Approvsllssue Course 228 - Module 3 - Selection and Specification of Materials

for Nuclear Applications

NOTES & REFERENCES

Module 3

SELECTiON AND
SPECIFICATION OF
MATERIALS FOR NUCLEAR
APPLICATIONS

OBJECTIVES:
After completing this module, you will be able to:
PlOW. , ~
3.1 State sbt important factors for selection of metals/alloys for in-core
• -0" - -...-
structural applications.
Page 5 <=> 3.2 State the code that has the ultimate authority for materials used for
nuclear and pressure vessel applications.
3.3 Identify from a given list of alloys, the alloy used for the following
system components in our CANDU generating stations:
Page 5 <=> a) calandria tubes,
Page 5 <=> b) pressure tubes,
Page 6 <=> c) fuel sheathing,
Page 6 ¢:::> d) end fittings,
ft ___ ,...--- __ 1 __ ....:I~ _______ 1
-~
rag.: 0 ~ C) ~WW1UCUl VC:S:SCl,

Page 7 ¢:::> t) primary heat transport system piping,

Page 8 <=> g) turbine blading,
Page 8 ¢:::> h) turbine shaft/casing,
Page 9 ¢:::> i) main condenser tubing (2 alloys).
Page 9 <=> j) heat exchangers (3 alloys),
Page 9 ¢:::> k) steam generator tubing (2 alloys).

* * *

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Approval Issue Course 228 - Module 3 - Selection and Specification of Materials
for Nuclear ApplicatioN

NOTES & REFERENCES

INSTRUCTIONAL TEXT

GENERAL MATERIAL REQUIREMENTS

Materials for nuclear reactors must simultaneously withstand the effects of
high temperature, intense gamma radiation and bombardment by neutrons.
At the same time they must be capable of containing the highly radioactive
fission product~ produced in the nuclear reaction. For thermal reactors,
efficient use of neutrons is essential and, therefore, core materials must
capture very few neutrons or, to maintain a chain reaction, the amount of
fissile material must be increased.
Obj. 3.1 ¢:::) The major requirements for an in-core structural material arc:
(a) good mechanical properties under all conditions of operation,
(b) low absorption of neutrons,
(c) stability under intense gamma and neutron irradiation,
(d) low rate of corrosion with fuel, moderator and coolant,
(e) ease of fabrication,
(0 acceptable cost.
Alloy and stainless steels would be ideal structural materials if it were not for
their high level of absorption of neutrons. To use these metals, the fuel would
have to be enriched with fissile material to ensure sufficient neutrons for a
continuous chain reaction. The selection of materials is basically limited to
carbon (graphite), beryllium, magnesium, zirconium and aluminum, if fuel
enrichment is not contemplated.
For the cANDU system graphite is porous and not suitable for containing
our moderator or coolant. In addition it has poor mechanical properties under
complex loading. Beryllium is brittle at room temperature, difficult to
fabricate and expensive. Aluminum and magnesium have relatively low
melting points (650-660°C) and have insufficient strength for components
such as pressure tubes, especially if elevated temperatures are involved.
Zirconium is the most suitable material for in-core components. Pure
zirconium has insufficient strength or creep resistance so it must be alloyed
to improve mechanical properties. Alloy additions are small, and do not
increase neutron absorption such that fuel enrichment is necessary.
Although we are principally interested in the materials used for structural
applications. it is ~lso wortJlwhile noting the major requirements for other
essential components of our thermal reactor.

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Approval Issue Course 228 - Module 3 - Selection lIIId Specification of MlIlerials
for Nuclear ApplicatioJII

NOTES & REFERENCES

They arc as follows:

i. Fuel Material Requirements:

(a) sufficient fissile material to maintain a chain reaction,
(b) dimensional stability,
(c) high melting point,
(d) compatibility with cladding and coolant

2. Control Material Requirements:

(a) high absorption of neutrons,
(b) dimensional stability under heat, intense gamma and neutron irradia-
tion,
(c) good corrosion resistance.

3. Coolant:
(a) efficient heat transfer,
(b) low neutron absorption,
(c) low rate of chemical reaction with surroundings,
(d) stability under heat, intense gamma and neutron irradiation.

REGULATION OF STANDARDS
Much of the work (design and operational) in our plants is regulated or
affected by standaids and codes. There arc a number of standards and codes
with complex inter-relationships but essentially there are two regulatory
authorities.

Atomic Energy Control Board (AECB)

This is a Federal authority authorized by "The Atomic Energy Control Act"
to develop, control, supervise and license the production, application and use
of atomic energy.

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Approval Issue Course 228 - Module 3 - Selection and Specification of Materials
for Nuclear Applications

NOTES & REFERENCES

Inspection Branch - Ministry of Consumer and Commercial

Relations (MCCR)
ntis is a plUvincial authoriLf which is authorized by ''The Boilers and
Pressure Vessels Act" to control the safety of boilers and pressure vessels in
Ontario.
The primary standards and codes listed below are those which the regulatory
authorities, AECB and MCCR, generally accept. They can refer to other
codes and standards and often are used as references in many other manuals
and standards.

Canadian Standards Association (CSA)

The CSA has a number of codes covering construction and inspection of
boilers, pressure vessels, and CANDU nuclear plant components, quality
assurance in nuclear plants and manufacturing of components, control and
safety systems, and environmental radiation protection.

Canadian General Specifications Board (CGSB)

These standards cover requirements for certification of non-destructive
testing personnel.

American National Standards Institute (ANSI)

ANSI publishes a code covering pn::ssure piping except nuclear piping.

Institute of Electrical and Electronics Engineers (IEEE)

IEEE has standards covering protection signal systems.

American Society of Mechanical Engineers (ASME)

ASME is the most extensive code to which many other standards refer. It has
been accepted across Canada as a basic document and has se~eral sections
covering design and operation of power boilers, heating boilers and pressure
vessels; material specifications, specifications for nuclear power plant
components and welding and brazing procedures.
ASME - Section II gives m~terial specifications for ferrous and non-ferrous
materials and for welding rods, electrodes and filler metals. These
specifications are often quite extensive covering chemical composition,
impurity limits. basic mechanical properties. method of manufacture and test
procedures.

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Approval Issue Course 228 - Module 3 - Selection and Specification of Mattrials
for Nuclear Appli..-tiona

NOTES & REFERENCES

American Society for Testing and Materials (ASTM)

Obi. 3.2 ¢::> ASTM also issues material specifications which in general are the same as
ASME specifications. However, ior nuciear and pressure vessei appiica-
tions ASME specifications are used in preference to ASTM specifications.

MATERIALS USED IN CANDU UNITS

Pressure and Calandrla Tube and Fuel Sheathing
Material
These components are all within the core, and require low absorption of
neutrons. They will. therefore, be zirconium alloys. Zircaloys were the first
zirconium alloys developed for reactor applications and are essentially
alloys of zirconium plus tin.
Obi. 3.3 a) ¢::> . Zircaloy-2 contains 1.5% Sn, 0.12% Fe. 0.10% Cr and 0.05% Ni. It is not
heat treatable and therefore. mechanical properties such as strength and
creep resistance are improved by cold working (defonnation). It is used for
all calandria tubes. Initially. pressure tubes were made from Zircaloy-2.
However. pressure tube perfonnance in an early (lower power) reactor
suggested that pressure tube creep rates would increase for higher power
reactors. This led to a change in pressure tube material and design. The new
material presented difficulties in welding and. since calandria tubes required
seam welding during fabrication. they remained as Zircaloy-2. This was
because the operating conditions for calandria tubes were much less severe
(lower temperature and pressure) than for pressure tubes.
Obi. 3.3 b) ¢::> The new matc;rial, Zirconium-21f1% by weight Niobium (Zr-21/2 Nb), was
developed primarily as a stronger zirconium. alloy for pressure tube
applications. Through cold work, higher strength could be developed in
thinner sections and pressure tube wall thickness was reduced by almost
I mm. This reduced parasitic absorption of neutrons by the pressure
tubes. and improved fuel burn-up. However, even though Zr-2 1h Nb was
stronger, the creep rate of the thinner pressure nibe has not been less

Zr-2 1h Nb is heat treatable. Strength can be increased by a process known as

precipitation hardening. This involves rapid cooling from a single phase
region (one type of crystal) into the two phase region (two types of crystal)
and allowing controlled precipitation and growth of the second type of
crjsta!. This may be a fut'w-c possibilirj in the production of press\U~ tubes.

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Approval Issue Course 228 - Module 3 - Selection and Specification of Materials
for Nuclev Applications

NOTES & REFERENCES

Obi. 3.3 c) c;:::> Zircaloy-4 contains 1.5% Sn, 0.20% Fe and 0.10% Cr. Leaving out the Ni
improved corrosion resistance and ductility and produced a material suitable
for fuel sheathing. The ductility is important so that the fuel sheathing can
accommodate volume changes in the fuel due to thennal expansion and
build-up of fission product gases. Oxidation resistance is similar to
Zircaloy-2 but hydrogen pick-up* is only about 1/3 - 112 ofZircaloy-2.

Calandrla, End Fittings and PHT Piping Material

These materials are all outside the core and, therefore, absorption ofneutrons
is not a concern. Mechanical properties, corrosion resistance and cost are the
h-nPOlUUit factors in selecting alloys for these applications, ai"id therefore,
.alloy steels and stainless steels are important contenders.
Stainl~ss
- ~~- - - ~te~l
- - - - i~- ~tm"'~
and VeTV corrosion resistant but has a wid~ rani!e of
- -- - IIIiiiII' -- -- - ~ - - - - - - - - - - ...

mechanical properties depending on the composition. The most influential

alloying element present in all stainless steels is chromium. Some also have
significant amounts of nickel. The amount of chromium and the presence or
absence of nickel detennine the type of crystal structure (and therefore.
material properties). In our nuclear stations we use two types of stainless
steel: martensitic and austenitic.
Obj. 3.3 d) <=> Martensitic stainless steels may also be called heat treatable or 400 Series**.
Generally they contain less than 14% Crwith variable carbon content. These
steels can be rapidly cooled from a high temperature to fonn martensite, a
non-equilibrium crystal structure in steels which is very hard, strong and
brittle. To relieve brittleness, the steels are tempered, allowing a controlled
decomposition of some of the martensite to equilibrium crystal structures.
Martensitic steels are weldable and easily fabricated and machined (these
operations are generally perfonned before heat treatment). End fittings on
pressure tubes must be very corrosion resistant to prevent deterioration of the
seal between fueling machine and pressure tube. They also must be strong
and hard as the joint between end fitting and pressure tube is fonned by
rolling or plastically defonning the pressure tube into grooves in the end
fitting to provide leak tightness. Martensitic stainless steel is used for
pressure tube end fittings. .
Obj. 3.3 e) <=> Austenitic stainless steels are also known as 300 Series...... or 18-8 stainless.
These steels contain nickel as well as chromium, are non-heat treatable and
have low carbon (less than 0.15%). Chromium provides the corrosion
resistance whereas nickel acts to stabilize the austenite crystal structure,
which nonnally exists only above 700-800°C.

* Hy4rogen ill zirconium alloys causes delayed hydride cracking and will be discussed in Mod~
5 a/this course.
.. American Iroll and Steel Institute (AlSl) designQtioli.

