Ensuring the Safety of Fukushima Daiichi Nuclear Power Station

Cooperation and support to ensure the safety of Fukushima Daiichi Nuclear Power Station

Toshiba Group has been conducting relief activities, sincerely hoping for the earliest recovery and reconstruction of the areas and people affected. Toshiba Group is also continuing to provide cooperation and support in maintaining the stability of the power station and advancing its decommissioning.

Since immediately after the accident at the Fukushima Daiichi Nuclear Power Station, Toshiba Group has been working to the best of its ability to eliminate the consequences and ensure safety in response to a request by the government and the Tokyo Electric Power Company, Incorporated (TEPCO) (currently Tokyo Electric Power Holdings, Inc.; hereinafter, this company will be referred to as TEPCO when discussing conditions prior to the name change).

As a company engaged in nuclear power generation, Toshiba Group takes the accident very seriously and have remained united in our resolve to overcome every difficulty.

1. Achieving a stable state of cold shutdown
(occurrence of the earthquake to December 2011)

In April 2011, the government and TEPCO developed a preliminary roadmap for termination of the accident of Fukushima Daiichi Nuclear Power Station.

In step 1, where we effectively reduced the high radiation dose rate, Toshiba Group mainly worked to prevent the nuclear-contaminated cooling water injected into the damaged plants from flowing out into the ocean.

Under harsh work conditions including blackouts and high levels of radiation, Toshiba Group built and began operating a coolant recirculation system designed to purify and recycle contaminated water in order to cool the reactor core, along with a radioactive substance removal system and a containment gas management system, within a short period of two months.

As a result, we were able to control the increase in contaminated water, preventing it from flowing outside the power plant premises.

In step 2, our goal was to control the release of radioactive substances and to greatly reduce the radiation dose.

Toshiba Group concluded that in order to process the massive amounts of contaminated water, it was necessary to improve the system's treatment capabilities and ensure its stable operation. Consequently, we proposed to TEPCO that we develop a new system in cooperation with partner companies in the United States and Japan. We commenced development in May 2011 and began operating the new system in August.

The new system, which we named SARRY™,*1 was highly valued for its stable operational performance and high decontamination ability, and came to be used as the primary decontamination device. The stable operation of SARRY™ allowed us to effectively cool the nuclear reactors through the coolant recirculation system, and to help achieving a state of cold shutdown for the Fukushima Daiichi Nuclear Power Station's Units 1 through 3 in December.

In step 2, Toshiba Group also contributed to verifying the cooling state of nuclear reactors by installing substitute thermometers for measuring nuclear reactor pressure vessel temperature and monitoring control systems and by restoring power systems inside the power station.

  • *1 SARRY™:Simplified Active Water Retrieve and Recovery System

2. Initiatives based on a medium- and long-term roadmap (from December 2011)

In December 2011, the Japanese government and TEPCO published a medium- and long-term roadmap for the decommissioning of Fukushima Daiichi Nuclear Power Station's Units 1 through 4. Thereafter, fourth revisions are being reviewed by September 2017. This roadmap provided details and a targeted schedule for contaminated water treatment, removal of fuel from the spent fuel pool, removal of fuel debris*2, and nuclear waste treatment.

  • *2 Fuel debris: Nuclear fuel melted in a nuclear reactor accident, which is cooled and solidified by being mixed with concrete and metals in the nuclear reactor.

Contaminated water treatment

To treat contaminated water, SARRY™ is in under stable operation to eliminate the cesium contained in treatment water. To reduce the concentration of strontium and 62 radioactive nuclides other than cesium below environmentally neutral levels, Toshiba Group newly developed and delivered a Multi Radio-nuclide Removal System (MRRS™*3) and an extended MRRS™ and we treated a total of 850,000 tons of retained water by August 2018. Toshiba Group will continue to inspect both facilities for stable operation and contribute to completion of processing in 2020 with the aim of starting operation of an expanded SARRY™ system before the end of 2018 (as of September 21, 2018).

To preserve treatment water, we installed 124 reliable welding tanks so far. Toshiba Group will contribute to inspection inside the tank already installed in the site and expansion.

  • *3 MRRS™:Multiple Radio-nuclides Removal System

Removal of fuel from the spent fuel pool

To remove fuel from the spent fuel pool of Unit 3, where the reactor building was severely damaged, it was essential to remove large rubbles from the operation floor of Unit 3 nuclear reactor building. Toshiba Group carried out repeated detailed simulations of rubbles removal operation by using 3D CAD programs to build a monitoring system, thereby completing remote removal of large rubbles in November 2015. In addition, other rubbles removal, decontamination and shielding installation were completed in December 2016, and the atmosphere radiation dose rate was reduced until manned work was possible with the introduction of a fuel removal system in November 2017. We will proceed with preparation work such as the testing and trial operation of a fuel removal system to start fuel removal.

Retrieval of fuel debris

Retrieving fuel debris requires conducting a survey on conditions around nuclear reactors. Toshiba Group examined how to conduct a survey of the conditions inside the nuclear reactor building for Unit 2 in the high radiation environment and developed survey equipment. By using the equipment, we took video inside the primary containment vessel and obtained temperature and radiation data in January 2012. In March 2013, we conducted a survey on the vent pipes on the bottom of the primary containment vessel by the quadruped robot to confirm that there was no leakage.

In order to design methods for retrieving fuel debris, it is necessary to continue detailed investigation in a harsh environment such as high radiation dose rate, high humidity, and darkness in the primary containment vessel. Therefore, we developed robots, etc. through the Japanese national projects. In February 2017 we put them in the Fukushima Daiichi Nuclear Power Plant and measured the temperature and radiation dose rate inside the primary containment vessel of Unit 2 and took video just under the reactor pressure vessel for the first time. In July 2017, we investigated inside the primary containment vessel of Unit 3 using a compact underwater Remotely Operated Vehicle (ROV), and in January 2018, we succeeded in taking images of objects with high probability of fuel debris for the first time using investigating equipment that can reach the bottom of the primary containment vessel of Unit 2. The data obtained will contribute to a decision on the retrieval method for the fuel debris in FY2019. Going forward, we will proceed with further detailed investigating to ensure the most appropriate retrieval method and with investigations to create a safety system for the entire plant based on the image data from inside the containment vessels that we have attained thus far.

Expanded Multi-Radionuclide Removal System (MRRS)
Expanded Multi-Radionuclide Removal System (MRRS)

Fuel removal system of Unit 3 (Photo courtesy of TEPCO)
Fuel removal system of Unit 3 (Photo courtesy of TEPCO)

System for investigating inside the primary containment vessel of Unit 2
System for investigating inside the primary containment vessel of Unit 2

Underwater ROV for investigating inside the primary containment vessel of Unit 3
Underwater ROV for investigating inside the primary containment vessel of Unit 3

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