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18 www.nuclearplantjournal.com Nuclear Plant Journal, March-April 2009
Steam Generators with Tight
Manufacturing Procedures By Ei Kadokami, Mitsubishi Heavy
Industries.Ei Kadokami Ei Kadokami is the deputy general
manager of Mitsubishi Heavy Industries
(MHI), Kobe Shipyard & Machinery
Works. In 1978, he graduated from
the faculty of nuclear engineering,
the University of Kyushu and joined
MHI. He has been working in the
nuclear engineering field throughout
his career. His expertise includes
entire Pressurized Water Reactor
power plants technology(basic plan,
design, manufacture, construction , and
maintenance).
Responses to questions by Newal
Agnihotri, Editor of Nuclear Plant
Journal.
1. What is the life expectancy of the
future generation steam generators cur-
rently manufactured by Mitsubishi Nucle-
ar Energy Systems?
Our steam generator is designed
and verified for 60 years of lifetime. “60
years” is just design lifetime and the
limitation of integrity is considered to be
much longer than that.
2. What are Mitsubishi Nuclear Energy
Systems’ recommendations to its clients for
ensuring optimum life and functionality
of its steam generators regarding the
following:
a. Maintaining the water chemistry
for the steam generators (primary as well
as secondary side).
MHI recommends that the clients
control the water chemistry in accordance
with EPRI guidelines.
b. Preventive maintenance practices
One of the most important preventive
maintenance practices is control of sec-
ondary water chemistry. Since tube ma-
terial of Inconel 690 has high resistance
against corrosion, the corrosion phenom-
ena is not a concern for our steam genera-
tor. On the other hand, the secondary side
scale management is important for degra-
dation of thermal hydraulic performance.
For this concern, MHI recommends that
the pH of secondary side is maintained
high (9.2 or more), which results in very
low iron concentration rate in the steam
generator. Even if the secondary water
pH control is performed, scale could be
deposited. MHI recommends that chemi-
cal cleanings should be performed in this
case.
c. Ensuring minimal leak rate of
reactor coolant into the secondary loop.
Our steam generator has Inconel 690
tubes to have enough corrosion resistance
against several degradation modes, which
could avoid the leakage due to tube
degradation.
For tube to tubesheet joint region,
seal weld is performed and the weld is
designed to have the structural integrity
to withstand the design pressure. Tube
expansion is also performed. This expan-
sion procedure is verified to avoid the
leakage even if the seal weld is not per-
formed.
d. Avoiding potential for in-service
rupture.
Our steam generators are designed
to have enough resistance for several
degradation modes as follows:
• Tube material is alloy 690 which has
high resistance against corrosion.
• Anti-Vibration bar (AVB) and tube
support plates (TSP) are designed to
have enough margins against fretting
wear.
On the other hand, all heat transfer
tubes are inspected by Eddy Current
Testing (ECT) in outage to find the
degraded tubes. The criteria for plugging
is determined by considering the tube
walls thinning growth until the next
outage, ECT measuring degradations and
other uncertainties, ensures that there is
no in-service tube rupture.
e. Chemical cleaning methods.
Chemical cleaning is taken into
account for material selection for US-
APWR. The material for the steam
generators of US-APWR is suitable for
general chemical cleaning so the owner
of the plants can use general commercial
chemical cleaning method.
f. Reactor coolant temperature to
insure least corrosion
There is no clear criterion of
reactor coolant temperature for tube
degradation.
No corrosion is observed when the
temperature is less than 617°F so the
thermal design temperature at full power
is 617°F.
g. Injection of chemicals into the
secondary side water.
High pH control is recommended
to maintain the low iron concentration
rate, which could avoid the performance
degradation.
h. Recommended tools and techniques
for sleeved tubes or other technologies to
defer replacing the steam generators.
MHI has techniques for plugging and
sleeving degraded tubes. Normally the
plugging is recommended. However, we
have several techniques and experiences
for sleevee tubes such as laser weld
sleeves.
To prevent performance degradation,
chemical cleaning and secondary water
chemistry control (high pH) is recom-
mended for long life operation.
3. What are the recommended techniques
for repairing tubes allowing degraded
tubes to remain in operation?
The sleeve tubes technique such as
Nuclear Plant Journal, March-April 2009 www.nuclearplantjournal.com 19
laser weld sleeves can allow degraded
tubes to remain in operation. The tubes
may also be plugged.
4. What instrumentations are provided
by Mitsubishi Nuclear Energy Systems
with its steam generators to monitor
degradation in a timely manner and
accurately?
Ceradyne Boron Products (formerly Eagle Picher)
has provided the global commercial nuclear power
industry with high purity stable isotopes
for more than 30 years.
- Enriched Boric Acid (E10BA)
- 7Lithium Hydroxide-Monohydrate
- Depleted Zinc
- Enriched Sodium Pentaborate (NaP10B)
Highest Purity Stable Isotopesfor Nuclear Power PlantChemistries
714-384-9465 [email protected]
www.ceradyneboron.com
3D-CAD Model of Steam Generator
for US-APWR
During operation,
• Steam pressure is monitored to
evaluate fouling factor of tubes.
• Leak rate is watched by N16
monitor.
During outage,
• Tube ECT inspection using Intelligent
ECT method can be applied for quick
and detailed tube inspections.
5. What in-service inspection is
recommended during:
Plant operation?
Leak rate, steam pressure and loose
parts are monitored.
Outages?
In outage, all heat transfer tubes can
be inspected by Eddy Current Testing and
the weld lines of pressure boundary can
be inspected by Ultrasonic Testing (UT).
6. How has Internet and the evolution
of Information Technology in the last
30 years helped Mitsubishi Nuclear
Energy Systems provide a state of the
art instrumentation to ensure efficient
and productive operation and detect
degradation during operation and
refueling?
The information on degradation ex-
perience is stored in the electronic data-
base, which can be instantaneously ac-
cessed by internet. MHI recommends the
best operation and maintenance method
based on the investigation of the data-
base.
Tube ECT source data can be
transferred from the job site to MHI Kobe
by internet and the data can be analyzed
immediately; earlier it used to take one or
more days to carry the data media from
job site to MHI.
7. Please provide any other design,
operation, and construction highlights,
which makes you believe that Mitsubishi
Nuclear Energy Systems future generation
steam generator has an edge over other
steam generator technologies?
Tube P/D (pitch/outer diameter)
is narrower than others so that the tube
bundle and the Steam Generator itself are
smaller, which is the first feature.
The second feature MHI would
like to emphasize is that MHI has not
experienced any significant degradation
in recent design.
Alloy 690 is used for almost
all steam generators and has high
resistance against corrosion, but some
steam generators fabricated by other
manufacturers have wear caused by tube
vibration in the U bend region. No tube
wear has been experienced in recent MHI
steam generators because not only AVB
and TSP are designed to have enough
margin against fretting wear but also the
manufacturing procedure is appropriate to
control gaps between the tube and AVB.
Contact: MHI Nuclear Energy
Systems Headquarters, 16-5, Konan2-
choume, Minato-ku, Tokyo, Japan; email: