Japan yesterday opened an investigation into its worst nuclear accident as the operator of the troubled uranium plant admitted it had violated safety standards for uranium-processing for four years.
The Science and Technology Agency separately searched the headquarters of the JCO Co firm operating the plant in Tokyo and facilities in Tokaimura late yesterday to seize documents.
"I ordered the agency to carry out a strict inspection," the Prime Minister, Mr Keizo Obuchi, told reporters.
"It is unforgivable that the firm operated, making light of safety, and made the accident happen," he added.
The operator admitted it violated standards, which may have helped cause last week's accident in which 49 people were exposed to potentially deadly radiation.
"Knowing that the standards did not meet legal requirements, the firm had operated for some four years," said Mr Hideki Motoki, a spokesman for the operator.
The company changed its procedure manual without government approval, allowing workers to move the uranium processed at the plant around in stainless steel containers similar to buckets.
"But it is not certain whether the violation caused the accident," Mr Motoki said. "We need more time to reach a conclusion as the workers involved in the accident remain in hospital."
Experts said although the use of stainless-steel containers may not be directly related to the accident, the violation was an example of the facility's insufficient attention to safety.
Police separately set up a special investigative team to look into allegations of negligence.
"We suspect the operator violated the nation's nuclear energy facilities law," said a police spokesman, adding that JCO Co managers were being questioned.
Mr Akito Arima, head of the Science and Technology Agency, said nuclear safety measures would be upgraded. "We have to assume a scenario that operators do not follow their operation manual," Mr Akita said in a television interview.
"We have to remember that unexpected things can happen," he said. "We need to review our stance toward all nuclear facilities and introduce a strict programme to train workers."
Questions are also being raised about the official supervision of the nuclear fuel industry.
The government allowed the operator to install a uranium-processing facility in 1993 despite insufficient safety measures for responding to a critical reaction, Kyodo News said, quoting JCO documents.
The documents showed the firm said in its application to the government that "there was no need to prepare for a possible critical mass accident," as it would take the appropriate preventive measures.
Critical mass occurs when enriched nuclear materials come into close proximity, triggering a chain reaction.
The chain reaction at the uranium plant in Tokaimura, the heart of Japan's nuclear industry, ran for more than 20 hours before it was contained early on Friday.
Three workers from the plant, which is 120km north-east of Tokyo, received heavy doses of radiation and two remained in a serious condition in hospital yesterday.