Evaluation of the report of the Tokyo Electric Power Company
regarding the leakage from the double strainer differential
manometer for the water transfer pipes of the desalination system at
Fukushima Daiichi Nuclear Power Station
15 April 2015
Nuclear Regulation Authority, Japan
1. Overview
Around 11:05 on 6 February 2014, the double strainer differential manometer of the water
transfer pipe connected to the reactor water-injection equipment (hereinafter, referred to as the
"differential manometer") was found to be leaking RO-treated water.1
RO-treated water that had leaked from the differential manometer spilled out of the drain
water receiver of the mount installed in the lower part of the double strainer and permeated
the soil. Tokyo Electric Power Company (hereinafter, referred to as "TEPCO") confirmed that
there was no indication that the leaked water had flowed outside the wall of sandbags placed
around the mount, and there was no trace of water flow in the side ditch near the location of
the leakage.
In the same day, the Nuclear Regulation Authority (hereinafter, referred to as "NRA")
received the report regarding accidents and failures based on the Article 62-3 of the Act on
Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors from TEPCO.
Subsequently, the NRA received the report regarding causes and countermeasures of the
aforementioned event (the final report) from TEPCO as of 30 June 2014 (partially corrected
on 31 October 2014) and the NRA reviewed the contents and summarized the evaluation
result.
Report from TEPCO
http://www.nsr.go.jp/activity/bousai/trouble/20141031-4.html1Desalinated water that has been treated by reverse osmosis wastewater treatment equipment and is used for
nuclear reactor water injection (refer to Figures 1 and 2).
2. Overview of the report submitted by TEPCO
(1) Environmental impact assessment (expansion of contaminated water)
The amount of RO-treated water that leaked from the differential manometer was
approximately 600 L based on the changes in flowmeter readings. According to the
analysis results, the radioactivity concentration was estimated to be approximately 9.3 ×ばつ104Bq/L for total beta and approximately 3.8 ×ばつ 105
Bq/L for tritium. Based on these
results, the amount of radioactivity of the leaked water was calculated to be
approximately 5.6 ×ばつ 107
Bq for total beta and the amount of radioactivity including
tritium was calculated to be approximately 2.8 ×ばつ 108Bq.Regarding the radiation dose on the ground surface before and after excavation and
collection of soil, the effective dose due to gamma rays dropped from 0.18 mSv/h to
0.016 mSv/h, and the equivalent dose to the skin due to beta rays dropped from 0.07
mSv/h to 0.00 mSv/h.
Although some of the leaked water permeated the soil, it did not flow outside the wall
of sandbags placed around the leakage location. TEPCO therefore concluded that this
event caused no ocean contamination that raises any concerns about effects on health or
the environment. Also, recovery of the soil that leaked water permeated has been
completed.
(2) Investigation of the leakage situation
Before the event occurred, low-temperature conditions persisted inside the lagging
material of the differential manometer due to the low outside air temperature (around
−6°C) from around 22:00 on 5 February 2014 to around 4:00 on 6 February 2014.
Because of this, the RO-treated water inside the differential manometer exposed to
outdoor air via the body froze and increased in volume, which led to deformation of the
bonnet flange and loosening of the bonnet flange bolts. It is assumed that the bonnet
flange thus lost its sealing function and leakage occurred from the seal portion when the
frozen RO-treated water melted. Moreover, due to the fact that other instruments having
operating temperature ranges similar to that of the relevant instrument (operating
temperature range: −5 to 40°C, however, the fluid should not be allowed to freeze) had
been used successfully outdoors at Fukushima Daiichi Nuclear Power Station, it was
considered sufficient to attach lagging material as a freeze-proofing measure when
choosing such an instrument.