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Approvsllssue Course 228 - Module 3 - Selection and Specification of Materials
for Nuclear Applications

NOTES & REFERENCES

Austenite has certain properties not found in the normal room temperature
crystal structures of steels. It is non-magnetic and does not show a ductile
brittle transition temperature. If we recall that intense neutron radiation
raises the ductile brittle transition temperature. it will become obvious that in
radiation environments it is desirable to use materials without this property.
especially if operating at ambient temperatures. The calandria is subjected
to intense radiation fields and operates generally around 60°C. so austenitic
stainless steel was selected as the best choice of material. The excellent
corrosion resista.l1.Ce of Litis staiPJess steel was also a factor LYl its selection.
Db}. 3.3 f) <=> Plain carbon steels are among the most versatile and least expensive
constnlction materials. However. they have very poor corrosion resistance.
especially in an aqueous environment with ready access to oxygen. This poor
corrosion resistance is a result of the brittle porous oxide film, which does
little to protect the metal from fw....u'1cr corrosive ar..ack. L, stainless steels, tliC
presence of chromium helps to fonn a strong adherent oxide film. which is
relatively impervious. In plain carbon steels. it is possible to improve the
perfonnance of the oxide film by providing optimum conditions for growth
and limiting the amount ofoxygen available. In the heat transport system this
treatment is possible and plain carbon steels were selected for the primary
heat transport piping. Corrosion control is effected by maintaining the heat
transport fluid at a pH of 10 (by addition of LiOD) and addition of Ih to
recombine with Oz formed by radiolysis*.

Turbine Shaft, Casing and Blading Materials

The many parts of a steam turbine work under varying conditions of service.
and effective design involves selection ofappropriate materials for each part.
The CANDU reactor (as do other water cooled reactors) produces low a
pressure. low temperature saturated stearn. The turbine should be capable of·
handling large volumes of wet steam for efficient power generation.
Maximum stearn pressure and temperature will be about 4 MFa and 250°C
and the highest moisture content expected about 12%. Important character-
istics for selection of materials for turbine components will include:
a) high strength and toughness to withstand the pressure of stearn and other
imposed loads.
b) good wear resistance to withstand the erosive effects of entrained
moisture in the steam.
c) good corrosion resistance.
d) good creep properties to withstand stresses developed by rotational
motion.

• This is discussed in 1M 224 Chemistry course.

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Approval Issue Course 228 - Module 3 - Selection and Specification of Materials
far Nuclear Applications

NOTES & REFERENCES

Low alloy and stainlcss stccls gcncrally exhibit the properties required, and
are important materials for turbine components.
Obi. 3.3 g) ¢:> Ma...-rensitic stainless steel, as noted earlier, has excellent corrosion
resistance, and can be treated to improve mechanical properties. Turbine
blading basically converts the heat energy ofthe stearn to kinetic energy. The
biading will be subjected to substantial cennifugal forces deveioped during
rotation, as well as the impact ofhigh pressure, high velocity steam. Moisture
entrained in the stearn and moisture from condensing stearn will tend to .
erode the blades. Martensitic stainless steel is hard and wear resistant and
has good creep properties as well as high strength. It is used as both high
pressure and low pressure turbine blading. Design and selection of
material for blading is so sophisticated that different blading stages use
different grades of martensitic stainless steel.
Obi. 3.3 h) <=> Low alloy steels are steels with alloying elements such as Ni, V, Cr, Mo, Si,
Mn, Nb, added to a total of less than 5% to improve mechanical properties.
Casings are essentially pressure vessels, but also transmit imposed loading
(thennal and static) to the foundations, while maintaining alignment of the
turbo-generator unit. They support flXed elements and, in the case of blade
or rotor failUre, act as containment for pieces of rotating equipment The
shaft (rotor) is the primary rotating element and carries the moving blades. It
is subjected to centrifugal loading through rotation, torque due to work done
on moving blading by the steam, and high temperatures and pressures. In
addition, because of its weight and support problems, it should have a high
degree of stiffness. The properties required in casings and shafts are found
in a group of chrome-moly steels. They contain essentially 1.5 - 2.5% Cr
and 0.5 - 1.0% Mo with smaller additions of V, Ni, etc. Chromium increases
corrosion resistance adds to high temperature strength and increases the
steels ability to harden. Molybdenum deepens hardening, raises high
temperature and creep strength and improves abrasion resistance.

