Subatomic Particles. Nuclear Energy. Mass and Energy. Chemistry in Context: Chapter 7: Fires of Nuclear Fission

Transcription

1 Nuclear Energy Chemistry in Context: Chapter 7: Fires of Nuclear Fission Assignment: All the problems with blue codes or answers on Page , the first nuclear power plant near Pittsburgh, PA. Seabrook plant first proposed in 1972, construction began in 1976, and operating license granted in /5 your electricity is generated by nuclear power 103 nuclear plants in US; 9 nuclear plants have closed since 1990 due to perceived risk. Mass and Energy E = mc 2 Where E = energy released, m = mass lost, and c = 3.0 x 10 8 m/s. Since c 2 =9.0 x m 2 /s 2 and 1 Joule is equal to 1 kg-m 2 /s 2, a tremendous amount of energy can be obtained from a very small amount of mass. 1 kg of U-235 with 0.1 % mass loss results in the production of 9.0 x Joules or the energy equivalent to tons of TNT. Subatomic Particles Electrons (Negatively charged particles outside nucleus; discovered by Thomson) Nucleons held together by very strong nuclear force Protons (Positively charged particles inside the nucleus; discovered by Rutherford) Neutrons (Neutral particles inside the nucleus; discovered by Chadwick)

2 History of Nuclear Fission 1938, Hahn and Strassmann found Ba atoms when bombarding U with neutrons Lise Meitner and Otto Frisch hypothesized that the U atoms were splitting to form atoms of lighter elements such as Ba in a nuclear fission process. Energy given off fission is described by Einstein s equation; mass of products less than mass of reactants; mass loss = energy! Neither mass nor energy is individually conserved; matter is a concentrated form of energy. Equations for Nuclear Reactions Mass number of an element is equal to the sum of protons (p) and neutrons (n). Atomic nuclei of heavier elements or higher atomic number tend to undergo nuclear reactions or decay due to the high p:n ratio. Balancing equations for nuclear reactions involves making sure that the sums of subscripts (charges) and superscripts (mass number) on both sides of the equation are equal. Decay Series of Radioactive Elements Transmutation of elements occur by nuclear emission of α or β particles that change the atomic number by 2 and +1, respectively. Nuclear Fission vs. Fusion Fission in which a large nucleus splits into smaller nuclei and neutrons is associated with loss of nuclear mass or release of a vast amount of energy in nuclear reactors. Fusion involves smaller nuclei combine to form larger nuclei with a greater release of energy. Sun and stars rely on fusion to achieve high temperature ~ 100 million degrees. Fusion produce ionized gas of electrons, protons and other nuclei. Advantages of fusion include low cost, abundance of deuterium, and no waste problems

3 Chain Fig reaction (i.e. exponential increase of neutrons) can occur spontaneously if a critical mass (~15 kg) of U-235 is available. Figure 7.5 on Page 291 How does this work? Each neutron hits a U-235 nucleus and causes its fission or splitting into lighter nuclei along with the production of 2-3 neutrons. The chain reaction of successive fission events lead to loss of mass (0.1 %) which is related to energy release (E=mc 2 ) Note that the Kr-Ba pathway releases 3 neutrons whereas the Rb-Cs and Xe-Sr pathways release 2 neutrons each. Nuclear Fuel UO 2 Fuel Pellets and Fuel Rod Assembly 200 UO 2 pellets of the size of pencil eraser are packed into metal tubes that are assembled into fuel assembly Critical mass of U-235 needed is about 33 pounds (15 kg). Plutonium or americium are used to generate neutrons needed to initial the fission as follows: 238 Pu 234 U + 4 He; 4 He + 9 Be 12 C + 1 n Fig. 7.1 on Page 284 Fig. 7.7 on Page 293

4 Coal and nuclear power plants are similar except for water being heated by energy from coal combustion as opposed to nuclear fission. How is fission controlled? Control rods made of cadmium, silver, indium serve as neutron absorbers to prevent uncontrollable chain reaction of the fission process. The fuel bundles and control rods are bathed in a primary coolant solution of boric acid in water to provide heat transfer and to absorb neutrons via borons and slow down the speed of neutrons. Contains boric acid, neutron absorber Fig. 7.6 on Page 292 Seabrook Nuclear Power Plant Fig. 7.2 Figure 7.12 Page 297 Natural waters does not come in contact with nuclear materials 400-ton reaction vessel with 44-ft wall of 8-in carbon steel; 4.5 ft thick inner concrete walls of containment and 15in outer wall.

5 Figure 7.8 on Page 293 Chernobyl Nuclear Accident Cooling towers Nuclear reactor Figure 7.9 on Page 295 Chernobyl Nuclear Accident Cooling water stopped as part of a safety test ; reactor temperature rose quickly. Due to insufficient number of graphite control rods and steam pressure to deliver coolant, a power surge led to the meltdown of reactor core and burning of graphite rods. Water sprayed on rods led to the production of hydrogen gas that exploded upon reaction with oxygen in the air Fig. 7.10: Chernobyl Reactor 4 In 1986, an explosion blew off the 4000-ton steel plate covering the reactor at the Chernobyl plant and spewed radioactive products in the vicinity.

