Cooling water at the plant’s No.2 reactor came up to just 2 feet from the bottom of the reactor’s containment vessel, a beaker-shaped structure that encases the fuel rods. That was far below the 10 meter-level (33 feet) estimated by officials when the government declared the plant stable in December.

The low water levels also raise concerns that radioactive water may be leaking out of the reactor at a higher rate than previously thought, possibly into a part of the reactor known as the suppression chamber, and into a network of pipes and chambers under the plant — or into the ocean. At the No. 2 reactor, workers still pump about 9 tons of water an hour into the core to keep it cool.

The investigation also found current radiation levels of 72.0 Sieverts inside the containment vessel, enough to kill a person in a matter of minutes, as well as for electronic equipment to malfunction.

Kazuhiko Kudo, a professor of nuclear engineering at Kyushu University in southwestern Japan, said it was now suspect whether the nuclear fuel was being adequately cooled. And if some parts of the fuel remained above water, there was a risk the fuel could again heat up and melt. That could trigger a dangerous spike in the pressure inside the containment vessel, and lead to more radiation escaping the reactor, he said.