The high radiation estimates in the No. 2 reactor of the stricken Fukushima No. 1 nuclear plant will probably force a rethink of the nationalized utility’s robot-based strategy for locating its molten fuel.
According to an analysis of Thursday’s abbreviated probe, the radiation in the primary containment vessel is about 650 sieverts per hour, more than the 530 sieverts estimated late last month, Tokyo Electric Power Company Holding Inc. said.
That level could kill a person quickly and indicates the fuel likely burned through the pressure vessel during the meltdown and is somewhere nearby.
Tepco, as the utility is known, halted Thursday’s robot after its camera went dark. The company suspects the problem was caused by the radiation.
A number of government officials had questioned the 530-sievert reading because it was calculated from camera interference, rather than measured by a dosimeter. Given the unorthodox method, some were reluctant to release the figure.
But Thursday’s analysis, also calculated via video footage, reinforced the experts’ findings, making it likely the radiation in that particular spot, near the pressure vessel, is high despite the considerable 30 percent margin of error.
“I had hoped that the previous results were wrong, but it is certain that there is an area with high radiation levels inside the reactor,” a government source said.
A Tepco official said a reading of 500 to 600 sieverts should be “basically correct,” especially given that the camera, which was designed for 1,000 sieverts of cumulative exposure, broke down within two hours.
On Thursday, the robot was equipped with a high-pressure water pump to wash off deposits up to 2 cm thick suspected to be the melted remains of paint and cable insulation from a 7-meter rail leading to an area beneath the pressure vessel, which holds the core.
Tepco hopes to send another robot along the rail to survey the bottom of the pressure vessel later this month.
A previous attempt on Tuesday to clear the rail was suspended because of a water pump malfunction.
The deposits cover 5 meters of the rail. The robot was able to clean about a meter of it close to the exterior of the primary containment vessel but it could not do any more because the deposits were too tough to remove, Tepco said.
If the deposits aren’t cleared, they might prevent the robot from getting beneath the pressure vessel, it said.
Three of the Fukushima No. 1 plant’s six reactors were hit by meltdowns after the March 2011 mega-quake and tsunami caused a station blackout that knocked out the plant’s cooling systems. It is the worst nuclear disaster since the Chernobyl in 1986.
Tepco is still in the early stage of assessing the conditions in and around the damaged reactors so the fuel can be removed. The decommissioning is expected to take decades.