Japanese Officials on Defensive as Nuclear Alert Level Rises

  A volunteer in Ofunato, Japan, cleaned photographs that were found in the tsunami debris.

TOKYO — Japanese officials struggled through the day on Tuesday to explain why it had taken them a month to disclose large-scale releases of radioactive material in mid-March at a crippled nuclear power plant, as the government and an electric utility disagreed on the extent of continuing problems there.

A policeman watched colleagues prepare to transport a body by van in Minamisoma, Japan, on Thursday, inside a deserted nuclear evacuation zone.

The government announced Tuesday morning that it had raised its rating of the severity of the accident at the Fukushima Daiichi Nuclear Power Station to 7, the worst on an international scale, from 5. Officials said that the reactor had released one-tenth as much radioactive material as the Chernobyl accident in 1986, but still qualified as a 7 according to a complex formula devised by the International Atomic Energy Agency.

Japan’s new assessment was based largely on computer models showing very heavy emissions of radioactive iodine and cesium from March 14 to 16, just after the earthquake and tsunami rendered the plant’s emergency cooling system inoperative. The nearly monthlong delay in acknowledging the extent of these emissions is a fresh example of confused data and analysis from the Japanese, and put the authorities on the defensive about whether they have delayed or blocked the release of information to avoid alarming the public.

Seiji Shiroya, a commissioner of Japan’s Nuclear Safety Commission, an independent government panel that oversees the country’s nuclear industry, said that the government had delayed issuing data on the extent of the radiation releases because of concern that the margins of error had been large in initial computer models. But he also suggested a public policy reason for having kept quiet.

“Some foreigners fled the country even when there appeared to be little risk,” he said. “If we immediately decided to label the situation as Level 7, we could have triggered a panicked reaction.”

The Japanese media, which has a reputation for passivity but has become more aggressive in response to public unhappiness about the nuclear accident, questioned government leaders through the day about what the government knew about the accident and when it knew it.

Prime Minister Naoto Kan gave a nationally televised speech and press conference in the early evening to call for national rebuilding, but ended up defending his government’s handling of information about the accident.

“What I can say for the information I obtained — of course the government is very large, so I don’t have all the information — is that no information was ever suppressed or hidden after the accident,” he said. “There are various ways of looking at this, and I know there are opinions saying that information could have been disclosed faster. However, as the head of the government, I never hid any information because it was inconvenient for us.”

Junichi Matsumoto, a senior nuclear power executive from the plant’s operator, the Tokyo Electric Power Company, fanned public fears about radiation when he said at a separate news conference on Tuesday morning that the radiation release from Daiichi could, in time, surpass levels seen in 1986.

“The radiation leak has not stopped completely, and our concern is that it could eventually exceed Chernobyl,” Mr. Matsumoto said.

But Hidehiko Nishiyama, deputy director general of Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency, said in an interview on Tuesday evening that he did not know how the company had come up with its estimate. “I cannot understand their position,” he said.

He speculated that Tokyo Electric was being “prudent and thinking about the worst-case scenario,” adding, “I think they don’t want to be seen as optimistic.”

Mr. Nishiyama said that his agency did not expect another big escape of radiation from Daiichi, saying that “almost all” the material that is going to escape has already come out. He said that the rate of radiation release had peaked in the early days after the March 11 earthquake, and that the rate of radiation had dropped by 90 percent since then.

The peak release in emissions of radioactive particles took place following hydrogen explosions at three reactors, as technicians desperately tried to pump in seawater to keep the uranium fuel rods cool, and bled radioactive gas from the reactors in order to make room for the seawater.

Mr. Nishiyama took pains to say — and other nuclear experts agreed — that the Japanese accident posed fewer health risks than Chernobyl.