WELCOME TO PERIOD 18: CONSEQUENCES OF NUCLEAR ENERGY

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1 WELCOME TO PERIOD 18: CONSEQUENCES OF NUCLEAR ENERGY Homework #17 is due today. Midterm 2: Weds, Mar 27, 7:45 8:55 pm (Same room as your midterm 1 exam.) Covers periods and videos 3 & 4 Review: Tues, 3/26, 7:00 8:00 pm 2005 SM Drop in: Weds, 3/27, 5:30 7:15 pm 2005 SM

2 PHYSICS 1104 PERIOD 18 How do nuclear reactors produce electricity? What type of reactor is safest? How is radioactive waste stored?

3 Nucleus stability Stable nuclei have. 83 or fewer protons. Large unstable nuclei with more than 83 protons are radioactive. As unstable nuclei decay (fission), they give off energy.

4 Fission reactions Uranium-235 nuclei can spontaneously fission (split) The products of the fission of a U-235 nucleus are smaller fission fragments, neutrons and energy. Other U-235 nuclei absorb the neutrons. These U-235 nuclei fission and emit neutrons. This process of neutron absorption resulting in fission produces an exothermic chain reaction.

5 Energy from fission

6 Components of conventional reactors Uranium-235 nuclei fission and emit fast moving neutrons. To produce a chain reaction, other uranium-235 nuclei absorb these neutrons. Neutrons are more easily absorbed when they move more slowly. a) A moderator (usually water or graphite) is needed to slow the velocity of the fast-moving neutrons. Fuel rods are placed in the moderator. b) Control rods control the rate of the reaction by absorbing neutrons. Control rods (usually cadmium or boron) can slow down or stop the reaction.

7 Components of conventional reactors 1) Fuel rods or ceramic fuel pellets filled with a mixture two isotopes of uranium: U-235 and U ) Coolant (usually water or helium gas) to transfer heat from the reactor core to the vessel that produces steam to turn the turbines. 3) Containment systems to prevent radiation from escaping into the environment. Double containment systems are safer. A heat exchanger transfers thermal energy from the fission reactions to a container of water. The water is converted into steam. The steam pressure turns turbines, which spin magnets near coils of wire.

8 Source:

9 Fuel for conventional nuclear reactors Naturally-occurring uranium ore consists of two isotopes of uranium: uranium-238 and uraniuim-235 Ore is about 99.3% uranium-238 and only 0.7% uranium-235. Since conventional reactors use uranium-235 as fuel, naturally occurring ore must be enriched to decrease the percentage of uranium-238 and increase the percentage of uranium-235.

10 Chernobyl reactor accident In April 1986, a mishandled safety exercise caused an uncontrolled fission chain reaction. The water coolant boiled into steam molecules, which dissociated into atoms of hydrogen and oxygen. These atoms recombined explosively, blowing the lid off the reactor core and causing the graphite moderator to catch on fire. Initially, there was concern that there might be enough fuel remaining below the reactor core to undergo another uncontrolled chain reaction. This was not the case. There is still a danger of radiation leaks from the crumbling concrete sarcophagus surrounding the damaged reactor. The massive reactor lid could fall and send radioactive dust into the atmosphere.

11 Fukushima nuclear accident What happened? The tsunami wave that inundated the Fukushima nuclear power plant lead to a series of explosions and a meltdown of three reactor cores. Radiation was released into the atmosphere and ocean. Why did this happen? The backup generators and electronics controlling the reactor cooling systems were located in the basement of the building. Because the flood walls were not tall enough to keep water out of the basement, the cooling systems flooded and failed. Battery backup systems lasted only a few days. The reactor cores overheated. Due to the intense heat, water dissociated into hydrogen and oxygen gas. There was insufficient ventilation in the buildings, and the hydrogen gas was trapped and exploded.

12 Fukushima nuclear accident What design changes could prevent Sea water from reaching the reactor? Sea water from flooding the generators and control systems? Hydrogen gas trapped inside the reactor buildings?

13 Types of conventional nuclear reactors Reactors that use fuel rods and are graphite moderated and water cooled. (Chernobyl type) Reactors that use fuel rods and are water moderated and water cooled. (type of reactor used in U.S.) Reactors that use small, widely separated, ceramiccoated fuel pellets. These reactors may be water moderated or graphite moderated and may be water cooled or gas cooled.

14 Pressurized water reactor

15 Boiling water reactors

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17 Breeder reactors Conventional reactors use relatively scarce uranium-235 (Only 0.7% of natural uranium is the U-235 isotope.) Breeder reactor fuel is plutonium-239, which has been created from uranium-238. Mixed in with the plutonium- 239 is uranium-238. Breeder reactors use fast neutrons, so no moderator is needed to slow down the neutrons. Advantage: Breeder reactors create additional plutonium- 239 from uraniuim-238 while they are operating. Breeder reactors produce more plutonium than they use. Disadvantage: Plutonium can be more easily converted into nuclear weapons than can the byproducts of conventional reactors.

18 Radioactive waste Used fuel rods from reactors are extremely radioactive. They remain radioactive for thousands of years. A plan to store spent radioactive fuel rods in canisters buried inside tunnels in Yucca Mountain, Nevada has been underway for several decades. This plan has been suspended. Currently, spent fuel rods are stored at the reactor site in concrete containers above ground or in pools of water.

19 Storage of spent radioactive fuel

20 Storage of less radioactive waste

21 Energy from fusion Fusion of smaller nuclei to make larger nuclei releases energy. A hydrogen nucleus (a proton) can fuse with neutron to form deuterium. Two deuterium nuclei can fuse into a helium nucleus.

22 Fusion reactors? Fusion occurs in the core of stars and some nuclear bombs. Advantages of fusion over fission: Hydrogen is plentiful (from water) Fusion produces more energy than fission. No radioactive uranium or plutonium waste. Problems? Temperatures of nearly a million degrees are required to provide the activation energy. This is too hot for material to hold the reactants. The reactants must be held in place using very strong (magnetic) forces. Nevertheless, there has been some limited success in producing controlled nuclear fusion.

23 BEFORE THE NEXT CLASS Read textbook chapter 19. Complete Homework Exercise 18. Print out Activity Sheet 19. Midterm 2: Weds, Mar 27, 7:45 8:55 pm (Same room as your midterm 1 exam.) Covers periods and videos 3 & 4 Review: Tues, 3/26, 7:00 8:00 pm 2005 SM Drop in: Weds, 3/27, 5:30 7:15 pm 2005 SM