Some Nuclear Regulation Authority members are skeptical of continuing to send robots into reactors in the crippled Fukushima No. 1 nuclear plant to collect vital data on the locations of melted nuclear fuel and radiation levels.
These regulators are increasingly calling for a new survey methodology after recent investigations utilizing robots controlled remotely generated few findings and were quickly terminated.
“We should come up with a method that will allow us to investigate in a short period of time and in a more sensible way,” said a senior member of the NRA, the government watchdog.
The suggestion followed the failure of the latest probe from March 18 to March 22 in which a robot was sent in the No. 1 reactor to ascertain the location of fuel debris, information crucial to preparing for the decommissioning.
Tokyo Electric Power Co., the operator of the plant, said on March 23 the robot was unable to deliver a camera to planned spots from where images of nuclear fuel debris could be taken.
The utility cited the piping and deposits of what looked like sand accumulating on the piping as impediments that hindered the robot surveyor's path.
The survey was designed for the robot to reach numerous locations inside the No. 1 reactor to determine the location of nuclear fuel debris and their radiation levels.
The lower part of the reactor’s containment vessel is submerged in water where deposits of fuel debris are believed to reside below the surface after melting through in the 2011 nuclear disaster, triggered by the Great East Japan Earthquake and tsunami.
At one location, the robot succeeded in placing a camera, which is combined with a dosimeter, to a depth 0.3 meter from the containment vessel floor.
The probe measured underwater radiation levels from 3.0 to 11 sieverts per hour during the five-day survey. But it was unable to take images of the debris in the water.
TEPCO and the government hope to start removing molten nuclear fuel from 2021. But they have yet to collect information on the location, amount and condition of the melted fuel.
In a survey of the No. 2 reactor in February, a robot became stuck in deposits and other debris after traveling only 2 meters inside.
Surveyor robots for the No. 1 through No. 3 reactors have been developed by the International Research Institute for Nuclear Decommissioning since 2014, a project costing 7 billion yen ($62 million) by the end of March 2018.
It takes time to develop such multifunctional robots, but the surveys centering around the robots so far have failed to produce meaningful results.
No survey has been conducted at the No. 3 reactor.
(This article was written by Kohei Tomida and Masanobu Higashiyama.)