24 Jul (NucNet): 24 Jul (NucNet): A submersible robot has obtained images of the interior of the primary containment vessel (PCV) of Unit 3 at the Fukushima-Daiichi nuclear power station, appearing to show deposits that could be melted nuclear fuel covering the floor. According to station owner and operator Tokyo Electric Power Company (Tepco), the robot found large amounts of solidified lava-like rocks and lumps in layers as thick as one metre on the bottom inside a main structure called the pedestal that sits underneath the core inside the PCV. Tepco said it could not confirm that the deposits were melted nuclear fuel, but it was “highly likely”. Tepco also said it had confirmed severe damage in several structures that had been damaged by the earthquake and subsequent tsunami that crippled the six-unit facility in March 2011. The submersible robot, made by Toshiba and known as “Little Sunfish”, was used for the inspection because the Unit 3 PCV contains a pool of water, originally used to cool the reactor, which is about 6.4 metres deep. On 21 July 2017, the robot spotted suspected debris of melted fuel for the first time since the earthquake and tsunami caused multiple meltdowns and destroyed the plant. The three-day investigation of Unit 3 ended on 22 July 2017. Locating and analysing the fuel debris and damage in each of the station’s three wrecked reactors is crucial for decommissioning the facility. The search for melted fuel in the two other reactors has so far been unsuccessful because of damage and extremely high radiation levels. During probe, cameras mounted on the robot showed extensive damage caused by the core meltdown, with fuel debris mixed with broken reactor components. Tepco said it would take time to analyse the debris in the images to figure out removal methods. Units 1, 2 and 3 at the station were in commercial operation at the time of the earthquake and tsunami and all suffered reactor core, fuel and containment damage. The other three units did not suffer fuel damage. Unit 4 was offline and was not loaded with fuel, but the reactor building was severely damaged by a hydrogen explosion. Units 5 and 6 were offline, but were still fuelled.
