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1st International Conference on Nuclear Risks
About the Possibility of 1F1 IC Piping Breakage
due to the Earthquake Jolts
Vienna
April 16, 2015
Mitsuhiko Tanaka
Mitsuhiko Tanaka
Science Journalist.
Born in Japan in 1943. Graduated from the Faculty of Mechanical
Engineering, Tokyo Institute of Technology.
Entered Babcock Hitachi K.K in 1968 and engaged in designing reactor
pressure vessels for BWRs as a structural analyst.
Performed detail stress analysis of the reactor pressure vessel of the
Fukushima Daiichi NPP Unit 4.
Since 1977, as a free-lance, lecturing science and technologies at colleges
and universities, and writing and translating scientific books.
Dec. 2011-July 2012, engaged in the investigation of the Fukushima
nuclear accident as a member of the NAIIC (The National Diet of Japan
Fukushima Nuclear Accident Independent Investigation Commission).
Topic
About the Possibility of 1F1 IC Piping Breakage
due to the Earthquake Jolts
The Point ;
• I suspect the hydrogen explosion might have
first occurred in the 4th floor, not in the 5th
floor, and then instantaneously triggered a
much more large-scale hydrogen explosion in
5th floor,
and if so, the possibility may surface that
the IC piping in the 4th floor might have
been broken by the earthquake jolts.
Contents
(1) Background of This Topic
(2) TEPCO’s Explanation about the Hydrogen Explosion in
Fukushima Daiichi Unit 1 Reactor Building
• Overview of Unit 1 Reactor Building
• TEPCO’s Explanation about How It Occurred
(3) Did It Really Occur in the 5th Floor?
(Some Enigmas Still Remain Unsolved)
(4) Discussion
abbreviation
•
•
•
•
•
•
•
•
•
•
1F1
Fukushima Daiichi NPP Unit 1
R/B
Reactor Building
IC
Isolation Condenser
RPV
Reactor Pressure Vessel
PCV
Primary Containment Vessel
D/W
Dry Well
S/C
Suppression Chamber
SRV Safety Relief Valve
PLR
Primary Loop Recirculation
(1) Background of the Topic
 The Japanese Government submitted a document to the
IAEA as early as June 20, 2011 (about three months after
the disaster) and said , “At present, damage caused by the
earthquake was concerned with off-site power supply
systems. Damage to safety-important systems and
components was not confirmed, and the plant was in a
manageable condition until the arrival of the tsunami.”
And , since then, this governmental view has been basically
unchanged.
 The NAIIC, on the other hand, says in its final report which
was sent to the National Diet of Japan in July 5, 2012, that,
particularly with regard to Unit 1 reactor, the
possibility of a small break LOCA due to the
earthquake ground motion could not be
completely denied.
 This judgment was deduced from the following very
important facts which were revealed by the NAIIC;
 A water leakage in the 4th floor was witnessed right
after the occurrence of the earthquake by several
subcontract workers who were, as it happened, working
in the 4th floor.
 In terms of the safety relief valves (SRVs) of Unit 1,
there isn’t any objective record to indicate that they
really functioned properly (such records are available
for Unit 2 & 3). In addition, no Unit 1 control room
operators heard loud SRV sound whereas the Unit 2 and
3 control room operators heard the sound repeatedly.
Saw Tooth-like Changes
in Unit 2 Reactor Pressure
Reactor Pressu re
13
(2) TEPCO’s Explanation about the Hydrogen
Explosion in Unit1 Reactor Building
• Overview of Unit1 Reactor Building
Fukushima Daiichi NPP
•
3
4
W
S
1
2
6
5
N
E
The Pacific Ocean
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Reactor Building for a BWR with Mark I Type PCV
Operating Floor
(5th Floor)
Spent Fuel Pool
PCV Dry Well
Ceiling Crane
Steel Plate
Reinforced Concrete
RPV
PCV Suppression Chamber
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Mark I Type PCV
Flnage Coupling
PCV Top Head
(Brwons Ferry Unit 1)
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Reactor Building Sections
During the Severe Accident
PCV Top Head
BWR3 (1F1) Reactor Building with the IC system
1F1 R/B
4th Floor Plan View
BWR 3 with the IC system
21
E
x
Equipment Hatch
• TEPCO’s Explanation about How It Occurred
Possible Hydrogen Leakage Paths
TEOCP says that 1F1 R/B Hydrogen Explosion occurred
in the 5th Floor.
Unit 1 Reactor Building right after the explosion
写真提供：（株）エアフォトサービス
東電公開写真より
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The Inside of the 4th Floor after the Explosion
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On-site Inspection of 1F1 R/B was performed on
February 22, 2015 by Niigate Prefecture’s Technical
Committee (*)
* TEPCO’s world largest Kashiwazaki-Kariwa NPP
is in Niigata prefecture, and this committee is
now re-examining the Fukushima accident from
various view points.
What caused this large scale destruction in the 4th floor?