Steam Generator, Condenser and Heat Exchanger

Tubing
The primary purpose of heat exchangers is to transfer heat energy from one
fluid to another without intimate contact or mixing between the fluids. The
materials used for heat exchangers must therefore have:
a) goodthennal conductivity,
b) adequate mechanical properties at the operating temperature,
c) good corrosion resistance.
d) reasonable wear/erosion resistance.

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ApprovallssU9 Course 228 - Module 3 - Selection II'Id Specification of Maleriala
for Nuclear Applicationa

NOTES & REFERENCES

Traditionally these properties have been best met by copper alloys, but the
more severe operating conditions encountered are forcing a move to other
materials such as Inconel, stainless steel and even titanium. In CANDU
stations, the major heat exchanger materials are Admiralty Brass, Cupnr-
Nickels, Monel and Inconel.
Db}. 3.3 i) <=> Admiralty Brass is basically a cartridge or 70/30 brass with 1 - 2% tin added
for improved corrosion resistance. It is very ductile with excellent cold
fanning characteristics. It has good thennal conductivity (1/3 that of copper)
and excellent corrosion resistance in fresh, salt and brackish water.
However, like all copper alloys it is susceptible to attack by dissolved oxygen
and carbon dioxide and it also suffers impingement or pitting at high fluid
velocities. Because 'of its advantages, Admiralty Brass was selected for
tubing in the main condensers. However, in practice we are finding that
there is excessive erosion of the tubing (primarily resulting from condenser
design) and, for Broce B and Darlington, stainless steel tubing has been
selected.
Obj. 3.3 JJ <=> CupnrNickels are alloys of copper and nickel. These alloys have lower
thennal conductivity than brasses eho that of copper) but much improved
resistance to impingement attack. They generally have the best resistance of
all copper alloys to aqueous corrosion and are more immune to stress
corrosion cracking. The 90/10 and 70/30 Cupro-nickels are used for heat
exchanger tubing in high pressure feedheaters and the moderator heat
exchanger. However, the trend in newer stations such as BNGS-B is to
stainless steel and nickel chromium alloys such as Incoloy.
Obj. 3.3 k) <=> Mooel, like Cupro-nickels, is an alloy of nickel and copper but is
predominantly nickel; average composition being 70% Ni and 30% Cu.
Thermal conductivity in monels is somewhat lower than Cupro-Nickels but
they have improved corrosion resistance under high flow conditions. Monels
...... "'..,...;,."1
.... W' yllAA -.. 1,,
J ......;
&'W'u:
..t .. nt tn ",..u-it..hnn
~ -"
..nA
,...
;n>",;nlY"'Yn...nt
y v_. _...........n
ow•••..,••• hnt w ""_ ..;h"...
_ _ .tn
.-

the presence of oxidizing agents such as Fe+++ or Cu++ ions or dissolved

oxygen. Steam generators and bleed coolers are heat exchangers with rather
severe service conditions and Monel was selected as tube material at
Pickering A&B.
Inconel, like some types of stainless steels, is an alloy of iron, chromium and
nickel but it is nickel based. It suffers almost no corrosion in flowing water
(salt, natural or brackish). In stagnant conditions, where chlorides, phos-
phates and hydroxides can concentrate, Inconel is susceptible to pining and
stress cOITOsion cracking. In pure water it is not as sensitive as Monel to the
presence of dissolved oxygen. This latter property led to the selection of
Ineonel as steam generator tubing at BNGS-A&B where boiling is
allowed in certain fuel channels. With boiling occurring, it becomes more
difficult to suppress dissolved oxygen which is produced by radiolytic
breakdown of water.

