UPDATE: Stratfor.com, the global intelligence site, is reporting that the Fukushima plant is in at least partial meltdown. The BBC reports that there's been a "huge" explosion at the plant.

This remains my main concern about nuclear power plants. People are always telling me how "safe" they are -- yes, they're safe. Until something unusual happens, like yesterday's massive earthquake:

TOKYO — Japan declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability in the aftermath of Friday's powerful earthquake. Thousands of residents were evacuated as workers struggled to get the reactors under control to prevent meltdowns.

Operators at the Fukushima Daiichi plant's Unit 1 scrambled ferociously to tamp down heat and pressure inside the reactor after the 8.9 magnitude quake and the tsunami that followed cut off electricity to the site and disabled emergency generators, knocking out the main cooling system.

Some 3,000 people within two miles (three kilometers) of the plant were urged to leave their homes, but the evacuation zone was more than tripled to 6.2 miles (10 kilometers) after authorities detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.

The government declared a state of emergency at the Daiichi unit — the first at a nuclear plant in Japan's history. But hours later, the Tokyo Electric Power Co., which operates the six-reactor Daiichi site, announced that it had lost cooling ability at a second reactor there and three units at its nearby Fukushima Daini site.

The government quickly declared states of emergency for those units, too, and thousands of residents near Fukushima Daini also were told to leave.

Japan's nuclear safety agency said the situation was most dire at Fukushima Daiichi's Unit 1, where pressure had risen to twice what is consider the normal level. The International Atomic Energy Agency said in a statement that diesel generators that normally would have kept cooling systems running at Fukushima Daiichi had been disabled by tsunami flooding.

Officials at the Daiichi facility began venting radioactive vapors from the unit to relieve pressure inside the reactor case. The loss of electricity had delayed that effort for several hours.

Plant workers there labored to cool down the reactor core, but there was no prospect for immediate success. They were temporarily cooling the reactor with a secondary system, but it wasn't working as well as the primary one, according to Yuji Kakizaki, an official at the Japanese nuclear safety agency.

The Atomic Insights blogger seems to think anyone who's worried is overreacting. Assuming he has correct information, he may be right -- but my skepticism is centered mostly around the fact that government officials often withhold accurate information in disasters. (Christine Todd Whitman assuring Ground Zero workers they were safe is just one that springs to mind.)

He cites this white paper on the safety of nuclear power plants as proof. I don't know how it works in other countries, but believing that for-profit nuclear power plants operated by companies whose first priority is the bottom line will build according to the highest specifications, and safely operate the plants exactly as they're supposed to, well, that seems a little naive.

So while experts assume the situation is under control, my position is that we may not know exactly what happened for a long, long time.