Below is TEPCO’s explanation;
On the fourth floor of the R/B where the IC main unit is installed, there was a
hole on the north ceiling due to the hydrogen explosion on the fifth floor, and
removed insulation and debris were scattered on the north side of the top
portion of the IC thought to be caused by the explosion’s blast. The insulation
on the south side of the IC was severely torn and removed on the R/B
equipment hatch side (opening). It
is considered that the hydrogen
explosion on the fifth floor blasted through the opening and
damaged the insulation on the IC.
No insulation on the third or second floor was removed or scattered. No damage
was found on the IC main unit. No ruptured pipes, leakage from flanges, or broken
valves were found. No conditions were present that would suggest a pipe failed
and caused release of massive amounts of high pressure reactor steam. Judging
from the above, it was confirmed that there was no damage to equipment located
outside of the PCV that could have caused loss of reactor coolant.
Equipment Hatch
⇐ 5m x 5 m Equipment Hatch
Explosion’s Blast
From 5th Floor
(3) Did it really occur in the 5th floor?
Some enigmas still remain unsolved
Enigma 1
It was revealed by the NAIIC investigation that the
equipment hatch was not open when the explosion
occurred. It was closed with a heavy iron hatch
cover.
When the hatch was not open, how could such a
thing like “the hydrogen explosion on the fifth floor
blasted through the opening (equipment hatch) and
damaged the insulation on the IC” happen?
The equipment hatch was not open when the explosion occurred.
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Enigma 2
What is the raindrop-like pattern found here and
there in the 4th floor?
Enigma 3
The surface of the reinforced concrete ceiling in the 4th
floor was partly destroyed, and some of the re-bars
emerged. Furthermore, the ventilation duct in the 4th
floor was thoroughly smashed, and obviously, some
parts of the duct were crushed by an upward strong
impact force.
It is hard to imagine that these destructions were , as
TEPCO explains, caused by the explosion’s blast which
blew in to the 4th floor from the 5th floor.
Upward Impact Force
IC Vent Pipe
(4) Discussion
a. TEPCO says in its “Fukushima Nuclear
Accident Analysis Report” (published in June,
2012) “No damage was found on the IC main
unit…….”
But, in fact, TEPCO performed only a brief
visual inspection. Besides, the IC piping is
mostly buried under the debris. Therefore,
nobody can say, “No ruptured pipes, leakage
from flanges, or broken valves were found.”
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b. Basically, I strongly stick to the fact the NAIIC
revealed that , in the dark silence caused by SBO,
none of the unit 1 control room operators heard
“SRV sound” (* ) even once whereas unit 2 & 3
control room operators heard it repeatedly.
(*) SRV sound; a banging sound generated when an
SRV opens. A unit 2 operator said to the NAIIC, “The
SRV of unit 2 was very frequently in motion and I
heard a loud banging noise each time.”
c. It is widely accepted now that right after the SBO,
the IC system stopped functioning. Then, the
reactor pressure must have begun to rise.
Unit 1 has four (4) SRVs. Therefore, if the reactor
pressure did begin to rise and attained to their
operating pressure, any one of the four SRVs
would have opened automatically and loud SRV
sound must have been caused. But, as stated
above, nobody in the unit 1 control room heard
such SRV sound. I believe this strongly suggests
that, in fact, the reactor pressure of Unit 1 never
rose to the SRVs operating pressure after the SBO.
Saw Tooth-like Changes
in Unit 2 Reactor Pressure
Reactor Pressu re
59
d. I also stick to another very important fact. That is,
when the hydrogen explosion occurred, the
equipment hatch in the 5th floor was closed with a
heavy iron cover. Then, how did the explosion’s
blast blow in to the 4th floor?
I discussed this problem with TEPCO in Niigata
Prefecture’s Technical Committee, but TEPCO fails
to show how it could happen.
Incidentally, so far, the 5m x 5m iron cover has not
been found anywhere. It is missing.
e. I suspect that the hydrogen explosion “first”
occurred in the 4th floor, not in the 5th floor. As a
result, the cover was blown off upward, and the
vast amount of hydrogen that had piled up in the
5th floor was almost simultaneously exploded.
f.
One more important fact we have to pay attention
to is the raindrop-like pattern on the surfaces of the
IC tanks, piping, etc. I suspect that, before the
hydrogen explosion, there was, in the fourth floor, a
considerable amount of steam, which condensed
later into water.
g. And if so, the possibility may surface that the IC piping
in the fourth floor was broken by the earthquake jolts.
Since the IC piping is connected directly to the reactor
pressure vessel, if it had even a small breakage, high
temperature hydrogen, mixed with steam, would leak
from the breakage and stay in the fourth floor. And if
that had happened, at some point, a hydrogen
explosion might have been caused in the 4th floor due
to high temperature spontaneous combustion, and
the iron cover of the equipment hatch might have
been blown off upwards by the blast, which might
have instantly triggered the large-scale hydrogen
explosion in the 5th floor.
Thank you for your attention!
Backup material
Isolation Condenser system
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Saw Tooth-like Changes
in Unit 2 Reactor Pressure
Reactor Pressu re
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1F1 Reactor Pressure and D/W Pressure
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02:45
3/12
15:36
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Unit 3 Reactor Building Explosion
Photo / K.K Air Photo Service
Photo by TEPCO