat to the public.

Although a meltdown will most likely not occur, this incident will surely result in significant financial harm and potential loss of public confidence. For example, it was less than four years ago, in July 2007, when the Kashiwazaki-Kariwa Nuclear Power Plant, Japan's largest, suffered shaking beyond its design basis acceleration. The plant's seven reactors were shut down for 21 months while authorities carefully investigated the extent of the damage. Fortunately, public safety was not harmed and the plant experienced no major damage. However, the government accepted responsibility for approving construction of the first reactor near a geological fault line, which was unknown at the time of construction. The biggest loss was financial. In particular, the fiscal year 2007 loss was estimated at $5.62 billion with about three-fourths of that to replace the 8,000 megawatts of generating capacity from the nuclear plant.

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