Nuclear fuel at the stricken Fukushima Daiichi power plant began melting just five hours after Japan’s March 11 earthquake, a Japanese nuclear engineer told a panel of U.S. scientists Thursday.

About 11 hours later, all of the uranium fuel in the facility’s unit 1 reactor had slumped to the bottom of its inner containment vessel, boring a hole through a thick steel lining, the University of Tokyo’s Naoto Sekimura told a committee of the National Academy of Sciences.

Sekimura’s assessment further damages the credibility of the plant’s operator, the Tokyo Electric Power Co. (Tepco). This week, the company admitted for the first time that nuclear fuel in three of the plant’s reactors had melted — a conclusion that independent scientists had reached long ago.

And in a rare insight into internal deliberations at the Nuclear Regulatory Commission, which oversees U.S. power plants, Commissioner George Apostolakis said that NRC staff members “thought the cores were melting” early in the Daiichi crisis. This conclusion — and the lack of information from Japanese authorities — drove the commission’s controversial recommendation to evacuate Americans within 50-mile radius of the facility, an area far larger than the 12.5-mile evacuation zone then enforced by the Japanese government.

“The 50 miles was very conservative,” Apostolakis told the academy scientists. “You can’t say someone was right or wrong in this situation.”

The six-reactor complex on Japan’s northeastern coast continues emitting radiation into the air and water, and Tepco has said that it will not be able to bring the three heavily damaged reactors under control until late this year or early next year.

The radioactive material already spewed by the plant could cause 120 cases of leukemia in Japanese children over the next 10 years, scientists from the National Cancer Institute said at the meeting.

“You can see there might be considerable number of leukemia cases,” said NCI’s Kiyohiko Mabuchi. He added that the estimate was very rough, based on the assumption that the non-evacuated population of Fukushima prefecture — nearly 2 million people — stays put for the next decade. The case figure was derived from scientists’ understanding of radiation’s dangers gleaned from studies of Japanese atomic bomb survivors.

At a second meeting Thursday related to the Fukushima Daiichi crisis, a U.S. Energy Department official warned that the nuclear facility still faces grave danger.

John E. Kelly, deputy assistant secretary for nuclear reactor technologies, said that protective components at the facility could crack because of high salt levels. There “is still a concern about more massive failure” of steel in the “lower head,” an important part of the containment system, Kelly told an NRC advisory committee. About 100 to 200 tons of salt left by the emergency pumping of salt water to cool the reactors are probably corroding the containment components.

Kelly also stressed that Tepco would have to continue pumping water into the damaged reactor units and venting radioactive steam for a year or more.

Tepco has built a low-level waste storage facility on the site but has no plans to move the waste elsewhere, he added. “It could be almost 30 years before they could use the site, so it’s almost permanent.”

Kelly made his comments during a hearing of the NRC’s advisory committee on reactor safety, which is drawing lessons from the disaster for the U.S. nuclear industry.

Kelly said an enormous number of unknowns, including the cause of an explosion at the unit 4 reactor, the safety of pools of used nuclear fuel and the condition of key protective components, remain.

More damaging revelations emerged earlier Thursday in Tokyo, where Tepco told reporters that a new leak in a storage container had dumped an additional 60 tons of radioactive water into the environment.

A high-level team of investigators from the International Atomic Energy Agency arrived in Japan this week to begin a 10-day investigation of the crisis.

In the United States, the NRC is in the midst of a wide-ranging 90-day review of U.S. nuclear power plant safety regulations in light of Japan’s crisis, the worst nuclear accident since the 1986 explosion at the Chernobyl power plant in Ukraine.