Progress toward decommissioning:Work related to circulation cooling and accumulated water treatment line
Stably continue reactor cooling and accumulated water treatment, and improve reliability
The cold shutdown condition will be maintained by cooling the reactor by water injection and measures to complement status monitoring will continue to be implemented.
To tackle the increase in accumulated water due to groundwater inflow, fundamental measures to prevent such inflow into the Reactor Buildings will be implemented, while improving the decontamination capability of water-treatment facilities and preparing facilities to control the contaminated water.
Status of Progress Points
(1) Improved reliability of circulating injection cooling
- To address leaks from pressure hoses and other such issues related to piping along the reactor injection line, the hoses have been replaced with more reliable polyethylene pipe.
- Appropriate backup facilities have been secured for the reactor coolant injection system (injection line pump: 3 systems; water source: 2 types; ensuring power sources from multiple buses, etc.). Even if the functionality of multiple systems involved in the injection of coolant into the reactor were to be lost simultaneously due to an accident, the injection of cooling water can be restarted using fire engines within three hours.
(2) Improved reliability of accumulated water treatment facility
- To address leaks from pressure hoses used for the main route of the accumulated water transfer line, the current hoses have been replaced with more reliable polyethylene and other types of pipe.
- As measures to prevent the spread of any leaks to the external environment, gates, earth dams and monitoring cameras have been installed and discharge channels have been covered with conduits.
- Reductions in transfer lines are being studied.
(3) Multifaceted response to accumulated water
- To reduce the amount of water continuing to accumulate due to the intrusion of groundwater into the reactor buildings, measures to check such inflow by means of underground water bypasses or the restoration of subdrains are being studied and construction is being performed on-site.
- Construction is being performed on-site to set up a multi-nuclide removal system to remove radioactive materials in contaminated water.
- Plans to augment storage tanks are being formulated and additional tanks are being constructed..
Work to improve the reliability of the circulation water injection cooling system and pipes to transfer accumulated water.
- Operation of the reactor water injection system using Unit 3 CST as a water source commenced (from July 5, 2013). Compared to the previous systems, the reliability of the reactor water injection system was enhanced, e.g. by increasing the amount of water-source storage and enhancing durability.
- To reduce the risk of contaminated-water leakage, the circulation loop was shortened by installing a reverse osmosis (RO) device in the Unit 4 Turbine Building within the circulation loop, comprising the transfer of contaminated water, water treatment and injection into the reactors. Operation of the installed RO device started from October 7 and 24-hour operation started from October 20. Installation of the new RO device inside the building shortened the circulation loop from approx. 3 to 0.8 km窶ｻ.
窶ｻ The entire length of contaminated water transfer pipes is approx. 2.1km, including the transfer line of surplus water to the upper heights (approx. 1.3km).
Preventing groundwater from flowing into the Reactor Buildings
Reducing groundwater inflow by pumping sub-drain water
- To reduce groundwater flowing into the buildings, pumping-up of groundwater from wells (subdrains) around the buildings started on September 3, 2015. Pumped-up groundwater was purified at dedicated facilities and released after TEPCO and a third-party organization confirmed that its quality met operational targets.
Via a groundwater bypass, reduce the groundwater level around the Building and groundwater inflow into the Building
- Measures to pump up groundwater flowing from the mountain side upstream of the Building to reduce the groundwater inflow (groundwater bypass) have been implemented. The pumped up groundwater is temporarily stored in tanks and released after TEPCO and a third-party organization have confirmed that its quality meets operational targets. Through periodical monitoring, pumping of wells and tanks is operated appropriately. At the observation holes installed at a height equivalent to the buildings, the trend showing a decline in groundwater levels is checked. The analytical results on groundwater inflow into the buildings based on existing data showed a declining trend.
Installing land-side impermeable walls around Units 1-4 to prevent the inflow of groundwater into R/B
- To prevent the inflow of groundwater into the Reactor Buildings, installation of impermeable walls on the land side is planned.
- Installation of frozen pipes commenced on June 2, 2014. Construction for freezing facilities was completed in February 2016.
- Freezing started on the sea side and at a part of the mountain side from March 2016 and at 95% of the mountain side from June 2016. Freezing started for two of seven unfrozen sections on the mountain side from December 2016.
- On the sea side, the underground temperature declined below 0°C throughout the scope requiring freezing except for the unfrozen parts under the sea-water pipe trenches and the areas In October 2016.
Basic Policy for Treatment of Accumulated Water
- For treating accumulated water, the following requirements will be reviewed and measures based on these will be implemented so that contaminated water will not be easily released to the sea.
- Fundamental measures to counter the flow of groundwater into reactor and other buildings, which is the causing an increase in the quantity of water.
- Measures to ensure stable operation including alternate facilities when there is a failure and to ensure improvements in the decontamination capacity of water treatment facility.
- Measures for further installation of land facilities, etc. for managing contaminated water.
- Any release into the sea will not be implemented without the consent of the relevant ministries and agencies.
Future Challenges and Direction of Responses
Continuation of Measures to Prevent Leakage to External Environment
- In preparation for any leaks, gates and earth dams will be set up around tanks installed in the future to prevent leaks into the sea, and patrols and inspections will be conducted to monitor for leaks.