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 water temperature in the containment vessels and
the spent fuel pools, and from the releases of Xenon-135, it can be
estimated that the state of the reactors is stable. Up to now, Tokyo
Electric Power Company (TEPCO) has assessed releases of radioactive
substances into the atmosphere at 10 million Bq/h However, from the
fact that the state of the reactors is stable and that releases
continue to be at a low level, from May TEPCO has used the expression
“less than” 10 million Bq/h. (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.
2. State of Operations concerning Spent Fuel Pools
The removal of fuel assemblies from Unit 4 began in
November 2013, and of the 1,535 assemblies that were being stored in
the Unit 4 spent fuel pool at the time of the accident, 1,331 spent
fuel assemblies have been transferred to the common pool (announcement
of November 5, 2014). The transfer of all spent fuel from Unit 4,
including the three damaged assemblies that had been stored in the
spent fuel pool since before the accident, is due to be completed by
the end of November. Due to lack of capacity in the common pool, fresh
fuel in the Unit 4 spent fuel pool will be transferred to the Unit 6
spent fuel pool (scheduled for completion in December).
Removal of large debris from the operating floor of Unit 3 is now
complete. At present, measures are being taken to reduce the radiation
dose level, but the level is still far above the target of 1 mSv/h and
additional measures are now being considered. The construction work to
put in place the cover for removal of the fuel is scheduled to begin in
the near future. Moreover, on August 29, an accident occurred in which
the operating console of the spent fuel pool fuel handling machine and
other large debris fell into the pool.
The dismantling of the cover now in place around the reactor building
of Unit 1 was due to begin in July 2014 in order to carry out the work
of removal of debris from the operating floor, but this task has been
delayed due to the inadequacy of measures to suppress the dispersal of
radioactive substances. (Application of an anti-dispersal agent is now
being implemented and it is planned to start debris removal in March
2015.) The schedule for removing spent fuel has been delayed from 2017
to 2019, and removal of molten fuel from 2020 to 2025.
While there is no great damage to the Unit 2 reactor building, high
dose rates have made it impossible to confirm details of the state of
the interior of the building. The construction method for the cover for
removal of the nuclear fuel was to be determined during the first half
of 2014, but this decision has now been delayed until mid-2016.
Additionally, the fuel assemblies removed from the spent fuel pools in
each of the reactor buildings are to be stored in the common pool, but
since there are concerns over storage of damaged fuel a special storage
rack for damaged fuel has been installed.
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.
The following measures are being planned to suppress the inflow of groundwater:
1) A groundwater bypass to reduce the inflow of groundwater by pumping
the water up using pumping wells installed on the land side of the site
and releasing the water into the ocean. The pumping of
underground water began in April 2014, and the water pumped up is being
released into the ocean after confirmation that the radioactivity in
the water is below the standard. According to the plan, the amount of
groundwater prevented from flowing into the buildings is 50 m3/day.
TEPCO says that in combination with the water prevention measures in
the high-temperature incinerator building, a reduction of 90 m3/day was seen as of October 14.
2) Pumping up groundwater from the subdrains in the vicinity of the buildings and releasing the water into the ocean. In
addition to the existing 27 subdrains, 15 new subdrains have been
constructed, and experimental operations with these began from
September 2014. TEPCO says that it is releasing this pumped up
groundwater into the ocean after purification, but as the pumped
groundwater is clearly contaminated, opposition from nearby fishery
cooperatives has been steadfast, and implementation has not yet begun
as of end October.
3) Construction of an inland water barrier. A frozen earth
barrier is to be created by burying 749 refrigeration pipes and 151
temperature measuring tubes at set intervals around Units 1 to 4.
Installation work began in June and the barrier is scheduled to begin
operation in the first half of FY2015.
4) Construction of an ocean side water barrier. This was
scheduled to begin operation around September 2014, but has not yet
been implemented since it is conditional upon the release of
groundwater from the subdrains.
5) Prevention of water leakage from the Unit 1 to 4 reactor buildings through holes, etc. in the outer walls of the buildings. (Scheduled to be completed in FY2017.)
6) Operation of the Advanced Liquid Processing System (ALPS).
The existing three ALPS plants were scheduled to be fully operational
in mid-2013, but are experiencing frequent problems. Hot experiments
with the added ALPS3 plant and the high-performance ALPS1 plant, built
with a government subsidy, began in September and October.
7) An increase in the number of waste water tanks. Tanks to
store contaminated water and purified water that still contains Tritium
were to be increased to a total of around 800,000 m3 by the end of FY2014, but a further 100,000 m3 was added to this in July, when it was announced that storage for a total of 930,000 m3 of water would be installed.
8) Removal of contaminated water from trenches. Removal of
contaminated water from branch trenches was completed in September
2013. Preparatory work to remove contaminated water from the main
trenches began with an attempt to prevent water leakage by freezing the
water in the junctions between the buildings and the trenches, but as
it proved impossible to freeze the water, the insertion of packing
material began on October 16.
In addition, in order to prevent direct flow into
the ocean when contaminated water leaks from the storage tanks, of the
drainage canals onsite, the C drainage canal, which is directly
connected to the ocean, was altered in July to flow into the harbor.
(Hajime Matsukubo, CNIC)
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