In order to investigate the inside of the pedestal holding the core of Unit 1, a remotely controlled vehicle known as ROV-A2 was launched underwater.
Concerns about the Fukushima nuclear power plant's ability to withstand another major earthquake were sparked by images captured by a robotic probe inside one of the three damaged reactors. The images showed exposed steel bars in the main supporting structure and parts of its thick external concrete wall were missing.
Tokyo Electric Power Company Holdings, the plant's operator, has been sending robotic probes into Unit 1's main containment chamber for the better part of a year. The most recent investigation, which was completed at the end of March, yielded the results that were disclosed on Tuesday.
ROV-A2, a submerged remotely controlled vehicle, was sent within the Unit 1 pedestal. This pedestal serves as a support structure directly under the core. The photographs it returned were first viewed 12 years ago, when an earthquake and tsunami destroyed the power lpant. The inside of the pedestal is likely to contain remnants of the molten fuel.
A five-minute clip from the robot's 39-hour-long footage revealed extensive damage to the pedestal's 120-centimetre (3.9-foot) thick concrete shell around its base, revealing the steel reinforcement inside.
On Tuesday, TEPCO spokesman Keisuke Matsuo told reporters that the steel reinforcement is generally intact, but that the firm still wants to analyse data and photographs over the next couple of months to see whether and how the reactor's earthquake protection may be enhanced.
The three reactors still contain around 880 tonnes of extremely hazardous melted nuclear fuel. Although robotic probes have revealed some details, the fate of the molten wreckage remains mostly a mystery. As compared to the quantity of damaged fuel removed during cleanup of the 1979 partial core meltdown at the Three Mile Island nuclear facility in the United States, this amount is around 10 times larger.
Matsuo said the robot's footage revealed a pile of debris 40-50 centimetres (1.3-1.6 feet) high at the bottom of the main containment chamber, which may have been hardened nuclear fuel that had fallen from the core. Company officials speculate that the meltdowns in each reactor may have evolved differently since the pile is lower than the mounds shown in photographs collected during prior interior examinations at two other reactors.
Matsuo has said that the information gathered by the most recent inquiry would assist specialists in devising strategies for clearing the debris and analysing the meltdowns of 2011. After a year of processing the information, TEPCO hopes to have a three-dimensional map of the melted fuel and debris details.
Experts believe that most of the melted fuel inside Unit 1 fell to the bottom of the primary containment chamber based on data collected from earlier probes and simulations, but some may have even fallen through into the concrete foundation, making the already daunting task of decommissioning even more so.
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The delayed trial cleanup of melted debris from Unit 2 is planned to begin later this year. After a 10-year delay, in 2027, workers will begin removing spent fuel from the Unit 1 reactor's cooling pool. The emphasis will shift in 2031 from removing melted debris from the reactors to removing spent fuel from the pools.
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