Chapter 30

1. Chapter 30 Nuclear Energy and Elementary Particles

2. Processes of Nuclear Energy • Fission • A nucleus of large mass number splits into two smaller nuclei • Fusion • Two light nuclei fuse to form a heavier nucleus • Large amounts of energy are released in either case

3. Nuclear Fission • A heavy nucleus splits into two smaller nuclei • The total mass of the products is less than the original mass of the heavy nucleus • First observed in 1939 by Otto Hahn and Fritz Strassman following basic studies by Fermi • Lisa Meitner and Otto Frisch soon explained what had happened

4. Fission Equation • Fission of 235U by a slow (low energy) neutron • 236U* is an intermediate, short-lived state • Lasts about 10-12 s • X and Y are called fission fragments • Many combinations of X and Y satisfy the requirements of conservation of energy and charge

5. More About Fission of 235U • About 90 different daughter nuclei can be formed • Several neutrons are also produced in each fission event • Example: • The fission fragments and the neutrons have a great deal of KE following the event

6. Sequence of Events in Fission • The 235U nucleus captures a thermal (slow-moving) neutron • This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations • The 236U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion • The nucleus splits into two fragments, emitting several neutrons in the process

7. Sequence of Events in Fission – Diagram

8. Energy in a Fission Process • Binding energy for heavy nuclei is about 7.2 MeV per nucleon • Binding energy for intermediate nuclei is about 8.2 MeV per nucleon • Therefore, the fission fragments have less mass than the nucleons in the original nuclei • This decrease in mass per nucleon appears as released energy in the fission event

9. Energy, cont • An estimate of the energy released • Assume a total of 240 nucleons • Releases about 1 MeV per nucleon • 8.2 MeV – 7.2 MeV • Total energy released is about 240 Mev • This is very large compared to the amount of energy released in chemical processes

10. Chain Reaction • Neutrons are emitted when 235U undergoes fission • These neutrons are then available to trigger fission in other nuclei • This process is called a chain reaction • If uncontrolled, a violent explosion can occur • The principle behind the nuclear bomb, where 1 kg of U can release energy equal to about 20 000 tons of TNT

11. Chain Reaction – Diagram

12. Nuclear Reactor • A nuclear reactor is a system designed to maintain a self-sustained chain reaction • The reproduction constant, K, is defined as the average number of neutrons from each fission event that will cause another fission event • The maximum value of K from uranium fission is 2.5 • In practice, K is less than this • A self-sustained reaction has K = 1

13. K Values • When K = 1, the reactor is said to be critical • The chain reaction is self-sustaining • When K < 1, the reactor is said to be subcritical • The reaction dies out • When K > 1, the reactor is said to be supercritical • A run-away chain reaction occurs

14. Basic Reactor Design • Fuel elements consist of enriched uranium • The moderator material helps to slow down the neutrons • The control rods absorb neutrons

15. Reactor Design Considerations – Neutron Leakage • Loss (or “leakage”) of neutrons from the core • These are not available to cause fission events • The fraction lost is a function of the ratio of surface area to volume • Small reactors have larger percentages lost • If too many neutrons are lost, the reactor will not be able to operate

16. Reactor Design Considerations – Neutron Energies • Slow neutrons are more likely to cause fission events • Most neutrons released in the fission process have energies of about 2 MeV • In order to sustain the chain reaction, the neutrons must be slowed down • A moderator surrounds the fuel • Collisions with the atoms of the moderator slow the neutrons down as some kinetic energy is transferred • Most modern reactors use heavy water as the moderator

17. Reactor Design Considerations – Neutron Capture • Neutrons may be captured by nuclei that do not undergo fission • Most commonly, neutrons are captured by 238U • The possibility of 238U capture is lower with slow neutrons • The moderator helps minimize the capture of neutrons by 238U

18. Reactor Design Considerations – Power Level Control • A method of control is needed to adjust the value of K to near 1 • If K >1, the heat produced in the runaway reaction can melt the reactor • Control rods are inserted into the core to control the power level • Control rods are made of materials that are very efficient at absorbing neutrons • Cadmium is an example • By adjusting the number and position of the control rods, various power levels can be maintained