6 What happened at Chernobyl? 2 H 2 O + C 2 H 2 + CO 2 2 H 2 + O 2 2 H 2 O + Energy Explosion spewed radioactive materials into the atmosphere across Ukraine, Belarus, and Scandinavia Tragic outcomes include several outright deaths, deaths of 31 firefighters due to acute radiation sickness, 190 patients with acute radiation sickness, and 200,000 liquidators, who buried the most hazardous waste and constructed the sarcophagus. Chernobyl closed in Could nuclear mishaps happen here? More regulations and different plant designs in the U. S. Three Mile Island Lost coolant, partial meltdown. Disasters result from the complex interplay of faulty plant design, human error, and political instability. Radioactivity Radioactivity is a spontaneous process of nuclei undergoing a change in atomic number or elemental identity by emitting particles or rays (transmutation). Nuclei continue to decay till stable nucleus is produced (Z [atomic number] < 83). Exceptions are Technetium (Z = 43) and Promethium (Z = 61). Radioactivity: History Discovered in 1896 by Antoine Henri Becquerel who noticed the radiation of a uranium sample on a photographic plate. Marie Curie applied the term Radioactivity and discovered 2 other radioactive elements, Radium and Polonium. Ernest Rutherford identified 2 types of radiation, namely the alpha (α) and the beta (β) particles.

7 Radioactive Emissions Page 301 Gamma ray is a form of electromagnetic radiation that is even more energetic than the X-ray discovered by Wilhelm Roentgen Penetration Powers of Nuclear Radiation Beta (β) particle is a form of high speed electron with a 1 charge. Alpha particle (α) is a helium nucleus with +2 charge. Gamma ray (γ) is a highly energetic photon with no mass and no charge. β = -1 o e 4 He α = 2 Nuclear Bomb vs. Nuclear Power Rapid and uncontrolled fission in a bomb compared to the slow and controlled energy release and use of control rods. Bomb uses highly enriched U-235 (>90%) relative to the 3-5% U-235. Separation of U-235 and U-238 is achieved by the diffusion of UF 6 at 56 C at Paducah, KY. Nuclear Fuel for Weapons Processing of U-235 is too complex and costly for use in nuclear weapons. Pu-239, which can be extracted from spent nuclear fuel with U-238, is a more likely fissionable material in bombs. Nuclear reaction in breeder reactor is: 1 n U 239 Pu e (Hanford, WA) PuO 2 is easily inhaled, causing lung cancer and damage to bone and liver via its solubility in blood; Nagasaki bomb is based on Pu-239.

8 Measurement of radioactivity Rate of emission of decay particles in counts per minute (cpm) can be measured by a Geiger counter. 1 Curie = 3.7 x disintegrations/sec Disintegrations refer to α, β, or γ emission Typically measured as mci, µci, and pci Radiation Doses and Health Alpha, beta, gamma, neutrons, x-rays are ionizing radiation that can damage living cells (white blood cells or bone marrow) or induce DNA transformation or cancer. One RAD is the absorption of 0.01 Joule of radiant energy per kg of tissue. Physiological damage is measured by rem or Roentgen equivalent applied to mammals. REM = n x (Number of Rads) where n = 10 for α and neutrons and n = 1 for β, γ, and X-rays. 1 Sv (sievert) = 100 rem; 1 µsv = 1 x 10-6 Sv = 0.10 mrem Table 7.2 Page 284 Radioactive Decay Nuclide refers to a nucleus with a unique atomic number and mass number Radionuclide decays spontaneously as follows: A B + b where A is a parent nuclide, B is a daughter nuclide, and b is the emitted radiation. The rate of decay is given by the half-life or the time required for the level of radioactivity to fall to ½ of its initial value.

9 Figure 7.15, 7.16, &7.17 Page 303 Table 7.4 Page 305 Applications of Radionuclides Smoke detectors - Americium-241 as an α emitter Food irradiation to kill harmful microbes Medical diagnosis and cancer treatment I-131 used to treat thyroid cancer and hyperthyroidism 85 Sr, 99 Tc, 197 Hg, and 123 I are used for diagnosis Research: Studies chemical reaction pathways and distribution of pollutants using radioactive labels Dating of archaeological objects by C-14 1 C-14 radionuclide for every C-12 atoms; C-14 concentration decreases by ½ every 5730 years. High-Level Nuclear Waste (HLW) HLW or mixed waste is hazardous due its radioactivity and toxicity; it consists of radioactive materials from the reprocessing of spent nuclear fuel. The harmful characteristics of HLW depend on the fission products, their half-lives, the type of radiation, and accumulation in the food chain

10 Spent fuel rods containing U-238, I-131, Cs-137, and Sr-90 are kept under onsite pools of water with neutron absorbers. Figure 7.20 Page 310 Active management Fig Figure 7.18 on Page 293 Burial of low-level nuclear wastes Figure 7.22 Page 312 Special glasses or ceramics are used to enclose high level nuclear wastes and then packed into metal canister for burial in repository (e.g. vitrification of Pu239)

11 Underground Disposal of Nuclear Waste Fig Figure 7.17 on Page 291 Ideal repository will be at least 1000 ft underground and 1000 ft above water table; the host rock formation should ideally be salt, basalt, tuff, and granite. Table 7.7 Page 318 Coal-fired power plants release more radioactivity than nuclear plants due to the vast amounts of coal consumed that contain both thorium and uranium. Usage of Nuclear Power in Various Countries The extent of usage of nuclear power is dependent upon the people s perception of nuclear risk and the existing resources for energy production Figure 7.25 Page 317