{"id":6276,"date":"2022-09-30T15:07:58","date_gmt":"2022-09-30T06:07:58","guid":{"rendered":"https:\/\/cnic.jp\/english\/?p=6276"},"modified":"2022-09-30T15:08:09","modified_gmt":"2022-09-30T06:08:09","slug":"inside-the-primary-containment-vessel-of-fukushima-daiichi-nuclear-power-station-unit-1","status":"publish","type":"post","link":"https:\/\/cnic.jp\/english\/?p=6276","title":{"rendered":"Inside the Primary Containment Vessel of Fukushima Daiichi Nuclear Power Station Unit 1"},"content":{"rendered":"\n

Pedestal with Exposed Reinforcing Steel<\/strong><\/h2>\n\n\n\n

By Kamisawa Chihiro (CNIC)<\/p>\n

Survey of the pedestal exterior in the Unit 1 primary containment vessel<\/strong><\/p>\n

On February 9, 2022, Tokyo Electric Power Company Holdings, Inc. (TEPCO) and the International Research Institute for Nuclear Decommissioning (IRID) conducted a survey into the underground level of the primary containment vessel (PCV) of Fukushima Daiichi Nuclear Power Station (FDNPS) Unit 1, using an underwater remotely operated vehicle (ROV). The survey was intended to be a preliminary to investigating the conditions inside the vessel. The survey revealed that there were vast amounts of deposits, which were attributed to melted nuclear fuel, and exposed reinforcing steel, which appeared to be the skeletons of damaged concrete structures.<\/p>\n

On March 14, the two parties started a close visual inspection along the periphery of the pedestal (reinforced concrete base on which the reactor pressure vessel was installed) with an ROV. However, on March 16, a serious earthquake of magnitude 7.4 occurred off the coast of Fukushima Prefecture. The water level in the PCV of Unit 1 declined due to the earthquake, and the inspection was suspended.<\/p>\n

The two parties thereafter adjusted the water level by increasing the rate of water feed to the reactor, realizing the required water level in the PCV. On May 17, close inspection of the PCV interior was resumed using the ROV. The visual inspection was continued until May 21. The thickness of the deposits were also measured and thermal neutron flux measurements were conducted to learn whether the deposits were from nuclear fuel. It was confirmed that a major part of the deposits was from the fuel.<\/p>\n

Fuel debris deposits and damaged wall concrete on the pedestal periphery<\/strong><\/p>\n

In the survey, the outside of the pedestal was visually inspected in detail with the underwater ROV camera along more than half the cylindrical surface of the pedestal (Figure 1).<\/p>\n

\"\"<\/a>

Fig. 1: Overview of Pedestal Periphery Survey Using an Underwater ROV\u00a0 \u00a0(Compiled from TEPCO and IRID Data)<\/p><\/div>\n

\"\"<\/a>

Photograph 1: Deposits in the containment vessel<\/p><\/div>\n

The videos and photographs taken from the pedestal periphery show there are deposits that have accumulated deep on the PCV floor (Photograph 1).<\/p>\n

The lower periphery of the drywell has eight vent pipes that connect to the suppression chamber. Each vent pipe is provided with a jet deflector to protect the pipe inlet. About one-third of the length of the jet deflectors are buried in the deposits from below (especially deflectors C through F). It was confirmed that deposits also exist behind the jet deflectors. This shows that fuel debris is likely to have entered the suppression chamber.<\/p>\n

TEPCO says that the piping and valves of the reactor auxiliary cooling system and the piping of the reactor recirculation system are not seriously damaged.<\/p>\n

\"\"<\/a>

Photograph 2: Reactor auxiliary cooling system valve<\/p><\/div>\n

However, as those photographs show, the piping and valves of the reactor auxiliary cooling system are covered with deposits, while keeping their original shape. Especially, the greater part of the piping looks to be buried under the deposits, and it is unknown whether the piping is damaged or not (Photograph 2).<\/p>\n

The valves and piping of recirculation system B do not look seriously damaged as far as the released videos and photographs show. The valves and piping of system A on the opposite side look vague, and it is unknown whether they are damaged or not. It seems possible that there is an anomaly in the proximity of the piping elbow near the pedestal opening.<\/p>\n

The pedestal opening lies nearly halfway around from the underwater ROV entry position. Before the 2011 accident, operators entered the space below the reactor through this opening to inspect and maintain the control rod drive and neutron instrumentation.<\/p>\n

Currently there is no concrete on either side of the pedestal opening, which must have been there before the accident. As the photographs show, the reinforcing steel skeletons are exposed, and deposits have settled on them (Photographs 3 and 4). The sheet-steel cylindrical structure called the inner skirt,<\/em> built inside the pedestal, is also exposed.<\/p>\n

\"\"<\/a>

Photograph 3: Exposed reinforcing steel on the pedestal opening (right)<\/p><\/div>\n

\"\"<\/a>

Photograph 4: Exposed reinforcing steel on the pedestal opening (left)<\/p><\/div>\n

This means that, on both the sides of the opening, the originally 120-centimeter-thick concrete wall of the pedestal has completely vanished down to the height of about 100 centimeters from the base floor of the PCV. The data by TEPCO and IRID, as shown in Figure 1, indicates the approximate positions of the exposed reinforced steel, but we believe that these are not the only positions where steel is exposed.<\/p>\n

\u00a0<\/p>\n

\"\"<\/a>

Fig. 2: Construction of the Unit 1 Pedestal and RCV Lower Part (compiled from TEPCO data)<\/p><\/div>\n

The Unit 1 pedestal and lower part of the PCV have the construction shown in Figure 2. The wall of the pedestal base is about 120 centimeters thick. According to the report of the Nuclear Regulation Authority’s investigative hearing with TEPCO, the inner skirt was originally buried in the pedestal base down to about the height of about 100 centimeters.<\/p>\n

This means that, on both the sides of the opening, the originally 120-centimeter-thick concrete wall of the pedestal has completely vanished down to the height of about 100 centimeters from the base floor of the PCV. The data by TEPCO and IRID, as shown in Figure 1, indicates the approximate positions of the exposed reinforced steel, but we believe that these are not the only positions where steel is exposed.<\/p>\n

How did the concrete vanish?<\/strong><\/p>\n

Concrete is produced from cement and aggregates such as sand and crushed stones, which are mixed with water and then solidified. Concrete and cement are known to have the following characteristics under elevated temperature (Kasami and Ohno, \u201cSolid-state Properties of Concrete in High-temperature Ranges\u201d).<\/p>\n