(3) Countermeasures
(i) Anti-freezing countermeasures
The entire double strainer, including the differential manometer, was covered with
an acrylic cover, and two fan heater units were placed under the cover. As a
countermeasure against leakage outside the system, a concrete dike was installed
around the double strainer. For instruments installed outdoors or in locations
where it is difficult to isolate them from outdoor air and which contain fluids that
may freeze, it was decided to install heaters to prevent freezing or draining of
water inside instrument main units and instrumentation piping in the future, and
these countermeasures were described in the "Flow chart for the implementation
of anti-freezing countermeasures." A total of 38 instruments (all installed in the
stagnant water storage facilities for Units 5 and 6) are subject to the
countermeasures based on said flow, and implementation of countermeasures has
been completed for 8 of these. Installation of freeze prevention heaters is
scheduled to be completed before the onset of winter in FY2014 for the 30
instruments for which countermeasures have not yet been implemented.
(ii) Establishment of the "Flow chart for the implementation of anti-freezing
countermeasures" as a manual
The "Flow chart for the implementation of anti-freezing countermeasures" will be
included among the "Anti-freezing countermeasures operation guidelines"
scheduled to be established in November 2014 as lower-tier documents in the
Basic Manual for Decommissioning and Basic Manual for Maintenance
Management.
3. NRA’s evaluation with regard to the report submitted by TEPCO and future
response
(1) Environmental impact (expansion of contaminated water)
Based on the fact that the leaked RO-treated water (radioactivity concentration: approx.
9.3 ×ばつ 104
Bq/L for total beta, approx. 3.8 ×ばつ 105
Bq/L for tritium) remained within the
sandbag wall placed around the leakage location and no contaminated water was
observed flowing outside the sandbag wall, so based on the facts above, the NRA
evaluates that there has been no contamination of the ocean that raises any concern
about influence to health and the environment.
(2) Exposure radiation dose
An evaluation of the effective dose due to gamma rays and the equivalent dose to the
skin due to beta rays for workers who patrolled the area where the differential
manometer was installed showed no significant differences between the values before
and after discovery of the leakage. The NRA therefore concludes that there has been no
exposure leading to concern (refer to Table 1).
(3) Countermeasures
TEPCO determined the assumed cause from the obtained information and formulated
anti-freezing countermeasures based on this cause. The NRA has evaluated them as
follows, countermeasures summarized by TEPCO shall be checked about its
implementation situation at an appropriate timing by safety inspection, etc.
(i) Anti-freezing countermeasures
Based on the "Flow chart for the implementation of anti-freezing
countermeasures," it was decided to install freeze prevention heaters and drain the
water inside the main units and piping of instruments for which countermeasures
were deemed necessary, and these countermeasures were described in the flow
chart.
The report stated that installation of freeze prevention heaters was scheduled to be
implemented before the winter of FY2014 for the 30 instruments for which
countermeasures had not yet been implemented among those subject to the
"Freeze prevention countermeasures implementation flow" (38 instruments in
total; all are installed in the stagnant water storage facilities for Units 5 and 6).
The NRA has confirmed in a meeting with TEPCO that implementation of the
countermeasures was completed on 28 November 2014.
TEPCO has analyzed the causes of the leakage and taken its measures responding
to them appropriately, the NRA thus considers them reasonable.
(ii) Establishment of the "Flow chart for the implementation of anti-freezing
countermeasures" as a manual
The report stated that the "Flow chart for the implementation of anti-freezing
countermeasures" would be included in the "Anti-freezing countermeasures
operation guidelines" scheduled to be established in November 2014 as lower tier
documents in the Decommissioning Basic Manual and Maintenance Management
Basic Manual. The NRA has confirmed in a meeting with TEPCO that the
guidelines were established on 27 November 2014 and came into force on 1
December 2014.
The relevant guideline states: "For instruments installed outdoors or in locations
where it is difficult to isolate them from outdoor air and which contain fluids that
may freeze, install heaters to prevent freezing and drain the water inside such
instrument main units and instrumentation piping in the future."
TEPCO has analyzed the causes of the leakage and taken its measures responding
to them appropriately, the NRA thus considers them reasonable.
