Nuclear fission and fusion

1. Nuclear fission and fusion Types of decay process Rates of decay Nuclear stability Energy changes Fission and fusion

2.     Forces at work in the nucleus • Electrostatic repulsion: pushes protons apart • Strong nuclear force: pulls protons together • Nuclear force is much shorter range: protons must be close together

3.   Neutrons only experience the strong nuclear force • Proton pair experiences both forces • Neutrons experience only the strong nuclear force • But: neutrons alone are unstable

4.     Neutrons act like nuclear glue • Helium nucleus contains 2 protons and 2 neutrons – increase attractive forces • Overall nucleus is stable  

5. As nuclear size increases, electrostatic repulsion builds up • There are electrostatic repulsions between protons that don’t have attractive forces • More neutrons required Long range repulsive force with no compensation from attraction      

6. Neutron to proton ratio increases with atomic number Upper limit of stability

7. Upper limit to nuclear stability • Beyond atomic number 83, all nuclei are unstable and decay via radioactivity • Radioactive decay (Transmutation) – formation of new element Mass number Atomic number decreases Alpha particle emitted Atomic number

8. Odds and sods • All elements have a radioactive isotope • Only H has fewer neutrons than protons • The neutron:proton ratio increases with Z • All isotopes heavier than bismuth-209 are radioactive • Most nonradioactive isotopes contain an even number of neutrons (207 out of 264). 156 have even protons and neutrons; 51 have even protons and odd neutrons; 4 have odd protons and neutrons

9. Nuclear processes relieve instability • Chemical reactions involve electrons; nuclear reactions involve the nucleus • Isotopes behave the same in chemical reactions but differently in nuclear ones • Rate of nuclear process independent of T,P, catalyst • Nuclear process independent of state of the atom – element, compound • Energy changes are massive

10. Types of radiation

11. Alpha particle emission 92 protons 146 neutrons 238 nucleons 2 protons 2 neutrons 4 nucleons 90 protons 144 neutrons 234 nucleons

12. Beta particle emission 53 protons 78 neutrons 131 nucleons 0 nucleons -1 charge 54 protons 77 neutrons 131 nucleons

13. Other decay processes • Positron emission: the conversion of a proton into a neutron plus positive electron • Decrease in z with no decrease in m • Electron capture: the capture of an electron by a proton to create a neutron • Decrease in z with no decrease in m 19 protons 21 neutrons 40 nucleons 18 protons 22 neutrons 40 nucleons 0 nucleons +1 charge 80 protons 117 neutrons 197 nucleons 0 nucleons -1 charge 79 protons 118 neutrons 197 nucleons

14. Summary of processes and notation

15. Measuring decay • Rates of radioactive decay vary enormously – from fractions of a second to billions of years • The rate equation is the same first order process Rate = k x N

16. Half-life measures rate of decay • Concentration of nuclide is halved after the same time interval regardless of the initial amount – Half-life • Can range from fractions of a second to millions of years

17. Mathematical jiggery pokery • Calculating half life from decay rate t = 0, N = No; t = t1/2, N = No/2 • Calculating residual amounts from half life

18. Magic numbers • Certain numbers of protons and/or neutrons convey unusual stability on the nucleus 2, 8, 20, 28, 50, 82, 126 • There are ten isotopes of Sn (Z=50); but only two of In (Z=49) and Sb (Z=51) • Magic numbers are associated with the nuclear structure, which is analogous to the electronic structure of atoms

19. Correlation of neutron:proton ratio and decay process

20. Stability is not achieved in one step: products also decay • Here atomic number actually increases, but serves to reduce the neutron:proton ratio • Beta particle emission occurs with neutron-excess nuclei • Alpha particle emission occurs with proton-heavy nuclei

21. Radioactive series are complex The decay series from uranium-238 to lead-206. Each nuclide except for the last is radioactive and undergoes nuclear decay. The left-pointing, longer arrows (red) represent alpha emissions, and the right-pointing, shorter arrows (blue) represent beta emissions.

22. Energy changes and nuclear decay • In principle there will be an energy associated with the binding of nuclear particles to form a nucleus • Experimentally demanding!

23. Use Einstein’s relationship E = mc2 • Consider the He nucleus: Mass of individual particles = 4.03188 amu Mass of He nucleus = 4.00150 amu Mass loss = 0.03038 amu • The “lost” mass is converted into energy – the binding energy, which is released during the nuclear process • For the example above, the energy is 2.73 x 109 kJ/mol

24. Inter-changeability of mass and energy • Loss in mass equals energy given out E = mc2 • Tiny amount of matter produces masses of energy: 1 gram  1014 J • Energy and mass are conserved, but can be inter-changed • Binding energy per nucleon presents the total binding energy as calculated previously per nuclear particle • Usually cited in eV, where 1 eV = 1.6x10-19J

25. Fe He Nucleon mass U H Average mass per nucleon varies with atomic number The binding energy per nucleon for the most stable isotope of each naturally occurring element. Binding energy reaches a maximum of 8.79 MeV/nucleon at 56Fe. As a result, there is an increase in stability when much lighter elements fuse together to yield heavier elements up to 56Fe and when much heavier elements split apart to yield lighter elements down to 56Fe, as indicated by the arrows.

26. Mass changes in chemical reactions? • Conservation of mass and energy means that energy changes in chemical processes involve concomitant changes in mass • Magnitude is so small as to be undetectable • A ΔH of -436 kJ/mol corresponds to a weight loss of 4.84 ng/mol

27. Fission and fusion: ways to harness nuclear energy • Attempts to grow larger nuclei by bombardment with neutrons yielded smaller atoms instead. • Distorting the nucleus causes the repulsive forces to overwhelm the attractive • The foundation of nuclear energy and the atomic bomb

28. Nuclear fission • Nuclear fission produces nuclei with lower nucleon mass • One neutron produces three: the basis for a chain reaction – explosive potential • Many fission pathways – 800 fission products from U-235

29. Chain reactions require rapid multiplication of species

30. Nuclear fusion: opposite of fission • Small nuclei fuse to yield larger ones • Nuclear mass is lost • Example is the deuterium – tritium reaction • About 0.7 % of the mass is converted into energy +E

31. The sun is a helium factory • The sun’s energy derives from the fusion of hydrogen atoms to give helium

32. Fusion would be the holy grail if... • The benefits: • High energy output (10 x more output than fission) • Clean products – no long-lived radioactive waste or toxic heavy metals • The challenge: • Providing enough energy to start the process – positive charges repel • Reproduce the center of the sun in the lab • Fusion is demonstrated but currently consumes rather than produces energy

33. Useful radioisotopes and half-lives

34. Radioisotopes have wide range of uses • H-3 Triggering nuclear weapons, luminous paints and gauges, biochemical tracer • I-131 Thyroid treatment and medical imaging • Co-60Food irradiation, industrial applications, radiotherapy • Sr-90 Tracer in medical and agricultural studies • U-235/238 Nuclear power generation, depleted U used in weapons and shielding • Am-241 Thickness and distance gauges, smoke detectors

35. Nuclear power prevalent in Europe

36. Different units for measuring radiation

37. Radiation is nasty