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Nuclear Reactors An Introduction. Overview. Nuclear Physics Neutrons, Fission and Criticality Reactor Components Fuel, Moderator and Coolant Types of Nuclear Reactors Generation III and Generation IV Reactors Advantages and Disadvantages of Nuclear Power. The Root of It All: The Atom.

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Nuclear reactors an introduction
Nuclear ReactorsAn Introduction


  • Nuclear Physics

    • Neutrons, Fission and Criticality

  • Reactor Components

    • Fuel, Moderator and Coolant

  • Types of Nuclear Reactors

    • Generation III and Generation IV Reactors

  • Advantages and Disadvantages of Nuclear Power

The root of it all the atom
The Root of It All: The Atom

  • Protons (p)

    • mp=1.673 x 10-27 kg

    • charge: +1e

  • Neutrons (n)

    • mn=1.675 x 10-27 kg

    • Charge: 0

  • Electrons (e)

    • me=9.109 x 10-31 kg

    • Charge:-1e

*e is the elementary charge, and it is about 1.602 x 10^-19 Coulomb.

A nuclear focus the nucleus
A Nuclear Focus: The Nucleus

  • The nucleus is comprised of nucleons

    • Protons

    • Neutrons

Mass number

Elemental symbol

Atomic number

(number of protons)

  • A=N+Z

    • N = number of neutrons

    • gives the total number of nucleons


  • Nuclei with same number of protons, different number of neutrons

    • Same Z, different A, thus different N

      Isotopes of Hydrogen

  • Similar chemical and physical properties

  • Very different nuclear properties!

Chart of Nuclides

Notice the trend’s digression from the line Z=N, why is this?

Nuclear glue
Nuclear Glue

  • There are four fundamental forces:

    • Gravitational (Universal Law of Gravitation)

    • Electromagnetic (Coulomb's Law)

    • Weak nuclear

    • Strong nuclear

  • Strong Nuclear Force

    • Binds the nucleus

    • Overcomes electromagnetic repulsion between protons

    • Neutrons (0 charge) facilitate strong interaction

    • Only acts over small distances, on the magnitude of

      10-15 meters!

So back to that trend:

  • 1:1 ratio of protons to neutrons sufficient for smaller nuclei ~Z≤20

  • As nuclei become larger:

    • Addition of protons increases electromagnetic repulsion

    • Strong force on the average weakens due to increase in distance

  • More neutrons are needed then to dilute proton-proton repulsion and increase average strong force!

  • Largest stable element is an isotope of Lead, Z=82 A=208

Nuclear binding energy
Nuclear Binding Energy

  • Amount of energy required to pull apart a nucleus

  • Usually expressed as binding energy per nucleon

  • Greater binding energy per nucleon means a more stable nucleus

Binding Energy

The maximum point on the graph occurs around atomic number 56, iron. Thus, iron is the most stable element. Ideally, everything is trying to become iron.


  • An exothermic reaction that involves the division of a heavy nucleus into smaller, lighter nuclei.

  • Utilized in commercial nuclear power

E mc 2

  • Einstein's Theory of Relativity, oh yeah it’s used.

  • Mass is conserved right?

    • mass-energy is conserved

  • Difference in mass between fission reactants and products shows up in the energy produced.

  • The essence of nuclear power!


  • Fission of Uranium 235 is utilized in power production

Extra neutrons are the key
Extra Neutrons are the key!

  • Notice the previous reaction started with a neutron and produced three more

    • Each fission produces ~2-3 neutrons

  • The product neutrons propagate and can cause other nuclei to fission

  • This is the key to fission as a power source, it allows for a chain reaction to occur.


  • Critical mass

    • Amount of fissionable material needed for a sustainable reaction

  • Three situations

    • Critical (equilibrium)

    • Subcritical (exponentially decreases)

    • Supercritical (exponentially increases)


These curves show the amount of neutrons present in the three situations. This correlates to the rate of the reaction

Reactor components
Reactor Components

  • A functional power reactor requires three basic components

    • Fuel

    • Moderator

    • Coolant

Reactor fuel
Reactor Fuel

  • All power reactors in the US use Uranium Fuel

  • This fuel must be enriched before it can be used as fuel


  • Enrichment is the process of raising the U235 content of natural uranium (U238 is a neutron absorber)

  • U235 is chemically identical to U238, so how is this done?


