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 inquiry@ceradyneboron.com

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:

reply-nuclear01@mhi.co.jp.