Current State of Post-Accident Operations at Fukushima Daiichi Nuclear Power Station (October 2013 to April 2014) Nuke Info Tokyo No. 161
State of the Plant
Many of the measuring instruments installed in the Fukushima Daiichi Nuclear Power Station (FDNPS) measuring system continue to malfunction as a result of the accident. Although there is no guarantee of the accuracy of values being measured, if these values are taken as the premise, from the temperature of the containment vessels and from the releases of Xenon-135, it can be estimated that the state of the reactors is stable. According to the assessment by Tokyo Electric Power Company (TEPCO), however, 10 million Bq/hr of radioactive substances continue to be released into the atmosphere (see Figure 1).
|Figure 1. Releases of radioactivity from Units 1 to 4 of Fukushima Daiichi Nuclear Power Station
From materials prepared and submitted by the secretariat to the Government and TEPCO’s Mid-to-Long Term Countermeasure Meeting and Secretariat of the Council for the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station. (However, this was for assessing leakage from exhaust outlets and gaps in building covers and does not include leakage to groundwater, etc.)
Current State of Post-Accident Operations
1. State of Operations concerning Molten Fuel
The current state is that for each of the reactors, surveys of the plant situation as preparation for decontamination of the buildings, surveys to reveal the locations of leaks in the containment vessels, as well as R&D work on various kinds of devices, are being implemented in parallel. Work to remove obstructing debris from the first floor of the Units 1 to 3 reactor buildings has now been completed.
2. State of Operations concerning Spent Fuel Pools
Removal of fuel assemblies from the Unit 4 spent fuel pool (SPF) began in November 2013. Of the 1,535 fuel assemblies present in the Unit 4 SPF at the time of the accident, 728 had been transferred to the common pool as of April 23, 2014. TEPCO has announced that the transfer to the common pool of the three damaged fuel assemblies found thus far (one has a deformed handle and a deformed and damaged channel box, while the other two show leakage of radioactive materials from the interior of the assemblies) is possible with the transfer vessel currently in use.
In February 2014, after the removal operations had begun, it was found that the air dose rate at the removal site had risen due to radioactive Cobalt-60 in the water of the SPF and the influence of radioactive materials in the surrounding area. By April, TEPCO had reduced the exposure rate of workers at the site after this was pointed out by the Nuclear Regulation Authority (NRA). However, it should have been possible to predict at the time when removal operations began that the dose rate in the working area would be high and to have considered methods for its reduction. This is one very clear example of how TEPCO prioritizes schedule and costs to the detriment of the safety of the workers at the site.
The removal of large debris from the operating floor in the upper part of the reactor building of Unit 3 is now complete and measures to reduce the dose rate are currently being implemented. From the first half of FY2014, it is planned to begin the work of installing a cover for removal of the nuclear fuel. Work to remove debris from the SPF has also begun, and was scheduled to be completed during the first quarter of FY2014.
Dismantling of the cover currently installed around the Unit 1 reactor building is due to begin around the beginning of FY2014 in order to carry out the removal of debris from the operating floor. The reactor building cover was installed to suppress releases of radioactive materials, and there are concerns that its dismantling will increase the amounts of radioactive material releases. Moreover, since the filtered ventilation equipment fitted on the reactor building cover has been turned off since September 2013, in preparation for the dismantling work, releases of radioactive material to the atmosphere from Unit 1 have increased since last October.
While there is no great damage to the reactor building of Unit 2, high dose rates have made it impossible to confirm details of the state of the interior of the building. According to the plan, the construction method for the cover for removal of the nuclear fuel is due to be determined during the first half of 2014.
Fuel assemblies removed from the Unit 4 SPF are to be transferred to the common pool for cooling, but the common pool itself is already close to its capacity limit. Sound fuel assemblies that were being kept in the common pool are therefore currently being transferred to dry casks, which are placed in a temporary storage facility that has been in operation since April 2013. (1,006 assemblies had been transferred as of April 23, 2014.)
3. The Problem of Contaminated Water
According to an estimate by TEPCO, roughly 800 m3 of underground water are flowing into the reactors of Units 1 to 4 per day, 400 m3 of which is flowing into the reactor buildings and the remaining 400 m3 being released into the ocean.
To suppress the inflow of underground water, measures that include the following eight are being planned:
Management of the water level by restoration of the subdrains (scheduled to be operable starting from around September 2014).
Construction of an inland frozen earth barrier (an ice barrier created by burying refrigeration pipes at set intervals around Units 1 to 4 – scheduled to begin operation in the first half of FY2015).
Construction of an ocean side water barrier (scheduled to begin operation around September 2014).
Preventing water leakage from the Unit 1 to 4 reactor buildings through the (roughly 880) holes, etc. in the outer walls of the buildings (scheduled to be completed in FY2017).
Reduction of the volume of contaminated water using the Advanced Liquid Processing System (ALPS) to separate out the treated water still containing Tritium from other nuclides included in the waste effluent (scheduled to be fully operational in mid-2013, but experiencing frequent problems).
An increase in the number of waste water tanks (to be increased from 490,000 m3 as of March 25, 2014 to around 800,000 m3 by the end of FY2014).
A 10 m square scaled-down experiment conducted in April 2014 confirmed that the creation of a frozen earth barrier was possible, but many issues still remain. These include whether or not it will be possible to create a 500 m north-south and 200 m east-west frozen earth barrier; whether or not it will be possible to maintain the barrier in the long term; and whether or not land subsidence due to cessation of the inflow of groundwater will remain within assumed limits.
Further, TEPCO claims that, by use of the inflow suppression measures mentioned above, the amount of accumulating contaminated water will reach equilibrium by January 2021 at around 800,000 m3. However, TEPCO also says that the premise for this 800,000 m3 figure is ocean release of the water pumped up by the groundwater bypass and from the subdrains. Water pumped up by the groundwater bypass has been released into the ocean since April 2014, but already Tritium exceeding government safety standards has been detected in water from some of the groundwater bypass pumping wells.
(Hajime Matsukubo, CNIC)