  • Methods include

    • Centrifuge

    • Gaseous Diffusion

    • Atomic Vapor Laser Isotope Separation

  • What these methods all have in common is that they are very expensive


  • Nuclear fission has a higher probability of occurring when the neutrons involved are at lower velocities

  • Neutrons are born in fission with velocities of about 30·106 m/s

  • We would like to slow them down to something closer to 2200 m/s, the velocity of particles in air


  • To do this we use a moderator where the neutrons can bounce around and lose their energy

  • A good moderator has several properties

    • High neutron scattering probability

    • High density

    • Low Atomic Weight

  • What might we use as a moderator?


  • The most commonly used moderators are:

    • Water (H2O)

    • Graphite (C)


  • Nuclear reactions produce massive amounts of heat (that’s the point!)

  • We need a way to remove this heat and turn it into electricity


  • Why water coolant?

    • Cheap

    • High thermal conductivity

    • High thermal capacity

    • We can use it as a moderator at the same time. Brilliant!

How is the reaction controlled
How is the reaction controlled?

  • Control rods:

    • Control rods are inserted or removed from the reactor to control the reaction rate

    • The control rods contain large amounts of Boron, a neutron absorber

Reactor control
Reactor Control

  • In the case of major reactor event, the reactor is scrammed and all the control rods are dropped into the core immediately

  • SCRAM stands for: Safety Cut Rope Axe Man

Reactor types

Reactor Types

American Reactors are Light Water Reactors (LWRs)

Coolant: Light Water

Moderator: Light Water

Light water

What is “light water”?

Regular, everyday water

Inexpensive, easy to obtain

Is there such thing as “heavy water”?

Heavy Water: the H atoms in water have an extra neutron

Much more expensive

Light Water

Light water reactors
Light Water Reactors

Basic Concept:

  • Nuclear fission in core generates heat

  • Heat boils water, creating steam

  • Steam turns a turbine, which powers a generator

  • Generator creates electricity

    Two Types of LWRs:

  • Boiling Water Reactor (BWR)

  • Pressurized Water Reactor (PWR)

Boiling water reactor bwr
Boiling Water Reactor (BWR)

  • Water boils in core

  • Single loop between core and turbine

Pressurized water reactor pwr
Pressurized Water Reactor (PWR)

  • Water kept under high pressure in core (>2000 psi)

  • Heat transferred to second loop, where the water can boil

  • Turbine not directly connected to core

Pros cons



Less components, simpler system


Fission products contained in reactor vessel



Containment needed for entire coolant loop


Complex cycle, pressure vessel needed

Pros / Cons

Candu reactors
CANDU Reactors

CANDU = Canadian Deuterium-Uranium

Primary Reactor Type used in Canada

Moderator: Heavy Water

Coolant: Heavy Water

Fuel: Natural Uranium

More on candus
More on CANDUs

  • Online refueling

  • CANDU reactors use Natural Uranium as fuel

  • They get away with this by using Heavy Water as moderator, which has better moderating properties than regular water (higher neutron scattering probibility)

Generation iii nuclear reactors
Generation III Nuclear Reactors

A generation III reactor design is a enhancement of any of the generation II reactor design incorporating improvements such as improved fuel technology and passive safety systems.

The Nuclear Regulatory Commission expects applications for about 24 new plant licenses in the next couple of years

These reactors will be Generation III designs

Net Output: 1350 MW electrical energy

Four ABWR’s are operational in Japan

Generation III- Advanced Boiling Water Reactor (ABWR)

