A probe touched melted nuclear fuel debris in a destroyed reactor at the Fukushima No. 1 nuclear plant, a long-awaited milestone in the battle toward decommissioning, Tokyo Electric Power Co. said Feb. 13.
The rod-like probe, fitted with 3-centimeter-long claws, lifted pieces of the nuclear fuel debris during the eight-hour operation at the bottom of the No. 2 reactor at the plant, the utility said.
Data obtained through the investigation, such as the hardness, temperature and radioactivity of the debris, will be used to develop equipment and containers for the eventual removal of the melted fuel. The probe, which was recovered after the investigation, also took pictures of the inside of the containment vessel.
No debris was taken outside the reactor, according to TEPCO.
The retrieval process will start in earnest at one of the reactors in 2021, according to the plan drawn up jointly by the central government and TEPCO.
It was the first physical contact with melted fuel at the plant since the Great East Japan Earthquake and tsunami on March 11, 2011, caused the meltdowns of three reactors there.
It took TEPCO years just to locate the melted fuel debris because of the high radiation levels in the reactor buildings.
The utility said workers manually inserted the probe through a crack created early in the disaster on the side of the No. 2 reactor’s containment vessel.
The probe, which resembles a prize-grabbing crane machine at a game arcade, was also equipped with a dosimeter and a camera. It can extend 15 meters and can lift pieces up to 8 centimeters in diameter and weighing 2 kilograms.
Workers remotely operated the probe from the operating room in a building close to the No. 2 reactor building, and lowered it to the bottom of the reactor’s containment vessel.
The probe lifted pebble-like nuclear fuel debris and structural parts up to 8 cm in diameter at five spots in the 2,500-square-cm area that was investigated. It failed to pick up debris at one spot.
TEPCO said the probe could not lift clay-like debris likely because it had adhered to the bottom of the containment vessel.
The probe also touched nuclear fuel debris lying at several spots on the lattice-shaped scaffold for workers directly below the reactor’s pressure vessel.
The previous investigation of the No. 2 reactor in 2017 located melted fuel debris on the scaffold. But a robot deployed for a further investigation broke down on its way to the debris.
In a survey last year, the utility used the rod-like probe to take images of the inside of the reactor.
TEPCO is expected to remove a small amount of nuclear fuel debris in the second half of fiscal 2019 as part of preparations for full-scale retrieval.