Figure 1 Overview of the processing route for accumulated
highly radioactive water (extracted from the Secretariat of
the NRA’s document for a meeting with TEPCO)
Figure 2 Approximate position of the leakage (extracted from the Secretariat of
the NRA’s document for a meeting with TEPCO)
Category
Highly radioactive water/waste
Treated water (saltwater)
Treated water (concentrated saltwater)
Treated water (freshwater)
Treated water from polynuclear species
removal system, etc.
Freshwater
Treated water
(storage tank)
Sr treated water
(storage tank)
Polynuclear species
removal system, etc.
(incl. other purification
systems)
Reactor building
Filtrate watertankInjection water
tank (CST)
(buffer tank)
Concentrated
liquid waste
(storage tank)
Evaporative
concentration
equipment
Evaporative concentration equipment-treated
water
Treated water
(concentrated saltwater)
(receiving tank)
Treated water (freshwater) (receiving tank)
Desalination system-treated water
Desalination
system
(RO type)
Liquid waste
supply tank
Treatment system
(cesium adsorption apparatus)
(secondary cesium absorption
apparatus)
(decontamination system)
WasteFromRadioactive waste treatment
facilities
(process main building)
Radioactive waste treatment
facilities
(high temp. incinerator bldg.)
Condenser
Location of
leakage
Turbine building
Containment
Pressure vessel
Reactor
water-injection
equipment
Location of
leakage
Buffer tank
Solid line: Circulation loop using the condensate storage tank as the water
source
Dotted line: Circulation loop using the upland buffer tank as the water source
Secondary cesium
absorption apparatus
(SARRY)
Common suppression
pool water surge tank
(SPT)
Stagnant water
transfer system
Reactor water-
injection pump
Water treatment
system Desalination
system (RO)
RO-treated water
temporary storage tank
Treated water transferpumpFigure 3 Overview of the differential manometer (extracted from the TEPCO
report)
External appearance of the affected differential manometer
Note: No traces of damage (scratches, deterioration, etc.) on
its exterior
(Top)
Low pressure
side Diaphragm bellows
High pressuresideOverview of the affected differential manometer
The portion shaded orange (low pressure side) and the portion
shaded blue (high pressure side) are filled with water, and the
diaphragm bellows expands or contracts due to each portion's
water pressure, causing the differential manometer to output a
value.
Condition of the bonnet flange (top)
Note: The flange is slightly swollen and the
O-ring is projecting out of the flange. Reading of the affected differential
manometer when leakage occurred
Note: The indicator is off the scale.
Table 1 Radiation exposure dose evaluation results (extracted from the
Secretariat of the NRA’s document for a meeting with TEPCO)
Differences in workers'
exposure doses
Effective dose
(gamma rays)
Equivalent radiation dose
(skin, beta rays)
Annual dose limit: 50 mSv Annual dose limit: 500 mSv
Avg. dose per a
single entry [mSv]
Max. dose per a
single entry [mSv]
Avg. dose per a
single entry [mSv]
Max. dose per a
single entry [mSv]
Patrols of the area where the differential manometer is installed (including patrols that also include other areas)
Before finding leakage
(30 Jan.–5 Feb.)
0.07 0.21 0.15 1.2
On the day the leakage
was found (6 Feb.)
0.04 0.11 0.02 0.1
After finding leakage
(7–13 Feb.)
0.04 0.19 0.01 0.1
Table 2 Content and completion time of countermeasures (extracted from the
Secretariat of the NRA’s document for a meeting with TEPCO)
Category Countermeasure Date completed
Anti-freezing
countermeasures
Replacement of the affected differential manometer with a new
product
9 April 2014
Installation of an overall cover (acrylic) and fan heaters (2 units) 17 June 2014
Replacement of the sandbag wall placed around the double
strainer differential manometer with a concrete dike
23 June 2014
Installation of freeze prevention heaters and draining of the
water inside instruments for Units 5 and 6 (30 locations)
28 November 2014
Establishment of the
"Flow chart for the
implementation of
anti-freezing
countermeasures" as
a manual
Revision of the "Flow chart for the implementation of
anti-freezing countermeasures"
4 September 2014
Formulation of the "Flow chart for the implementation of
anti-freezing countermeasures" 27 November 2014