The crippled Fukushima Daiichi nuclear plant, one of the oldest in Japan, had a record of accidents and radiation exposure even before being damaged by this month's earthquake and tsunami.

Between 2005 and 2009, Fukushima Daiichi had 15 accidents, more than any other large Japanese nuclear plant, according to a Wall Street Journal analysis of data from the Japan Nuclear Energy Safety Operation. The accidents were mostly minor, but some involved important safety equipment, including emergency diesel generators.

Data shows that the plant's workers were also exposed to more radiation than at other nuclear plants in the country. Tokyo Electric Power Co., which owns the plant, said the plant was operated safely and that the higher accident rate is due to the plant's old age. Fukushima Daiichi's reactors date to the 1970s.

According to nuclear engineers, a large contribution to the crisis at the plant was the practice of removing fresh fuel from a reactor and storing it for weeks in a spent-fuel pool during maintenance. In the United States, reactors generally do not store fresh fuel in pools during repair work and instead put it in a thick pressure vessel. Only the oldest fuel rods, which are less radioactive, are typically stored in pools for years while they are cooled by water.

But in Japan, at the time the earthquake struck, Unit 4 had been offline for repairs, and its fresh, highly radioactive fuel rods were stored in a spent-fuel pool.

"We were carrying out checks on the inside of the reactor" and workers "had to remove the nuclear fuel from the reactor," said Takeshi Makigami, head of TEPCO's nuclear-equipment-management section.

When the tsunami hit and knocked out emergency generators, the rods in the pools of Unit 4 overheated, causing a fire and destroying the roof above the pool.

"The Japanese argue it's safer to move all the fuel to the pool, but the practice of full-core discharge caused a problem, in this case," said Andy Kadak, a former professor of nuclear engineering at Massachusetts Institute of Technology.

TEPCO officials initially hesitated in using seawater to cool the reactors because it could destroy the plant. But yesterday, disaster response teams, including the Hyper Rescue Squad from Tokyo, restored electrical power and prepared to restart cooling systems.

Over the weekend, "the most important thing we were able to do was to fill the spent-fuel pools at No. 3 and No. 4," said Hidehiko Nishiyama, an official at the Ministry of Economy, Trade and Industry (Smith/Casselman/Obe, Wall Street Journal, March 21). -- AP