Init 9/16/2008 by Daniel R. Barnes Section 25.3 Fission and Fusion of Atomic Nuclei 25.3 Section Assessment, page 813 15. Explain what happens in a nuclear chain reaction. Neutrons produced by fissioning atoms strike other fissionable atoms, causing them to split, which produces even more neutrons, which can then strike even more fissionable atoms. Lather, rinse, repeat. Ba-142 U-235 U-236 Kr-91 Ba-142 U-235 U-236 Kr-91 Ba-142 U-235 U-236 Kr-91 Ba-142 U-235 U-236 Kr-91 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 The energy released by nuclear reactions is much larger, per gram of explosive material, than the energy relased by chemical explosions. c2 = 34,500,000,000 mi2/s2 c = 186,000 mi/s e= 2 mc The atom bomb dropped on Hiroshima contained 64 kg of uranium, of which 0.7 kg underwent nuclear fission, and of this mass only 0.6 g was transformed into energy. The 1.5 pounds of uranium that split that day yielded the same explosive energy as 15,000 tons of TNT. 1.5 pounds of uranium = 30,000,000 pounds of TNT. 25.3 Section Assessment, page 813 16. Why are spent fuel rods from a nuclear reaction stored in water? Water cools spent fuel rods and provides a radiation shield. 25.3 Section Assessment, page 813 17. How are fusion reactions different from fission reactions? Fission reactions involve splitting nuclei. In fusion reactions, small nuclei combine and release much more energy. In fission reactions, big atoms fall apart. In fusion reactions, little atoms come together. Fission reactions make atoms smaller and more numerous. Fusion reactions make atoms bigger but less numerous. Ba-142 U-235 U-236 Kr-91 Protons donвЂ™t like other protons. Positives donвЂ™t like positives. The closer two charged particles get, the stronger the force between them. Remember how small a nucleus is compared to the atom as a whole? In a nucleus, protons are VERY close together. Protons close enough to join and form one nucleus will repel each other with extreme force. To get protons to get close enough to fuse together, you need to overpower this electrostatic repulsion. Extreme heat and/or extreme pressure, both of which are found in the centers of stars, can make fusion happen. If protons are smashed together by exterme heat and/or pressure, they will get close enough for something magical to happen . . . The strong force has a strength of zero until protons get VERY close together, and then it gets VERY strong, very suddenly. The вЂњstrong forceвЂќ turns on. The strong force is so powerful, that it overpowers the electrostatic repulsion the protons feel for each other . . . (The вЂњstrong forceвЂќ is sometimes called the вЂњstrong nuclear force.) . . . and it вЂњgluesвЂќ them together. H H He Ba-142 U-235 U-236 Kr-91 H H 25.3 Section Assessment, page 813 18. What does nuclear [sic] moderation accomplish in a nuclear reactor? Neutron moderation slows down neutrons so that they can be absorbed by fissile atoms. If you donвЂ™t slow down the neutrons, the chain reaction stops. If you throw the baseball too fast, the next guy canвЂ™t catch it. Without a moderator . . . Ba-142 U-235 U-236 Kr-91 U-235 . . . fission can not continue But with a moderator . . . Ba-142 U-235 U-236 Kr-91 MODERATOR Nice throw! Nice and slow! Ba-142 U-235 U-236 Kr-91 . . . the chain reaction can continue. 25.3 Section Assessment, page 813 19. What is the source of the radioactive nuclei in spent fuel rods? Unused nuclear fuel and fission products are the radioactive nuclei in spent fuel rods. Because вЂњspentвЂќ fuel rods still have some unsplit fissile nuclei in them (U-235 or Pu-239), some day, when uranium and plutonium are more expensive, we may need to recycle our spent fuel rods to get the remaining U-235 & Pu-239 out of them. вЂњSpentвЂќ fuel rods arenвЂ™t 100% spent. ThereвЂ™s still unused fuel in them. Now thatвЂ™s nuclear waste, isnвЂ™t it? 25.3 Section Assessment, page 813 20. Assuming technical problems could be overcome, what are some advantages to using a fusion reactor to produce electricity? Potential fuels are inexpensive and readily available. Fusion uses heavy hydrogen, and the world is covered with H2O. Not every hydrogen atom is deuterium or tritium, but when youвЂ™ve got as much water as we do in our oceans, it adds up to quite a bit of these less abundant hydrogen isotopes. Also, fusion produces helium instead of radioactive fission products. Helium is a very safe by-product. Perfecting nuclear fusion power will help us fill party balloons. Section 25.4 Radiation in Your Life 25.4 Section Assessment, page 813 21. Describe three methods of detecting radiation. If you want to detect radiation, you could use a Geiger counter, a scintillation counter, or a film badge. 25.4 Section Assessment, page 813 22. Describe two applications of radioisotopes in medicine. Radioisotopes can be used to both diagnose and cure disease. вЂњDiagnosisвЂќ means figuring out whatвЂ™s wrong with a sick person. For example, the radioisotope iondine-131 can be used to detect thyroid problems in a patient. As harmful as radiation is, radioisotopes can also be used, ironically, to help cure a sick person. For instance, salts of radioisotopes can be sealed in gold tubes and implanted in tumors. With any luck, this kills tumor cells much more than it kills healthy cells nearby, but, sometimes, the cure is worse than the disease and the patient suffers horribly. 25.4 Section Assessment, page 813 23. If you work regularly near a radiation source, why might your employer want to monitor your exposure to radiation by havnig you use a film badge rather than a Geiger counter? A Geiger counter only detects radiation being given off by radioactive atoms in you or on you. It doesnвЂ™t measure how badly youвЂ™ve been hit by radiation in the past. You can be hit by lots of damaging radiation but not become radioactive. Therefore, a dose of radiation might make your film badge change color, but you wouldnвЂ™t make a Geiger counter click. DonвЂ™t confuse radioactive atoms with radiation. DonвЂ™t confuse the gun with the bullet. You can be full of bullet holes, but with no guns anywhere on you. 25.4 Section Assessment, page 813 24. What is an advantage of using a radioactive seed, consisting of a radioisotope in a small gold tube, to treat a cancerous tumor? The radioisotope inside the вЂњseedвЂќ will emit beta and gamma rays into the surrounding tumor cells, killing them. The seed will not move from its inserted location. Why gold? Gold does not rust, so the tube is leakproof. You donвЂ™t want that radioactive stuff leaking out and wandering around the body. Keep the cobra in its cage!