MATERIALS FOR NUCLEAR POWER

GENERATION

BY

B.SRIKANTH

13ETMM06

M.TECH. (MATERIALS ENGINEERING)

S.E.S.T.

OUTLINEIntroduction to Nuclear ProcessesIntroduction to Nuclear Power GenerationCross-SectionsMechanisms of Radiation Damage Effects of Radiation DamageMaterial Selection For Fuel and Cladding ModeratorsSummary & Conclusion

INTRODUCTION TO NUCLEAR PROCESSES Each nucleus, consisting of protons and neutrons (collectively known as nucleons), has an associated binding energy, it is the energy released when a nucleus is assembled from individual nucleons.

A graph of the binding energy per nucleon, in MeV, for common nuclides.

INTRODUCTION TO NUCLEAR POWER GENERATION There are two main types of nuclear reactor, characterized by the speed of the neutrons which induce fission.

1.Thermal reactors: These are the predominant kind, using slower neutrons to induce fission, the basic fissile nuclide being U-235.

2.Fast breeder reactors: In these less-common reactors, the fast neutrons are used directly to create (breed) fissile nuclides from fissionable nuclides; most commonly Pu-239 is bred from U-238.  Pu-239 is also used in nuclear weapons

There are many varieties of nuclear reactor, but all have the following common elements:

1.Fuel: The material that undergoes fission. This needn’t have the fissionable nuclides in the form of the element. The fuel is often in the form of a ceramic.

2.Cladding:This encases the nuclear fuel, isolating it mechanically and chemically from its immediate environment.

3.Moderators:Necessary in thermal reactors to slow down the neutrons produced by the fission process. Commonly, the moderator is in the form of a rod, but can be in liquid form or even be mixed with the fuel itself.4.Control: This can be used to absorb excess neutrons, or even shut down the reactor in an emergency. Most often, the control material is in the form of a rod.

5.Core: The heart of the reactor, containing the fuel.  The fuel is encased in cladding, and core must also accommodate the coolant and allow for more moderating rods or control rods to be added.

6.Coolant: The coolant removes heat from the reactor core into a heat exchanger. Note that the coolant itself is not cool, just that it removes heat from the core.Reactor vessel: This contains the reactor core and the coolant. It often also acts as a reflector, reducing the loss of neutrons to the outside environment.

7.Generator/turbine: The heat generated by the reactor core generates steam, used to drive a turbine, which can generate electricity.

A cross-section quantifies the probability that a particle passing through a material will interact with the material. 

nuclear cross section σ For neutrons passing through a plate of thickness δx (m), C is the number of events occurring per unit area (m−2), N is the number of nuclei per unit volume, or nuclear number density (m−3), and I is the number of neutrons passing through a unit area (m−2).  Types of Cross-Section: Several different cross-sections having Standard notation is used below, where (a,b) means an atomic interaction in which a is absorbed and b is emitted.

1. Elastic scattering (n,n) 2. Inelastic scattering (n,n') 3. Radiative capture (n,γ) 4. Fission (n,f) 5. Alpha decay (n,α)

MECHANISMS OF RADIATION DAMAGE Most of the radiation damage in a reactor is from the neutron flux being produced in the core.

Material Property Effect of Radiation DamageYield strength Increases on irradiation, along with a decrease in plastic flow

range.Ultimate tensile strength This also increases on irradiation, but less than the yield

strength.Ductile-brittle transition temperature It increases significantly on irradiation, which can present a

problem when the reactor vessel cools on shut down when internal pressure within the reactor is still high, and so fracture can occur if this is not taken into account.

Young’s modulus Small increase on irradiation.Hardness Increase.High-temperature creep rate Increase during irradiation.Ductility Decrease.Density Decrease as the material swells on irradiation.Impact strength Decrease.Thermal conductivity Decrease on irradiation since lattice disorder increases, thus

increasing phonon scattering.Electrical conductivity Decrease for similar reasons to thermal conductivity.

FUEL AND CLADDING There are several important factors when choosing a nuclear fuel:The fuel itself must be easily fissionable, preferably fissile.The fuel must release sufficient quantities of neutrons per neutron captured to be able to sustain a fission chain reaction.The fuel must have a sufficiently long half-life , otherwise  the nuclei would decay into a useless form before fission could be induced in a controlled manner.Economic factors are also important. The fuels must be abundant and readily available. Form of Fuel: Metallic uranium is not favoured as a fuel since it is dimensionally unstable under irradiation, flammable, can readily corrode in oxygen-containing atmospheres, and can produce uranium dust which has a low-temperature flash-point and which can cause serious health problems if inhaled.

There is some interest in using metallic U alloys as fuel when a particularly high density of fissile or fissionable nuclides is required.

As an alternative, ceramic forms can be used, including UO2, U3O8, UC, U2C3, UN, U3Si and USi. The most common of these is uranium dioxide, which has the calcium fluorite structure shown in the image below

Choice of Cladding: The nuclear fuel cannot be allowed to make direct contact with the coolant inside the reactor vessel, due to the potential for radioactivity to be released into the environment. Instead, cladding has to be used to surround the fuel.

design criteria of cladding should be transparent to neutrons, so that it doesn’t absorb neutrons that could be used to induce further fission and have a high thermal conductivity, and not have a high thermal expansion coefficient.

Key problems include hydrogen embrittlement due to (n, p) reactions inside cladding and swelling due to release of fission product gases.

Common choices for cladding material are stainless steel (in FBRs) Zircaloy (in PWRs) and, in the past, Magnox.

MODERATORS A moderator is designed to slow down fast neutrons such that they are more easily absorbed by fissile nuclei. There are two main factors in choosing a moderator:

1. The moderator must not absorb neutrons itself. This means it should have a relatively low neutron absorption cross-section.

2. The moderator should efficiently slow down the neutrons. Modelling neutron-nuclei collisions as a classical elastic collision, in much the same way as gas molecules are modelled, gives the result that the closer the nucleus’ mass is to that of the neutron, the more energy will be transferred in the collision. This means that lighter elements are favoured.

Material selection for Moderators: Graphite has been a very popular neutron moderator, other common choices are light water, heavy water, beryllium, lithium flouride

SUMMARY & CONCLUSIONThe process of nuclear fission has been described, thus explaining the common choices for nuclear fuel used commercially. Materials selection for the major components of a nuclear reactor have also been explored, including1. Moderators, and how they work best when they consist of light nuclides

with relatively low absorption cross-sections.2. Control rods, which require high absorption cross-sections, and how the

same nuclides found in control rods, e.g. boron, can act as poisons significantly reducing the efficiency of a reactor if found elsewhere, such as in moderators.

3. Cladding, which experiences much stronger radiation fluxes and extremes of temperature than any other structural material in the reactor, and so must be able to withstand these conditions.

Radiation materials science is a mature field, but there any many challenges for materials to permit more efficient operation, improve safety and reliability and reduce costs.

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