Chapter 4 for nuclear engineering DU.pptx

Chapter 4 for nuclear engineering DU.pptx

1. Chapter 4 Nuclear Chemistry

2. Content

3. A study of the nuclear changes in atoms is termed Nuclear Chemistry The disintegration or decay of unstable atoms accompanied by emission of radiation is called Radioactivity/ Radioactive decay.

4. Nuclear Definitions Radiation =Particles and/or electromagnetic “energy” emitted from atomic nuclei in various nuclear processes. The important nuclear radiations are alpha and beta particles, positrons, gamma rays, and neutrons. Non-Ionizing Radiation = Low frequency radiation in the form of microwaves, infrared radiation, radio waves, and cell phones. Ionizing Radiation = High frequency radiation with enough energy to break chemical bonds, remove electrons from atoms, break apart atomic nuclei, and damage DNA. Ex. Alpha, beta, positron, gamma, UV, and X-rays.

5. Unstable Isotopes Kelter, Carr , Scott, Chemistry A World of Choices 1999, page 439 Excited nucleus Stable nucleus Energy Particles + and or Radiation The original nucleus is called the parent nucleus and the product is called the daughter nucleus

6. Initial Discovery of Radiation Rutherford (1902) two oppositely charged plates

7. Alpha decay : The nucleus releases and alpha particle. This decreases the mass number by 4 and the atomic number by 2. An alpha particle = He nucleus . 4 Beta decay : The nucleus releases a beta particle. This does not decrease the mass number, but decreases the atomic number by one. (neutron is converted into a proton and an electron) Positron Emission : Similar to beta emission. A proton is converted to a neutron and a positive particle similar to an electron. Ex. 22 Na Gamma decay : The nucleus release a gamma ray (high energy photon). Gamma decay can accompany alpha and beta decay. In gamma decay the nucleus does not change, it makes a transition to a lower energy state.

8. Absorption of Radiation Timberlake, Chemistry 7 th Edition, page 84

9. When we see a radioactive decay? The exact mode of radioactive transformation depends on the energy available for the transition. The available energy, in turn, depends on two factors: on the particular type of nuclear instability that is, whether the neutron-to-proton ratio is too high or too low for the particular nuclide under consideration-and on the mass-energy relationship among the parent nucleus, daughter nucleus, and emitted particle.

10. Nuclear Stability Atomic Numbers 83 and greater are unstable and radioactive Neutron/proton ratio affects stability Isotopes with too few or too many neutrons are unstable Ex 9 C 12 C 13 C 14 C Carbon 9 and 14 are radioactive

11. Nuclear Stability Why nuclides decay: need a stable ratio of p/n Neutron/Proton Ratios: where Z = atomic number (protons) When Z<20, the most stable nuclei have n/p ratio of 1:1 When Z>20, most stable isotopes have n/p ratio approaching 1.5/1 Unstable atoms with a high n/p ratio (more neutrons) tend to be beta emitters. Unstable atoms with a low n/p ratio (more protons) tend to be positron emitters.

12. Particles of equal chargerepel each other in the nucleus? The STRONG FORCE Proton to Neutron Ratio

13. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

14. Types of Radioactive Decay

15. Alpha and Beta Emission Alpha Decay Beta Decay

16. Positron Emission Beta Emission electron Positron Emission positron Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

17. Fajans -Soddy Group Displacement Law in an α-emission, the parent element will be displaced to a Group two places to the left and in a β-emission, it will be displaced to a Group one place to the right. RADIOACTIVE DISINTEGRATION SERIES The whole series of elements starting with the parent radioactive element to the stable end-product is called a Radioactive Disintegration Series (1) The Uranium Series (4n+2) (2) The Thorium Series (4n) (3) The Actinium Series (4n+3) (4) The Neptunium Series (4n+1)

18. Nuclear Reaction Nuclear Fusion is the energy-producing process taking place in the core of the Sun and stars The core temperature of the Sun is about 15 million °C. At these temperatures Hydrogen nuclei fuse to give Helium and Energy . The energy sustains life on Earth via sunlight

19. Nuclear Reaction Nuclear reactions deal with interactions between the nuclei of atoms The focus of this presentation are the processes of nuclear fission and nuclear fusion Both fission and fusion processes deal with matter and energy Matter can be changed into Energy Einstein’s formula above tells us how the change occurs In the equation above: E = Energy m = Mass c = Speed of Light (Universal Constant ) Energy Mass Light Speed

20. Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Atoms remain unchanged, although they may be rearranged Atoms often converted into atoms of another element Involve only valence electrons May involve protons, neutrons, and electrons Associated with small energy changes Associated with large energy changes Reaction rate influenced by temperature, particle size, concentration, etc. Reaction rate is not influenced by temperature, particle size, concentration, etc.

