NPTEL – Chemical Engineering – Nuclear Reactor Technology
Selection of Materials for Reactor
Internals
K.S. Rajan
Professor, School of Chemical & Biotechnology
SASTRA University
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
Table of Contents
1 CLADDING ....................................................................................................................................... 3 2 REFLECTOR .................................................................................................................................... 4 3 CONTROL ELEMENTS .................................................................................................................. 4 4 SHIELDING MATERIALS ............................................................................................................. 5 5 REFERENCE/ADDITIONAL READING ..................................................................................... 5 Joint Initiative of IITs and IISc – Funded by MHRD
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
In this lecture, we shall discuss the selection criteria for materials used in internals of
nuclear reactors.
At the end of this lecture, learners will be able to
(i)
(ii)
(iii)
list the desirable properties of cladding material
list the materials suitable for cladding in nuclear reactors
list the reflector materials
1 Cladding
The purpose of cladding in a nuclear reactor is of two-folds:
(i)
(ii)
Cladding gives the physical configuration by housing fuel pellets
Cladding retains the fission products and prevents direct contact between
coolant and fuel
The material of choice for cladding must possess ductility, impact strength and creep
adequate enough to maintain cladding unaltered during the operation of the reactor.
The material must be suitable for fabrication in the desired form. The resistance to
corrosion by coolant must be high for a material to be used as cladding. The cladding
material must possess high melting point. Since cladding separates fuel and coolant,
the cladding acts as additional resistance to heat transfer from fuel to coolant. To
ensure rapid removal of fission heat, the material of cladding must offer low
resistance for thermal conduction. In other words, the material used for cladding must
possess high thermal conductivity. The material of cladding must not be damaged due
to sustained neutron irradiation.
Apart from the above mechanical and thermo-physical properties, the nuclear
properties of clad material are also of immense importance. This holds true especially
for thermal reactors. The clad material used in thermal reactors must possess low
absorption cross section for neutrons. Aluminum, beryllium, magnesium and
zirconium have low absorption cross section for neutrons and possess high melting
point. The mechanical properties of beryllium are poor. Moreover, beryllium is
expensive. Aluminum is used in small, research reactors. Magnox, an alloy of
Magnesium is used in gas-cooled reactors. Zircaloy, the alloy of zirconium is used in
most thermal reactors. These alloys (Zircaloy-2 and Zircaloy-4) possess good
mechanical properties and have superior resistance to corrosion.
The resistance of zirconium to corrosion is increased by addition of tin, iron,
chromium and nickel. This zirconium alloy is zircaloy-2. The distribution by mass of
these metals in zircaloy-2 is as follows: Tin ~ 1.2-1.7; Iron ~ 0.07-0.2; Cr ~ 0.05-0.15
and Ni ~ 0.03-0.08. By addition of these metals to zirconium, the low absorption
cross section for neutrons was not comprised to a large extent. However, zircaloy-2
has tendency to react with hydrogen, an undesirable event. This problem is overcome
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
in zircaloy-4, whose maximum nickel and iron concentrations are restricted to 0.007
% and 0.12 % respectively by weight.
The requirement of low absorption cross section need not be satisfied for fast reactors.
However a material with low swelling under higher fast neutron fluence is required.
Stainless steel has both low swelling and good resistance to corrosion by liquid
sodium. Hence stainless steel is used as cladding material in sodium cooled fast
reactors.
2 Reflector
The purpose of reflector in a nuclear reactor is to reflect the neutrons escaping or
leaving, back to the core. This serves to flatten the flux, which is essential to increase
the reactor power without excessive heating of fuel elements. The following are the
essential characteristics required for a reflector material:
(i)
(ii)
(iii)
(iv)
Low cross section for neutron capture or absorption
High cross section for neutron scattering
High energy loss per collision event between neutron and reflector
Temperature and radiation stability
Most thermal reactors use water as both the moderator as well as reflector. The
reflector in PHWR is the heavy water. In graphite-moderated reactors, graphite acts as
reflector also. These common reflector materials, which are also good moderators,
cannot be used in fast reactors.
3 Control elements
These elements are meant to control the reactor power through absorption of neutrons.
These materials are ‘poisons’. Hafnium, silver-indium-cadmium alloys and boron
carbide are the widely used control materials.
The neutronic, physical and mechanical properties of Hafnium make it suitable for
use as control material in water-cooled reactors. However, the availability of Hafnium
is limited and hence it is expensive.
Silver-indium-cadmium alloys are as effective as Hafnium. These materials can be
easily fabricated in the desired form. However, these materials must be enclosed in a
stainless steel enclosure to protect the same from corrosion.
Boron has very high absorption cross section and is of low cost. However, these
materials have to be incorporated in a metallic enclosure.
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
4 Shielding materials
The types of radiations in a nuclear reactor are neutrons, gamma, alpha and beta
radiation. For absorption of neutron radiation, a material with low mass number and
high cross section is suitable. Water possesses the above properties, apart from being
of low cost and capable of removing heat.
The material used to shield gamma radiation must be composed of high mass number
element. High material density is also a desirable quality. Lead, iron and concrete are
candidates for shielding gamma radiation. Lead is attractive due to its low cost.
However its lower melting point is a disadvantage. Iron and concrete are good
neutron absorbers. They can be easily fabricated as well. Shielding materials are not
required for α and β radiation.
5 Reference/Additional Reading
1. http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1017v2.pdf
2. http://www.ornl.gov/info/reports/1962/3445605716311.pdf
3. http://nuclearpowertraining.tpub.com/h1017v2/css/h1017v2_71.htm
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