Progress Status and Future Challenges of the Mid-and-Long-Term Roadmap
toward Decommissioning of TEPCO Holdings' Fukushima Daiichi Nuclear Power Station Units 1-4 (Outline)
The temperatures of the Reactor Pressure Vessel (RPV) and the Primary Containment Vessel (PCV) of Units 1-3 have been maintained within the range of approx. 15-25°C*1 for the past month. There was no significant change in the density of radioactive materials newly released from Reactor Buildings in the air*2. It was evaluated that the comprehensive cold shutdown condition had been maintained.
*1 The values varied somewhat depending on the unit and location of the thermometer.
*2 In January 2017, the radiation exposure dose due to the release of radioactive materials from the Unit 1-4 Reactor Buildings was evaluated as less than 0.00029 mSv/year at the site boundary.The annual radiation dose by natural radiation is approx. 2.1 mSv/year (average in Japan).
Completion of dismantling of the Unit 1 R/B cover wall panels
- To help remove rubble on the Unit 1 Reactor Building (R/B) top floor, dismantling of building cover wall panels started from September 13 and all 18 panels had been dismantled by November 10.
- No significant variation attributable to the work was identified at the dust monitors installed in the workplace and near the site boundary.
- Currently the status of rubble under the fallen roof is being investigated from the side of the building. Following the investigation, pillars and beams of the building cover will be modified and windbreak sheets installed from March 2017.
Dismantling of wall panels
Status of the land-side impermeable walls
- As for the land-side impermeable walls (on the mountain side), freezing and closure of seven unfrozen sections have been advanced with a phased approach. Regarding two of the seven sections, as a result of freezing started on December 3 and a supplementary method implemented (completed on February 8), the temperature declined to below 0°C in the area under the supplementary method. In the next phase, freezing of four of the remaining five sections will start. Prior to the freezing, a supplementary method has been implemented from February 22.
- The effect of the land-side impermeable walls (on the mountain side) will be evaluated in terms of groundwater level, inflow volume into buildings, groundwater volume pumped from subdrains and groundwater volume transferred to the area 4 m above sea level. The effect of the land-side impermeable walls (on the sea side) will also continue to be evaluated.
Freezing scope of the land-side impermeable walls
Reduction of water injection volume to the reactor
- The volume of water injected into the Unit 1 reactor was reduced from 4.5 to 4.0m3/h from December 14,2016; 4.0 to 3.5m3/h from January 5; and 3.5 to 3.0m3h from January 24, 2017. In all these cases, temperatures of the RPV bottom, etc. remained within the anticipated range.
- To facilitate purification of contaminated water in buildings, the water injection volume to the Unit 3 reactor was reduced from 4.5 m3/h as was done for Unit 1, and reached the target volume of 3.0 m3/h on February 22. Temperatures in the Reactor Pressure Vessel (RPV), etc. remained within the anticipated range.
- Prior to reducing the Unit 3 reactor water injection volume, real-time data disclosure of plant parameters such as Unit 1-3 RPV bottom temperatures started from February 7. The graphs of temperature changes, etc. are updated hourly.
Toward investigation inside the Unit 1 PCV
- To investigate the status of debris spreading to the basement floor outside the pedestal, based on the results of an investigation conducted on the 1st floor grating in April 2015, a self-propelled investigation device will be injected in March.
- In the investigation, a camera, etc. will be suspended from the 1st floor grating to inspect the status of the basement floor.
Image of investigation
Results of investigation inside the Unit 2 PCV
- To investigate the status inside the Unit 2 Primary Containment Vessel (PCV) pedestal* including the fall of fuel debris, prior investigation inside the PCV using a guide pipe was conducted on January 26 and 30, deposit on the access route of the self-propelled investigation device was removed on February 9, and an investigation inside the PCV using the device was conducted on February 16.
- The results of this series of investigations confirmed fallen and deformed gratings and a quantity of deposit inside the pedestal.
- The evaluation results of the collected information will be utilized when considering the policy for fuel debris removal.
*The base supporting the RPV
(Reference) Inside the Unit5 pedestal
Status inside the pedestal
Installation progress of a cover for Unit 3 fuel removal
- Toward fuel removal from Unit 3, work to install a cover for fuel removal, etc. started from January. Installation (hanging) of stoppers was completed on February 7 and 13 on the west and east sides respectively. The FHM girders will be installed from March.
- Work will continue with safety first.
Installation of stoppers
Completion of marine soil coverage inside the port
- For marine soil inside the port, in addition to the first layer coverage installed to prevent the spread of radioactive materials (completed in April 2015), the second layer coverage to improve durability was completed on December 26, 2016.
Image of marine soil coverage
Results of the questionnaire survey for workers to improve the work environment
- With the aim of improving the work environment for workers at the power station, the 7th questionnaire survey was conducted, to which approx. 89% (6,182) of workers responded. The evaluation results on the work environment showed further improvement compared to those of the previous survey.
- For potentially inappropriate working situations (such as cases labor conditions were not explained to the workers) were identified in the questionnaire answers, a follow-up investigation was conducted into the items for which the prime contractors and employer companies identified.
- The investigative results confirmed that the working situations were appropriate. Efforts to improve the work environment will continue, based on opinions and requests received from workers.
* Data of Monitoring Posts (MP1-MP8.)
Data (10-minute value) of Monitoring Posts (MPs) measuring airborne radiation rate around site boundaries show 0.503 - 2.100 ﾎｼSv/h January 25 - February 21, 2017).
We improved the measurement conditions of monitoring posts 2 to 8 for precise measurement of air dose rate. Construction works such as tree-clearing, surface soil removal and shield wall setting were implemented from Feburary 10 to April 18, 2012.
Therefore monitoring results at these points are lower than elsewhere in the power plant site.
The radiation shielding panel around monitoring post No. 6, which is one of the instruments used to measure the radiation dose of the power station site boundary, were taken off from July 10-11, 2013, since the surrounding radiation dose has largely declined due to further deforestation, etc.