21. Nuclear reactions are different than chemical reactions Chemical Reactions Mass is conserved (doesn’t change) Small energy changes No changes in the nuclei Nuclear Reactions Small changes in mass Huge energy changes protons, neutrons, electrons and gamma rays can be lost or gained Fission = the splitting of nuclei Fusion = the joining of nuclei (they fuse together) Both reactions involve extremely large amounts of energy

22. Nuclear Reactions Characteristics: Isotopes of one element are into isotopes of another element Contents of the change amounts of are released changed nucleus Large energy

23. Induced Nuclear Reactions Scientists can also force ( = induce) nuclear reactions by smashing nuclei with alpha, beta and gamma radiation to make the nuclei unstable or

24. Types of Nuclear Reaction Elastic Scattering Inelastic Scattering Photonuclear Reactions Radiative Capture Fission Fusion Other Types of Nuclear Reactions Special Nuclear Reaction Evaporation Spallation Fragmentation Transfer reaction: Stripping and Pick-up

25. Nuclear Fission As a nuclear reaction occurs, it has the ability to produce a chain reaction A chain reaction is a reaction where the products are able to produce more products in a self-sustaining reaction series. In order to achieve a chain reaction there must be: A sufficient mass. A large concentration of fissionable nuclei The critical mass is when the mass and concentration are high enough to sustain a chain reaction. A sub-critical mass is one that is too small to achieve a chain reaction.

26. The fission reaction occurring when a neutron is absorbed by a uranium-235 nucleus. The deformed nucleus splits any number of ways into lighter nuclei, releasing neutrons in the process.

27. Induced Nuclear Fission of Uranium-235 is the origin of nuclear power and nuclear bombs. A neutron, , crashes into an atom of stable uranium-235 to create unstable uranium-236, which then decays. After several steps, atoms of Krypton and Barium are formed, along with the release of 3 neutrons and huge quantities of energy.

28. A schematic representation of a chain reaction. Each fissioned nucleus releases neutrons, which move out to fission other nuclei. The number of neutrons can increase quickly with each series .

29. Chain Reactions The neutrons released in the induced reaction can then trigger more reactions on other uranium-235 atoms…causing a CHAIN REACTION

30. A chain reaction can quickly get out of control materials that absorb some neutrons can help to control the chain reaction. Nuclear reactors have complex systems to ensure the chain reaction stays at safe levels. An uncontrolled chain reaction can result in the release of excess energy as harmful radiation It is on this concept that nuclear bombs are created. Nuclear “meltdown” occurs if the chain reactions cannot be controlled

31. Nuclear Fusion Nuclear fusion is the source of the energy from the Sun and other stars. Fusion is a very desirable energy source as: Two isotopes of hydrogen (deuterium and tritium) undergo fusion at a relatively low temperature. The supply of deuterium is unlimited with seawater being a very large source Enormous amounts of energy are released with no radioactive byproducts.

32. Nuclear Fusion joining of two light nuclei into one heavier nucleus. In the core of the Sun, two hydrogen nuclei join under tremendous heat and pressure to form a helium nucleus. When the helium atom is formed, huge amounts of energy are released. The fusion of hydrogen nuclei

33. The problems with utilizing fusion as an energy source are: Temperature . The amount of energy required to bring two nuclei together is enormous. Density The density of the reacting hydrogen nuclei must be significantly high so that there are enough reactions occurring in a short period of time. time These nuclei need to be confined to up to a second or more at 10 atmospheres of pressure in order for enough reactions to take place. Scientists cannot yet find a safe, and manageable method to harness the energy of nuclear fusion. “cold fusion” would occur at temperatures and pressures that could be controlled (but we haven’t figured out how to get it to happen)

34. A fusion reaction between a tritium nucleus and a deuterium nucleus requires a certain temperature, density, and time of containment to take place.

35. Plasma. A very hot gas consisting of atoms that have been stripped of their electrons and utilized as a confining mechanism Inertial confinement An attempt to heat and compress small frozen pellets of deuterium and tritium with energetic laser beams or particle beams, producing fusion.

36. 20 g 10 g 5 g 2.5 g after 1 half-life Start after 2 half-lives after 3 half-lives Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 757 10 g 5 g Half-life (t ½ ) Time required for the amount of radioactive atoms to decrease by half. Shorter half-life = less stable.

37. 0 1 2 3 4 Number of half-lives Radioisotope remaining (%) 100 50 25 12.5 Half-life Initial amount of radioisotope t 1/2 t 1/2 t 1/2 After 1 half-life After 2 half-lives After 3 half-lives

38. Half-Lives of Some Isotopes of Carbon Nuclide Half-Life Carbon-9 0.127 s Carbon-10 19.3 s Carbon-11 10.3 m Carbon-12 Stable Carbon-13 Stable Carbon-14 5715 y Carbon-15 2.45 s Carbon-16 0.75 s