A 2007 earthquake that spilled radioactive material into the Sea of Japan was a "wake-up call that reverberated around the globe," according to a report by the International Atomic Energy Agency.

About 20 percent of the world's nuclear reactors are in areas of significant seismic activity, according to the IAEA.

The agency set up an International Seismic Safety Centre after the 2007 quake at the Kashiwazaki-Kariwa nuclear plant in central Japan. Four reactors at the plant shut down automatically, but water that contained radioactive material was spilled, "though without an adverse effect on human health or the environment," the IAEA reported. The plant was about 11 miles from the epicenter of that quake, magnitude 6.6.

The tremors in that 2007 earthquake were two and one-half times the levels that the plant had been designed for, the IAEA found, but the reactor did withstand the quake. An inspection by IAEA found no significant damage to the plant.

"There has been a misconception since the early days of nuclear power," the IAEA reported, "that human error or mechanical failure, in other words risk factors within the plant itself, are the most significant variables regarding possible radiological release to the environment. In fact, the greatest threat to a plant´s operation may lie outside its walls. Nuclear power plants all over the world are exposed to natural hazards, such as hurricanes, floods, fires, tsunamis, volcanoes and earthquakes. With safety always a key concern, engineers, safety specialists and architects also have to take extreme natural forces into consideration."

In the U.S., the Nuclear Regulatory Commission says that all nuclear power plants are designed to take into account historical data on earthquakes and other dangers at each location, plus a "margin for error." A 2008 study by the NRC found that the risk of hazards from seismic activity had increased, but was "still small." A summary is here.

Nuclear industry groups say that nuclear reactors have proven they can withstand earthquakes. They point to the 2009 earthquake at Japan's Hamaoka plant, where two reactors shut down automatically without damage, and were restarted safely. Seismic regulations for new plants were strengthened in Japan after the 2007 quake. Industry makes these points:

• Japanese, and most other, nuclear plants are designed to withstand earthquakes, and in the event of major earth movement, to shut down safely.
• In 1995, the closest nuclear power plants, some 110 km north of Kobe, were unaffected by the severe Kobe-Osaka earthquake, but in 2004, 2005, 2007 and 2009 Japanese reactors shut down automatically due to ground acceleration exceeding their trip settings. 
• In 1999, three nuclear reactors shut down automatically during the devastating Taiwan earthquake, and were restarted two days later.

The anti-nuclear power group Beyond Nuclear put out a statement with the most dire forecast for the Fukushima Daiichi nuclear plant northeast of Tokyo: "Given the large quantity of irradiated nuclear fuel in the pool, the radioactivity release could be worse than the Chernobyl nuclear reactor catastrophe of 25 years ago."

A citizens group opposing nuclear power in Japan, Citizens' Nuclear Information Center, has its own arguments online.

After the 2007 quake the IAEA began testing a monitoring system for nuclear plants after earthquakes and tsunamis. Participating countries included the United States, Japan, India, Indonesia, South Korea, Pakistan and Turkey.

The Japanese utility company, TEPCO, is providing updates on the nuclear reactor. (Scroll down that page to "press releases.") At midday Eastern time, it reported, "Currently, there is a possibility of a release of radioactive materials due to decrease in reactor water level. Therefore, the national government has instructed evacuation for those local residents within 3km radius of the periphery and indoor standby for those local residents between 3km and 10km radius of the periphery."

Here are other resources on the topic of nuclear reactors and earthquakes. These open in a new browser window.

• The IAEA summary of its seismic efforts.
• The IAEA's International Seismic Safety Centre.
• IAEA conference and follow-up with lessons learned from the 2007 quake.
• The Nuclear Safety Review for 2009.
• Industry view from the World Nuclear Association.
• The latest on the Fukushima Daiichi nuclear plant from msnbc.com and NBC News.

The TEPCO nuclear plants have had other problems. This info comes from a June 2010 report from Nuclear Energy Insider:

In late June, the Tokyo government’s Nuclear and Industrial Safety Agency (NISA) announced that nearly half of Japan’s commercial reactors had problems that needed to be addressed and further inspections were deemed necessary.

NISA’s report noted that none of the reactors, "…had a problem that is not 'tolerable,”’ and that the majority of the country’s reactors got a passing grade.

But, the report did cast doubt over Japan’s nuclear safety record.

A particular problem seems to exist with the reactors operated by the Tokyo Electric Power Corporation (TEPCO) — 14 of their 17 reactors were considered to need additional inspections, with the No. 1 to 4 reactors of the Fukushima Prefecture No. 2 plant considered to have had ‘‘significant’’ problems following the mistaken discharge of radioactive materials into the sea through a drainage pipe that came to light in October 2009.

The Fukushima reactors have suffered a host of problems including in January 1989, when an impeller blade on one of the reactor coolant pumps in Unit 3 broke at a weld forcing a reactor shut down while in 2006 Fukushima’s Unit 1 was shuttered following leaking irradiated water.