19. Pressurized Water Reactor – Diagram

20. Pressurized Water Reactor – Operation Notes • This type of reactor is commonly used in electric power plants in the US • Fission events in the reactor core supply heat to the water contained in the primary system • The primary system is a closed system • This water is maintained at a high pressure to keep it from boiling • The hot water is pumped through a heat exchanger

21. Pressurized Water Reactor – Operation Notes, cont • The heat is transferred to the water contained in a secondary system • This water is converted into steam • The steam is used to drive a turbine-generator to create electric power • The water in the secondary system is isolated from the water in the primary system • This prevents contamination of the secondary water and steam by the radioactive nuclei in the core

22. Reactor Safety – Containment • Radiation exposure, and its potential health risks, are controlled by three levels of containment • Reactor vessel • Contains the fuel and radioactive fission products • Reactor building • Acts as a second containment structure should the reactor vessel rupture • Location • Reactor facilities are in remote locations

23. Reactor Safety – Loss of Water • If the water flow was interrupted, the nuclear reaction could stop immediately • However, there could be enough residual heat to build up and melt the fuel elements • The molten core could also melt through the containment vessel and into the ground • Called the China Syndrome • If the molten core struck ground water, a steam explosion could spread the radioactive material to areas surrounding the power plant • Reactors are built with emergency cooling systems that automatically flood the core if coolant is lost

24. Reactor Safety – Radioactive Materials • Disposal of waste material • Waste material contains long-lived, highly radioactive isotopes • Must be stored over long periods in ways that protect the environment • Present solution is sealing the waste in waterproof containers and burying them in deep salt mines • Transportation of fuel and wastes • Accidents during transportation could expose the public to harmful levels of radiation • Department of Energy requires crash tests and manufacturers must demonstrate that their containers will not rupture during high speed collisions

25. Nuclear Fusion • Nuclear fusion occurs when two light nuclei combine to form a heavier nucleus • The mass of the final nucleus is less than the masses of the original nuclei • This loss of mass is accompanied by a release of energy

26. Fusion in the Sun • All stars generate energy through fusion • The Sun, along with about 90% of other stars, fuses hydrogen • Some stars fuse heavier elements • Two conditions must be met before fusion can occur in a star • The temperature must be high enough • The density of the nuclei must be high enough to ensure a high rate of collisions

27. The proton-proton cycle is a series of three nuclear reactions believed to operate in the Sun Energy liberated is primarily in the form of gamma rays, positrons and neutrinos 21H is deuterium, and may be written as 21D Proton-Proton Cycle

28. Fusion Reactors • Energy releasing fusion reactions are called thermonuclear fusion reactions • A great deal of effort is being directed at developing a sustained and controllable thermonuclear reaction • A thermonuclear reactor that can deliver a net power output over a reasonable time interval is not yet a reality

29. Advantages of a Fusion Reactor • Inexpensive fuel source • Water is the ultimate fuel source • If deuterium is used as fuel, 0.06 g of it can be extracted from 1 gal of water for about 4 cents • Comparatively few radioactive by-products are formed

30. Considerations for a Fusion Reactor • The proton-proton cycle is not feasible for a fusion reactor • The high temperature and density required are not suitable for a fusion reactor • The most promising reactions involve deuterium (D) and tritium (T)

31. Considerations for a Fusion Reactor, cont • Deuterium is available in almost unlimited quantities in water and is inexpensive to extract • Tritium is radioactive and must be produced artificially • The Coulomb repulsion between two charged nuclei must be overcome before they can fuse

32. Requirements for Successful Thermonuclear Reactor • High temperature  108 K • Needed to give nuclei enough energy to overcome Coulomb forces • At these temperatures, the atoms are ionized, forming a plasma • Plasma ion density, n • The number of ions present • Plasma confinement time,  • The time the interacting ions are maintained at a temperature equal to or greater than that required for the reaction to proceed successfully