Generation iii abwr
Generation III- ABWR

1. Vessel Flange and Closure Head

2. Vent and Head Spray

3. Steam Outlet Flow Restrictor

4. RPV Stabilizer

5. Feedwater Nozzle

6. Forged Shell Rings

7. Vessel Support Skirt

8. Vessel Bottom Head

9. RIP Penetrations

10. Thermal Insulation

11. Core Shroud

12. Core Plate

13. Top Guide

14. Fuel Supports

15. Control Rod Drive Housings

16. Control Rod Guide Tubes

17. In Core Housing

18. In-Core Instrument Guide Tubes

and Stabilizers

19. FeedwaterSparger

20. High Pressure Core Flooder

(HPCF) Sparger

21. HPCF Coupling

22. Low Pressure Flooder (LPFL)

23. Shutdown Cooling Outlet

24. Steam Separators

25. Steam Dryer

26. Reactor Internal Pumps (RIP)

27. RIP Motor Casing

28. Core and RIP Differential

Pressure Line

29. Fine Motion Control Rod Drives

30. Fuel Assemblies

31. Control Rods

32. Local Power Range Monitor

Net Output: 1600 MW electrical energy

Two units are under construction in Finland and France

Generation III- Evolutionary Pressurized Reactor (EPR)

Provide up to 70MW electrical or 300MW heat energy that would satisfy a population of 200,000 people

Can be modified as a desalination plant producing 240,000 cubic meters of fresh water

Generation III- Russian Floating Nuclear Power Station

Generation iv nuclear reactors
Generation IV Nuclear Reactors would satisfy a population of 200,000 people

  • The next big thing in Nuclear reactor design (possible deployment in 2030)

  • Possible designs include:

    • Very-High-Temperature Reactor (VHTR)

    • Molten Salt Reactor (MSR)

    • Sodium-Cooled Fast Reactor (SFR)

utilizes a graphite-moderated Helium cooled core would satisfy a population of 200,000 people

outlet temperature of 1,000 °C.

high temperatures enables hydrogen production and allows for high thermal efficiency

Generation IV: Very-High-Temperature Reactor (VHTR)

the coolant is a molten salt (why?) would satisfy a population of 200,000 people

nuclear fuel is dissolved in the molten fluoride salt as uranium tetrafluoride (UF4),

the fluid would reach criticality by flowing into a graphite core which serves as the moderator

Generation IV: Molten Salt Reactor (MSR)

increase the efficiency of uranium usage by breeding plutonium

uses an unmoderated core running on fast neutrons

Burns both Uranium and Plutonium as fuel

Generation IV: Sodium-Cooled Fast Reactor (SFR)

PROS Production?

  • Cheap

  • Zero greenhouse gas emissions

  • Reliable

Costs Production?

Emissions Production?

Coal, natural gas and oil power facilities all release harmful pollutants, excess heat and greenhouse gases into the atmosphere.

Emissions Production?

The only emission of a nuclear power station is heat and water vapor. Its ‘carbon footprint’ is negligible.

Reliability Production?

Since nuclear power plants are capable of delivering a very consistent amount of electricity, and only have to be refueled periodically, they are relied upon by electric utilities to provide the ‘base load’ of power generation

Base load
Base Load Production?

Cons Production?

Expensive start up costs

Waste Disposal

Safety Issues

Start up cost
Start up Cost Production?

  • A nuclear power facility costs about 3-4 billion dollars to build. That’s a lot!

  • Why so much?

    • Each plant produces about 3x the amount of electricity as a conventional coal or gas power plant

    • Plants are designed and built to rigorous safety criteria, which has a cost

    • Nuclear Power plants are complicated!

Waste disposal
Waste Disposal Production?

  • The fuel waste from a nuclear power plant is highly radioactive, and must be properly stored

  • This is done by placing the fuel in an on-site facility called the spent fuel pool.

Safety issues
Safety Issues Production?

  • Nuclear power plants make use of super-heated steam, which must be held under pressure and is relatively corrosive

  • An intense radiation environment exists inside the reactor core, thus extra safety precautions and must be observed

Safety issues1
Safety Issues Production?

  • The possibility exists that a criticality accident, commonly referred to as a meltdown may occur

  • The most notable meltdown occurred at Chernobyl Power Station on April 26, 1986.

Conclusions Production?

  • Nuclear Power has its own unique advantages and disadvantages, and is a viable alternative to fossil fuels

  • Expect to see the addition of new nuclear power generation to the US power grid within the next 5-10 years (they’ve already started working on it)

  • If you are considering a career in math, physics or engineering, you may want to take a look at the nuclear power industry

Fun links

Fun Links Production?

Nuclear Energy Information Service

Energy Information Administration

Nuclear Regulatory Commission

GE Nuclear

Los Alamos National Lab