It is unclear why there is less radioactivity under the reactor vessel, when it is where there should be the most.
A robot was expected to solidify ways to clean up the No. 2 reactor at the Fukushima No. 1 nuclear plant, but its short-lived mission raised puzzling questions that could derail existing decommissioning plans.
The robot, Sasori, was abandoned in the melted-down reactor after it became stuck in deposits and other debris that are believed to have interfered with its drive system.
But it did take radiation measurements that indicate Tokyo Electric Power Co., operator of the plant, was too optimistic about the state and location of the melted fuel within the reactor. The melted fuel, in fact, may be spread out all over the reactor’s containment vessel.
Scientists had believed the melted nuclear fuel fell through the reactor’s pressure vessel and landed on metal grating and the floor of the containment vessel.
The results of Sasori’s investigation, coupled with previous data taken from possible images of the melted fuel, show the situation within the reactor is much worse than expected. And a fresh investigation into the reactor is now nowhere in sight.
A remote-controlled video camera inserted into the reactor on Jan. 30 took what are believed to be the first images of melted fuel at the plant, which suffered a triple meltdown after the March 2011 Great East Japan Earthquake and tsunami.
Based on the images, TEPCO estimated 530 sieverts per hour at a point almost halfway between the metal grating directly beneath the pressure vessel and the wall of the containment vessel. Black lumps on the grating are believed to be melted fuel.
A different robot sent in on Feb. 9 to take pictures and prepare for Sasori’s mission estimated 650 sieverts per hour near the same spot.
Both 530 and 650 sieverts per hour can kill a person within a minute.
Sasori, equipped with a dosimeter and two cameras, on Feb. 16 recorded a reading of 210 sieverts per hour near the same location, the highest figure measured with instruments in the aftermath of the disaster.
Sasori was supposed to travel along a rail connecting the outer wall of the containment vessel with the metal grating to measure radiation doses and shoot pictures inside, essential parts of work toward decommissioning the reactor.
After traveling only 2 meters, the robot became stuck before it could reach the metal grating.
TEPCO at a news conference repeatedly said that Sasori’s investigation was not a “failure” but had produced “meaningful” results.
However, an official close to TEPCO said, “I had great expectations for Sasori, so I was shocked by how it turned out.”
Hiroaki Abe, professor of nuclear materials at the University of Tokyo who has studied TEPCO’s footage, tried to explain why high doses were estimated between the pressure vessel and the containment vessel.
“Instead of directly landing on the rail, the melted nuclear fuel may have flown off after it reacted violently with the concrete, which had a high moisture content, at the bottom of the containment vessel, just like what happens when lava pours into the sea,” Abe said.
But he said this scenario raises a puzzling question, considering the estimated radiation readings near the area below the pressure vessel were down to 20 sieverts per hour, according to an analysis of the video footage.
“If nuclear fuel debris had splattered around, the radiation levels at the central area below the pressure vessel must be extremely high,” he said. “In addition, deposits on the rail would have taken the shape of small pieces if they were, in fact, flying nuclear fuel debris. The findings are puzzling.”
Images by the remote-controlled camera also showed that equipment in the lower part of the pressure vessel was relatively well preserved, indicating that the hole at the bottom of the vessel is not very large.
“How to remove nuclear fuel debris will all depend on how much remains inside the pressure vessel and how much fell out,” Abe said.
Toru Obara, professor of nuclear engineering at the Tokyo Institute of Technology, stressed the need to retrieve substances from the bottom of the robots or elsewhere.
“We could get clues as to the state of the melted nuclear fuel and the development of a meltdown if we could figure out which materials mixed with the nuclear fuel,” he said.
The surveys by the camera and robots were conducted from a makeshift center at the No. 2 reactor. The center’s walls are made from radiation-blocking metal.
TEPCO and the government plan to determine a method to remove nuclear fuel debris in fiscal 2018 before they proceed with the actual retrieval process at one of the three destroyed reactors.
One possible method involves filling the containment vessels with water to prevent radioactive substances from escaping.