33. Lawson’s Criteria • Lawson’s criteria states that a net power output in a fusion reactor is possible under the following conditions • n  1014 s/cm3 for deuterium-tritium • n  1016 s/cm3 for deuterium-deuterium • The plasma confinement time is still a problem

34. Magnetic Confinement • One magnetic confinement device is called a tokamak • Two magnetic fields confine the plasma inside the doughnut • A strong magnetic field is produced in the windings • A weak magnetic field is produced in the toroid • The field lines are helical, spiral around the plasma, and prevent it from touching the wall of the vacuum chamber

35. Some Fusion Reactors • TFTR • Tokamak Fusion Test Reactor • Princeton • Central ion temperature of 510 million degrees C • The nt values were close to Lawson criteria • JET • Tokamak at Abington, England • 6 x 1017 DT fusions per second were achieved

36. Current Research in Fusion Reactors • NSTX – National Spherical Torus Experiment • Produces a spherical plasma with a hole in the center • Is able to confine the plasma with a high pressure • ITER – International Thermonuclear Experimental Reactor • An international collaboration involving four major fusion programs is working on building this reactor • It will address remaining technological and scientific issues concerning the feasibility of fusion power

37. Other Methods of Creating Fusion Events • Inertial laser confinement • Fuel is put into the form of a small pellet • It is collapsed by ultrahigh power lasers • Inertial electrostatic confinement • Positively charged particles are rapidly attracted toward an negatively charged grid • Some of the positive particles collide and fuse

38. Elementary Particles • Atoms • From the Greek for “indivisible” • Were once thought to the elementary particles • Atom constituents • Proton, neutron, and electron • Were viewed as elementary because they are very stable

39. Discovery of New Particles • New particles • Beginning in 1937, many new particles were discovered in experiments involving high-energy collisions • Characteristically unstable with short lifetimes • Over 300 have been cataloged • A pattern was needed to understand all these new particles

40. Quarks • Physicists recognize that most particles are made up of quarks • Exceptions include photons, electrons and a few others • The quark model has reduced the array of particles to a manageable few • The quark model has successfully predicted new quark combinations that were subsequently found in many experiments

41. Fundamental Forces • All particles in nature are subject to four fundamental forces • Strong force • Electromagnetic force • Weak force • Gravitational force

42. Strong Force • Is responsible for the tight binding of the quarks to form neutrons and protons • Also responsible for the nuclear force binding the neutrons and the protons together in the nucleus • Strongest of all the fundamental forces • Very short-ranged • Less than 10-15 m

43. Electromagnetic Force • Is responsible for the binding of atoms and molecules • About 10-2 times the strength of the strong force • A long-range force that decreases in strength as the inverse square of the separation between interacting particles

44. Weak Force • Is responsible for instability in certain nuclei • Is responsible for beta decay • A short-ranged force • Its strength is about 10-6 times that of the strong force • Scientists now believe the weak and electromagnetic forces are two manifestations of a single force, the electroweak force

45. Gravitational Force • A familiar force that holds the planets, stars and galaxies together • Its effect on elementary particles is negligible • A long-range force • It is about 10-43 times the strength of the strong force • Weakest of the four fundamental forces

46. Explanation of Forces • Forces between particles are often described in terms of the actions of field particles or quanta • For electromagnetic force, the photon is the field particle • The electromagnetic force is mediated, or carried, by photons

47. Forces and Mediating Particles (also see table 30.1)

48. Paul Adrien Maurice Dirac • 1902 – 1984 • Instrumental in understanding antimatter • Aided in the unification of quantum mechanics and relativity • Contributions to quantum physics and cosmology • Nobel Prize in 1933

49. Antiparticles • For every particle, there is an antiparticle • From Dirac’s version of quantum mechanics that incorporated special relativity • An antiparticle has the same mass as the particle, but the opposite charge • The positron (electron’s antiparticle) was discovered by Anderson in 1932 • Since then, it has been observed in numerous experiments • Practically every known elementary particle has a distinct antiparticle • Exceptions – the photon and the neutral pi particles are their own antiparticles

50. Hideki Yukawa • 1907 – 1981 • Predicted the existence of mesons • Nobel Prize in 1949