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CHM 585 Chapter 22

CHM 585 Chapter 22. Nuclear. Radioactive. When nuclei change spontaneously, emitting radiation, they are said to be radioactive. Radioactive elements are widely used in medicine as diagnostic tools and as a means of treatment, especially for cancer. Nuclear Power.

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CHM 585 Chapter 22

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  1. CHM 585Chapter 22 Nuclear

  2. Radioactive • When nuclei change spontaneously, emitting radiation, they are said to be radioactive. • Radioactive elements are widely used in medicine as diagnostic tools and as a means of treatment, especially for cancer

  3. Nuclear Power • Accounts for about 20% of the total electricity generated in the U.S.

  4. The source of nuclear energy is the rearrangement of protons and neutrons to form more stable nuclei. • In this manner, nuclear reactions are akin to chemical reactions; the latter occur in nature — often spontaneously — and except for the relatively few spontaneous endothermic reactions, the products have less internal energy than the reactants and can therefore exist in a more stable form.

  5. Fusion is the combination of light nuclei such as the hydrogen nucleus, a single proton, to produce something heavier such as the helium nucleus, two protons and two neutrons. • 2H + 3H  4He + 1n • Fission is the process of breaking apart large nuclei such as uranium into some smaller pieces such as krypton and iodine. • Both nuclear reactions function under the principle that more stable nuclei result, both release very large amounts of energy in the process

  6. E=mc2 • 1 amu = 4.2 x 10-17 kwh = 931 MEV • The energy evolved from grams of nuclear fuel is equivalent to that evolved from tons of chemical fuels

  7. Energy Changes in Nuclear Reactions • Einstein showed that mass and energy are proportional: E = mc2 • If a system loses mass it loses energy (exothermic). • If a system gains mass it gains energy (endothermic). • Since c2 is a large number (8.99  1016 m2/s2) small changes in mass cause large changes in energy. • Mass and energy changed in nuclear reactions are much greater than chemical reactions.

  8. Energy Changes in Nuclear Reactions 23892U 23490Th + 42He • for 1 mol of the masses are 238.0003 g  233.9942 g + 4.015 g. • The change in mass during reaction is 233.9942 g + 4.015 g - 238.0003 g = -0.0046 g. • The process is exothermic because the system has lost mass.

  9. Continued • To calculate the energy change per mole of 23892U:

  10. Terms • The 4He particles are known as alpha particles • High speed electrons emitted by an unstable nucleus are known as beta particles • Gamma radiation consists of high-energy photons – electromagnetic radiation of very short wavelength • A positron is a particle that has the same mass as an electron, but an opposite charge

  11. Nuclear Fission • Splitting of heavy nuclei is exothermic for large mass numbers. • Consider a neutron bombarding a 235U nucleus:

  12. Nuclear Fission • The heavy 235U nucleus can split into many different daughter nuclei, e.g. 10n + 23892U 14256Ba + 9136Kr + 310n releases 3.5  10-11 J per 235U nucleus. • For every 235U fission 2.4 neutrons are produced. • Each neutron produced can cause the fission of another 235U nucleus.

  13. Nuclear Fission

  14. Nuclear Fission • The number of fissions and the energy increase rapidly. • Eventually, a chain reaction forms. • Without controls, an explosion results. • Consider the fission of a nucleus that results in daughter neutrons. • Each neutron can cause another fission. • Eventually, a chain reaction forms. • A minimum mass of fissionable material is required for a chain reaction (or neutrons escape before they cause another fission).

  15. Uranium • Three naturally occurring isotopes • Uranium-233 • Uranium 235 • Uranium 238 • Vary in number of neutrons; all have 92 protons • 99.3% is 238U; 0.7% 235U; only trace 233U

  16. 235U is the only naturally occurring nuclide that is fissionable with thermal neutrons • Other fissionable materials are 239Pu and 233U • 235U from uranium ore

  17. Uranium Ore • Ore contains 0.05 – 0.2% U3O8 • U3O8 concentrated from ore by extraction with sulfuric acid followed by either ion exchange or solvent extraction • For solvent extraction, typically use organic amines to give about 90% U3O8

  18. UF6 • U3O8 + 2H2  3 UO2 + 2 H2O • UO2 + 4 HF  UF4 + 2 H2O • UF4 + F2 UF6 ( distillable)

  19. UF6 • A mixture of 238U and 235U • About 0.7% 235U • 235U content is enriched to about 2 – 4% by a variety of techniques – typically gaseous diffusion through a membrane • Enriched 235UF6 is hydrolyzed and reduced to UO2 – ground for preparation of high density pellets.

  20. FG21_020.JPG Nuclear Power Plant

  21. Enriched uranium pellets encased in Zr or stainless steel tubes are used for fuel in nuclear power plants. • Control rods made of Cd or B control the fission process by absorbing neutrons. • A moderator slows down the neutrons so they are more likely to be captured by the fuel. • A cooling liquid circulates through the reactor and its heat is used to produce steam which generates electricity by driving a steam turbine.

  22. Some Question Nuclear Power Plants, but.

  23. A Few Other Concepts: • Radioactive decay • Radon • Radiotracers • PET ( Positron Emission Tomography)

  24. Rates of Radioactive Decay • 90Sr has a half-life of 28.8 yr. If 10 g of sample is present at t = 0, then 5.0 g is present after 28.8 years, 2.5 g after 57.6 years, etc. 90Sr decays as follows 9038Sr 9039Y + 0-1e • Each isotope has a characteristic half-life. • Half-lives are not affected by temperature, pressure or chemical composition. • Natural radioisotopes tend to have longer half-lives than synthetic radioisotopes.

  25. Rates of Radioactive Decay

  26. Rates of Radioactive Decay • Half-lives can range from fractions of a second to millions of years. • Naturally occurring radioisotopes can be used to determine how old a sample is. • This process is radioactive dating.

  27. Radon Radon • The nucleus 22286Rn is a product of 23892U. • Radon exposure accounts for more than half the 360 mrem annual exposure to ionizing radiation. • Rn is a noble gas so is extremely stable. • Therefore, it is inhaled and exhaled without any chemical reactions occurring. • The half-life of is 3.82 days. • It decays as follows: 22286Rn 21884Po + 42He

  28. The -particles produced have a high RBE(Relative Biological Effectiveness). • Therefore, inhaled Rn is thought to cause lung cancer. • The picture is complicated by realizing that 218Po has a short half-life (3.11 min) also: 21884Po 21482Pb + 42He • The 218Po gets trapped in the lungs where it continually produces -particles. • The EPA recommends 222Rn levels in homes to be kept below 4 pCi per liter of air.

  29. Radiotracers • 131I has been used to test the activity of the thyroid gland • Patient drinks a solution of NaI containing a small amount of 131I. • A Geiger counter determines the ability of the thyroid to take up iodine. A normal thyroid will absorb about 12% of the iodine within a few hours.

  30. PET (Positron Emission Tomography) • Can be used to monitor various compounds such as glucose. • Compound to be detected in the patient must be labelled with a radionuclide that is a positron emitter. • Most widely used nuclides are 11C, 18F, 15O, and 13N.

  31. For example, glucose can be labeled with 11C • Because half life is short, must then quickly inject the compound into the patient and then place patient in PET instrument

  32. The End

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