An ongoing operation to learn more about the melted nuclear fuel at the crippled Fukushima Daiichi nuclear plant in Japan may have helped the decommissioning project—estimated to take up to 40 years—reach an important milestone.
Tokyo Electric Power Company (TEPCO), the plant operator, said on Monday 30 January that a complicated maneuver employing a 10.5-meter-long telescopic rod with a pan-tilt camera attached has yielded images of a dark mass of rubble inside the containment vessel and under the reactor vessel that houses the nuclear fuel. The images are now being analyzed in an effort to ascertain what the material might be.
“If the mass captured on camera is melted nuclear fuel, it would be a big step in helping the decommissioning work,” Yoshiyuki Ishizaki, executive vice president of TEPCO, said on 30 January, following the discovery.
Should the presence of nuclear fuel be confirmed, nuclear engineers could then work up a strategy for removing the highly radioactive rubble. However, if the material proves to be part of the damaged pressure vessel, or remains of cables or pipes, then more robot-aided searches of the surrounding area—including the concrete base supporting the containment vessel—will be required.
The rubble was found at the end of an access rail and on the adjoining platform located under the pressure vessel that, during normal reactor operation, is used to inspect the Control Rod Drive (CDR) assembly. The assembly helps control the fission rate of the reactor’s fuel rods. TEPCO says the CRD assembly and cables remained in their original locations.
Close-up images of the rubble show black globs of material covering part of the grating making up the platform, with areas of the grating dissolved and warped. Monday’s inspection is the first to find signs of this kind of damage. Last April’s 360-degree inspection of the No. 1 containment vessel, using two snake robots, showed no signs of melted fuel.
TEPCO had planned to send a scorpion robot into the No. 2 containment vessel next month to conduct further inspections. Monday’s inspection by remote camera was a precursor to that operation. However, a TEPCO official told IEEE Spectrum that obstacles had been discovered that may prevent the use of the scorpion bot. The official added that more analysis of the images is necessary before engineers can decide what type of robot will be used.
Meltdowns similar to what occurred in the No. 2 Reactor happened in Reactors 1 and 3 after a tsunami resulting from a March 2011 earthquake knocked out the plant’s safety features. The No. 4 Reactor, which was off-line and undergoing an inspection at the time, did not suffer the same fate. Though the building housing the spent fuel rods was badly damaged by a hydrogen explosion, TEPCO has since managed to remove all the rods from the pool.