Chapter 30

1. Chapter 30 Nuclear Energy Fission and Fusion Reactions

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

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. 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

19. 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

20. 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

21. 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

22. 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

23. 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)

24. 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

25. 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

26. 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

27. 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