In March 2011, due to the tsunami caused by the Great East Japan Earthquake, reactors 1, 2, and 3 of the Fukushima Daiichi Nuclear Power Plant caused core melt.
It is estimated that almost all of the nuclear fuel melted due to the high temperature inside the reactor pressure vessel and penetrated the bottom and flowed down the containment vessel. never had.
In the midst of this, Tokyo Electric Power, the operator of the Fukushima nuclear power plant, resumed an investigation into the inside of the containment vessel of Unit 1 on the 8th.
This is the first resumption of the investigation in 5 years since March 2017, and although it was originally scheduled to resume in 2019, the route to put the robot used for the investigation into Unit 1 had to be changed, and there is also the effect of the corona crisis, so the on-site investigation is next day It had been delayed.
At present, near the bottom of the containment vessel of nuclear power plant No. 1, radioactively contaminated water is stagnant at a depth of about 1.8m.
TEPCO plans to use a remotely controlled underwater robot to investigate the situation on site, including investigation of debris from molten nuclear fuel.
There are six types of robots prepared according to their purpose, and the first robot has the task of installing a 'guide ring' where other investigation robots can safely enter in the future after approaching the bottom of the containment vessel through the water. received.
The main purpose of this 'guide ring' is to prevent the control cables of other investigation robots from getting tangled.
Robot No. 1 was put into Unit No. 1 on the afternoon of the 8th.
A nuclear reactor has a structure in which a concrete cylinder named 'pedestal' surrounds a pressure vessel with a hole through which molten nuclear fuel flows, and a containment vessel surrounds the outside again.
Originally, there was an open part (opening) in the pedestal adjacent to the pressure vessel, so if the robot was put in this way, the work could be quite comfortable (in the case of Units 2 and 3, this was the case), but in the case of Unit 1, the radiation near the opening The volume was so high that the robot was inserted through a pipe through the middle part of the containment vessel, which is on the opposite side.
Robot No. 1 went down to the bottom of the containment vessel and slowly moved underwater, installed four 'guide rings' sequentially, took a video near the bottom of the containment vessel, and transmitted it to the outside.
TEPCO has released a picture of the inside of the Fukushima Daiichi Nuclear Power Plant Unit 1 taken by the No.
You can see the orange lumpy sediment at the bottom, which was adsorbed to the structure inside the containment vessel.
It is presumed to be the remains (debris) of the molten nuclear fuel identified in Units 2 and 3, but TEPCO has drawn a line, saying, "At this point (the identity of the material) is not known for sure."
In addition, it is also unclear how much sediment settles to the bottom from this investigation alone.
In a 2017 survey, a sedimentary layer with a depth of 90 cm was confirmed in this vicinity, but even at this time, it was not possible to confirm whether the deposited material was molten nuclear fuel.
What is surprising is the radiation dose of the contaminated water moved by Robot No.
TEPCO said the radiation dose in the water was 1 to 2 sieverts per hour in this measurement.
Considering that the exposure limit of nuclear power plant workers is 50 millisieverts per year, if a person enters this water, they will reach the exposure limit of a year's worth in just 1 to 3 minutes.
Of course, it is an environment in which humans cannot directly enter and investigate, so it means that robots have no choice but to put in.
In the future, Tokyo Electric Power Corporation plans to sequentially introduce the remaining five types of remote-controlled robots to begin an investigation into the presence of molten nuclear fuel through sensors and photographing the interior of the containment vessel.
However, the timing of the deployment of the second robot has not yet been determined, so it will take a considerable amount of time to determine whether the sediment photographed by the first robot is, as expected, the remains of the nuclear fuel melted in the nuclear accident.
One of the most difficult tasks in the decommissioning process of the Fukushima nuclear power plant is extracting the molten nuclear fuel debris.
In the midst of this, the Japanese government has announced that it will strengthen monitoring of radioactively contaminated water stored at nuclear power plant sites.
They are taking steps one after another with the release of contaminated water into the ocean, which will begin as early as next spring.
Currently, there are 12 points measuring tritium in the waters off the nuclear power plant, and the goal of the strengthening policy is to increase the number to 50.
The Japanese government explains that more than half of these will be installed in the sea within 10 km of the Fukushima nuclear power plant, and the amount of tritium will be measured by collecting seawater from the sea level as well as from the sea floor within 50 km from the outside.
The number of measurements is based on four times a year, and immediately after the ocean discharge starts, the measurement interval will be narrowed to determine the effect on seawater.
The Japanese government will also explain these plans to the International Atomic Energy Agency's (IAEA) investigation team, which is scheduled for next week.
Although IAEA experts will have to carefully examine the plan to see if the investigation will be carried out according to the Japanese government's plan, there seems to be no change at all in the Japanese government and TEPCO's insistence on enforcing the policy of 'discharging to the sea', which is also expressed concern by local fishermen.