Tokyo (CNN) -- As the reactors at Japan's Fukushima Daiichi nuclear power plant overheated, poorly trained operators misread a key backup system and waited too long to start pumping water into the units, investigators reported Monday.
The Tokyo Electric Power Company didn't train its operators well enough to deal with severe accidents, according to an interim report from the government committee probing the worst nuclear accident since Chernobyl. And neither Tokyo Electric nor government regulators prepared for the chance that a tsunami could trigger a nuclear disaster, the panel concluded.
So when a roughly 15-meter (49-foot) tsunami hit the plant after the historic earthquake that struck Japan in March, operators misjudged the status of Unit 1's isolation condenser, which had shut down when the tsunami knocked out the plant's electrical system.
The device is designed to remove heat from the reactor in the event of a crisis. When it shut down, "appropriate corrective action was not taken nor instruction was given," the report states.
And when operators began to suspect the isolation condenser wasn't working, they didn't report that move to the officials managing the emergency response, who believed the system was still operating normally.
Those steps suggest that officials both at the scene and at Tokyo Electric's headquarters "did not fully understand" the backup system.
"Such a situation is quite inappropriate for nuclear operators," the report states. As a result, "an earlier opportunity for core cooling was missed," it found.
A hydrogen explosion -- a symptom of melting fuel rods -- blew the upper walls off the Unit 1 reactor building on March 12, the day after the earthquake. A similar explosion two days later ripped apart the housing around Unit 3, where operators had failed to recognize how badly the reactor needed outside water injections to keep cool, the report states.
Operators and managers at the plant didn't start pumping water into the No. 3 reactor until the morning before the explosion. However, the report adds, "It is still too early to judge" whether those steps could have prevented the explosions.
In the end, units 1, 2 and 3 at Fukushima suffered meltdowns, the company acknowledged in June. And Unit 4, which had been shut down at the time, suffered extensive damage from used but still-energetic fuel rods that were stored in a cooling pond in the building. The plant's two other reactors were not operating at the time the quake hit.
The 10-member panel, led by Tokyo University engineering professor Yotaro Hatamura, plans to issue a final report in summer of 2012. In the meantime, it noted, about 110,000 people "are still obliged to spend restricted life in evacuation for a long period of time."
Though no deaths have been attributed to the nuclear accident, the earthquake and tsunami killed more than 15,000 people in northeastern Japan.
Monday's 500-page report also criticized the government for failing to follow its own manuals for handling a nuclear accident and for not being able to make decisions involving people's safety in the aftermath of the nuclear accident. It also criticized the speed of the government's response and the vague statements given by company and government officials as the disaster unfolded.
"Transmission and public announcement of information on urgent matters was delayed, press releases were withheld and explanations were kept ambiguous," the report states. "Whatever the reasons, such tendency was hardly appropriate, in view of communication in an emergency."
In addition, the plant's emergency command center -- located about 5 kilometers (3 miles) away -- "was not designed to withstand elevated radiation levels, although it was intended for use in nuclear emergencies," the report states. A 2009 government report recommended improvements to the command center, but they weren't made, the report states.
And plans for managing nuclear accidents were not only voluntary, they ignored the possibility of an earthquake or tsunami, it said. In the future, "Measures against severe accident should not be left with the operator's voluntary activities," the report states.
There was no immediate response to the report from Tokyo Electric, Japan's largest utility.
In mid-December, the company declared that the reactors where the meltdowns occurred had finally reached "cold shutdown," meaning temperatures inside the crippled units had been kept under the boiling point for a sustained period of time. But experts say the declaration is largely symbolic, and decades of work remain to scrap the damaged reactors.
Tokyo Electric released its plan for decommissioning the plant December 21, projecting a 30- to 40-year timeline for the job.