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:
Operation of an “underground water
bypass” to reduce the inflow of underground water by pumping up underground water
using pumping wells installed on the land side of the site and
releasing the water into the ocean. (The pumping of groundwater began
in April 2014, after consent to the plan was obtained from the local
and prefecture fisheries cooperatives and JF Zengyoren, the national
Federation of Japan Fisheries Cooperatives).
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).
Removal of contaminated
water from trenches. (Removal of contaminated water from branch
trenches was completed in September 2013 and preparatory work to remove
contaminated water from the main trenches is now underway.)
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)
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