Unit 1: Unit 1 was examined in 2015. The reactor core was almost completely displaced out of the RPV and there is molten material at the bottom of the containment. By comparing the examination results with existing construction documentation, it could be determined that the RPV and the containment are still in their original position and that there are no nuclear assembly components larger than 1 m in the RPV. The examination results confirm the results of the accident analyses. During the accident, a major part of the nuclear fuel was displaced from the RPV to the lower part of the containment and began reacting with the concrete structures, resulting in a solidified mixture of concrete, metal structural material and nuclear fuel. In March 2017, results showed that, among other things, there was also molten material outside the control rod drive room. A material sample was taken from the bottom of the containment. No major damage was detected at the outer wall of the control rod drive room and other structures.
To facilitate further inspections of the inner part of the containment, on 8 April 2019 work started to gain access via an existing personnel airlock. The personnel airlock was drilled open for this. Measures to prevent the outflow of radioactive dusts from the containment included reducing the overpressure in the containment.
Unit 2: According to the accident analysis for Unit 2, only small amounts of corium were formed at the bottom of the containment during the accident. The major part of the molten nuclear fuel and the molten structural material are stated to have remained in the RPV and re-solidified. There are approximately 160 tonnes of material at the bottom of the RPV, 20 to 50 tonnes in their original core position and 70 to 100 tonnes of structural material in the area above it. There may also be smaller amounts in the outer area of the RPV. The results of the examination confirm the results of the accident analyses.
No major damage was detected at the inner wall of the control rod drive room, which is located below the RPV, nor any molten cable runs or metal support struts of the handling system for control rod drives. There are also deposits in higher areas which presumably have dripped down various structures. No outflow of molten material from the control rod drive room via its access was detected. On 13 February 2019, a robot was deployed in the containment of Unit 2 to facilitate further planning of the dismantling work. The robot was equipped with two gripping claws, a swivelling camera, a thermometer, a dose meter and LED lights; it was said to be capable of lifting up to 2 kilograms. The robot was introduced into the containment with the help of a rod and then lowered into the lower part of the control rod drive room. It was sent to six different positions to test the plasticity of the deposits. Tokyo Electric Power Holdings, Inc. (TEPCO) reported that the deposits were malleable and could be grasped with the gripping claws at five of the positions. This meant that the malleable material could be removed with a similar mechanism. TEPCO published a video of the examination of Unit 2 in February 2019 at the link TEPCO - Examination of Unit 2 in February 2019 (in Japanese). Another sample for additional laboratory tests was to be taken with the help of another robot in the second half of 2019.
Block 3: According to official Japanese statements relating to Unit 3, part of the molten core of Unit 3 is still in the RPV and corium has moved to the bottom of the containment.
In September 2017, a scan showed that a major part of the nuclear fuel was outside the RPV. TEPCO assumes, on the basis of the scan images, that there were no large accumulations of contaminated and very dense material in the RPV as is the case in Unit 2.
The deployment of a robot in the containment in July 2017 provided images of the control rod drives. The underwater images clearly showed parts of the control rod drives. In addition, the scans showed numerous damaged and/or molten structural building elements in the control rod drive room. Pieces of a control rod guide tube from the inside of the reactor with a diameter of several centimetres could be seen on the scans, which indicated a correspondingly sized opening in the RPV. In the control rod drive room corium was detected close to the access to the room, however, no inspection of the access itself was possible. Therefore, the possibility could not be excluded that part of the corium flowed out of the control rod drive room via the access. No major damage at the inner wall of the control rod drive room was detected.