CHM 585Chapter 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 • Accounts for about 20% of the total electricity generated in the U.S.
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.
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
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
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.
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.
Continued • To calculate the energy change per mole of 23892U:
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
Nuclear Fission • Splitting of heavy nuclei is exothermic for large mass numbers. • Consider a neutron bombarding a 235U nucleus:
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.
Nuclear Fission
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).
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
235U is the only naturally occurring nuclide that is fissionable with thermal neutrons • Other fissionable materials are 239Pu and 233U • 235U from uranium ore
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
UF6 • U3O8 + 2H2 3 UO2 + 2 H2O • UO2 + 4 HF UF4 + 2 H2O • UF4 + F2 UF6 ( distillable)
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.
FG21_020.JPG Nuclear Power Plant
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.
Some Question Nuclear Power Plants, but.
A Few Other Concepts: • Radioactive decay • Radon • Radiotracers • PET ( Positron Emission Tomography)
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.
Rates of Radioactive Decay
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.
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
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.
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.
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.
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
The End
