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 • 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 • 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
Isotopes • 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 • 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 • 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.
Fission • 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 • Fission of Uranium 235 is utilized in power production
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.
Criticality • Critical mass • Amount of fissionable material needed for a sustainable reaction • Three situations • Critical (equilibrium) • Subcritical (exponentially decreases) • Supercritical (exponentially increases)
Criticality These curves show the amount of neutrons present in the three situations. This correlates to the rate of the reaction
Reactor Components • A functional power reactor requires three basic components • Fuel • Moderator • Coolant
Reactor Fuel • All power reactors in the US use Uranium Fuel • This fuel must be enriched before it can be used as fuel
Enrichment • 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?
Enrichment • Methods include • Centrifuge • Gaseous Diffusion • Atomic Vapor Laser Isotope Separation • What these methods all have in common is that they are very expensive
Moderator • 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
Moderator • 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?
Moderator • The most commonly used moderators are: • Water (H2O) • Graphite (C)
Coolant • Nuclear reactions produce massive amounts of heat (that’s the point!) • We need a way to remove this heat and turn it into electricity
Coolant • 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? • 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 • 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 American Reactors are Light Water Reactors (LWRs) Coolant: Light Water Moderator: 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 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) • Water boils in core • Single loop between core and turbine
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: BWR: Less components, simpler system PWR: Fission products contained in reactor vessel Cons: BWR: Containment needed for entire coolant loop PWR: Complex cycle, pressure vessel needed Pros / Cons
CANDU Reactors CANDU = Canadian Deuterium-Uranium Primary Reactor Type used in Canada Moderator: Heavy Water Coolant: Heavy Water Fuel: Natural Uranium
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 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 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 • 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 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?) 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)