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Approval Issue Course 228 - Module 3 - Selection and Specification of Materials
for Nuclear Applications

NOTES & REFERENCES

SUMMARY OF THE KEY CONCEPTS

• For metal/alloy in-<:ore structural applications, six important factors for
selection exist:
a) good mechanical properties under all conditions of operation,
b) low absorption of neutrons,
c) stability under intense gamma and neutron iITadiation,
d) low rate of corrosion with fuel, moderator and coolant,
e) ease of fabrication,
t) acceptable cost.
• For nuclear and pressure vessel applications, ASME specifications are
the ultimate authority.
• Calandria tubes, which must be seam welded during fabrication, are
made from Zr-2. The less severe operating conditions experienced by
the calandria tubes allow them to be made from this lower strength Zr
alloy.
• A strong, heat treatable alloy of Zirconium known as Zr-2 1h.Nb is used
for pressure tubes in CANDU reactors. It offers lower parasitic
absorption of neutrons than earlier pressure tube materials because its
higher strength allows a thinner tube wall.
• Zircaloy-4, a more cOITOsion resistant and ductile Zr alloy, is ideal for
use as fuel sheathing.
• CANDU end fittings must have high cOITOsion resistance, strength and
hardness. Martensitic stainless steel is suitable, since it is easily
machined and is heat treatable to give excellent strength and hardness.
• Since austenitic stainless steel does not show a ductile/brittle transition
. temperature under neutron irradiation, it is used for the calandria vessel.
• Primary heat transport piping is made from plain carbon steel. This is a
relatively inexpensive material and its corrosion can be readily
controlled by chemical conditioning of the coolant.
• The excellent hardness and creep resistance of martensitic stainless steel
allows it to perform well as both high and low pressure turbine blading.
• The strength and operating temperature demands of turbine shafts and
casings require them to be made from special chrome-moly low alloy
steels.
• Main condenser tubing in newer stations is made from stainless steel
because of its improved erosion resistance. Older stations used
Aif'11iralty Brass, chosen for its good therm!ll conductivity 2.'1d corrosion
resistance.

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Approval Issue Course 228 - Module 3 - Selection and Specification of Materials
for Nuclear Applications

NOTES & REFERENCES

• Heat exchanger tubing material selection has undergone a similar

evolution from cupro-nickels in early stations to stainless steel in newer
stations.
• Pickering stations have stearn generator tubing made from a nickel-cop-
per.alloy known as Monel. With boiling present in fuel channels at the
Bruce stations, stearn generator tubing is made from Inconel. Inconel
has improved resistance to effects caused by dissolved oxygen.

Page 12 ¢:> You can now do assignment questions 1-13.

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Approval Issue Course 228 - Modulo 3 - Selection and Specification of MaterialJ
for Nuclear Applications

NOTES & REFERENCES

ASSIGNMENT
1. State six factors considered important in the selection of material for
core structural components.
2. State the code that has the ultimate authority for materials used for
nuclear and pressure vessel applications.
3. State the alloy used for calandria tubes in CANDU generating stations.
4. State the alloy used for pressure tubes in CANDU generating stations.
S. State the alloy used for fuel sheathing in CANDU generating stations.
6. State the alloy used for end fittings in CANDU generating stations.
7. State the alloy used for calandria vessels in CANDU generating stations.
8. State the alloy used for primary heat transpon system piping in CANDU
generating stations.
9. State Lite alloy used for t-~bine bladLl'1g L'l C&J\NDU generating stations.
10. State the alloy used for turbine shafts/casings in CANDU generating
stations.
11. State two alloys used for main condenser tubing in CANDU generating
stations.
12. State three alloys used for heat exchangers in CANDU generating
stations.
13. State two alloys used for steam generator tubing in CANDU generating
stations.

Before you move on to the next module, review the objectives and make
sure that you can meet their requirements.

Original by: A. Wadham, ENTD

Revised by: P. Bird, WNTD
Revision: R-2. June 1993

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