Nuclear Energy Fundamentals

Transcription

1 Nuclear Energy Fundamentals PREPARED BY Academic Services April 2012 Institute of Applied Technology, 2012

2 Module Objectives Upon successful completion of this module, students will be able to: Identify the different processess comprising uranium fuel cycle. Describe the three different methods used in mining uranium. Explain the milling operation of uranium. Describe the conversion operation of uranium. Explain the three methods used for uranium enrichment. Distiguish between the different uranium grades and their uses. Describe the fuel fabrication process of uranium. Identify and explain the two method of the spent fuel storage. Explain the reprocessing operation of uranium. Module Contents: Topic Page No. 1. Introduction 3 2. Nuclear Fuel 3 3. Uranium Fuel Cycle 4 4. Uranium Mining 4 5. Uranium Milling 6 6. Uranium Conversion 6 7. Uranium Enrichment 7 8. Uranium Grades 8 9. Fuel Fabrication Spent Fuel Storage Reprocessing Summary of Fuel Cycle Activities References 14

3 1. Introduction As explained in module 1 uranium is the most commonly used fuel in nuclear power plants (NPP). In this module we are going to focus on the fuel cycle of uranium which comprises many processes to convert uranium from the ore stage to the fuel stage. 2. Nuclear Fuel In the last 50 years, uranium (Fig. 2.1) has become one of the world s most important energy minerals. Traces of it occur almost every where, including oceans. There are uranium mines in about 20 countries, although more than two third of the world production comes from just 10 mines. Today there are strict controls on the buying and selling of uranium. It is only sold to countries that have signed the Nuclear Non-Proliferation Treaty. This allows international inspectors to check that it is used only for peaceful purposes. Table 2.1 shows which countries have the largest uranium resources and the percentage of uranium they have in relation to that of the whole world. Fig. 2.1: Uranium ore. Table 2.1: World resources of uranium. No. Country Tons Approximate % of world 1 Australia % 2 Kazakhstan % 3 Canada % 4 USA % 5 South Africa % 6 Namibia % 7 Brazil % 8 Niger % 9 Russia % 10 Uzbekistan % 11 Ukraine % 12 Jordan % 13 India % 14 China % 15 Others %

4 3. Uranium Fuel Cycle The first step before the uranium fuel cycle starts is the exploration of ore. The ore bodies containing uranium are first located by drilling and through other geological techniques. The preparation of uranium to change to a usable fuel involves five main processes. These are mining, milling, conversion, enrichment and fuel fabrication. After being used in the power plant the spent fuel has to be stored for several months to several years in order to reduce the radiation levels. In a reprocessing facility the used fuel is separated into different components to produce fresh fuel and to reduce the amount of waste. Fig. 2.2 shows the nuclear fuel cycle. High Level Waste Fig. 2.2: Uranium fuel cycle. 4. Uranium Mining Uranium is mined from underground and the ore is crushed and extracted. The natural uranium is composed of 99.28% U-238 and 0.72% U-235. Uranium ore is removed from the ground in one of three ways depending

5 on the characteristics of the deposit. Uranium deposits close to the surface can be recovered using the open pit mining method (Fig. 2.3), and underground mining methods (Fig. 2.4) are used for deep deposits. In some circumstances the ore may be mined by in-situ recovery (Fig. 2.5), a process that dissolves the uranium while still underground and then pumps a uranium-bearing solution to the surface. Fig. 2.3: Uranium open pit mining. 2.4: Uranium underground mining. Fig. 2.5: Uranium in-situ recovery.

6 5. Uranium Milling At uranium mills, usually located near the mines, uranium ores are crushed and ground, and the uranium oxide is chemically extracted. The mill product, called uranium concentrates or yellowcake (Fig. 2.6), is then marketed and sold as pounds of U3O8 or kilograms of uranium Fig. 2.6: Uranium Yellowcake. 6. Uranium Conversion After the yellowcake is produced at the mill, the next step is conversion into pure uranium hexafluoride (UF6) gas suitable for use in enrichment operations. During this conversion, impurities are removed and the uranium is combined with fluorine to create the UF6 gas. The UF6 is then pressurized and cooled to a liquid. In its liquid state it is drained into 14-ton cylinders where it solidifies after cooling for approximately five days. The UF6 cylinder, in the solid form, is then shipped to an enrichment plant (Fig. 2.7). UF6 is the only uranium compound that exists as a gas at a suitable temperature. Fig. 2.7: The cylinders of uranium hexafluoride are transported to the enrichment facility.

7 7. Uranium Enrichment Throughout the global nuclear industry, uranium is enriched by one of two methods: gaseous diffusion or gas centrifuge. Many methods like laser enrichment and others have been proposed for use in the recent years. Gaseous diffusion Fig.2.8: Gaseous diffusion. Combining uranium with fluorine is followed by gaseous diffusion to increase the percentage of uranium fissionable isotope U-235. The UF 6 output from gaseous diffusion is divided in two streams (Fig. 2.8). One is increased, or enriched, in its percentage of U-235, and the other is reduced, or depleted, in its percentage of U-235. The depleted uranium hexafluoride product is referred to as "depleted UF 6." After gaseous diffusion, the enriched uranium hexafluoride is subjected to further processing, while the depleted UF 6 is generally stored. Fig. 2.9: Gas centrifuge cylinders. Gaseous diffusion is based on the separation effect caused by the flow of gas through small holes. Fig. 2.10: Gas centrifuge process.

8 Gas centrifuge process The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations (Fig. 2.9). Each cylinder's rotation creates a strong centrifugal force so that the heavier gas molecules containing U-238 move toward the outside of the cylinder and the lighter gas molecules rich in U-235 collect closer to the center (Fig. 2.10). It requires much less energy to achieve the same separation than the older gaseous diffusion process, which it has largely replaced and so is the current method of choice and is termed second generation. 8. Uranium Grades The enrichment can produce several grades. Each grade has it is own use. The following explains each grade and where it is used: Slightly enriched uranium: has a U-235 concentration of 0.9% to 2% and is used in some heavy water reactors. Reprocessed uranium is a product of nuclear fuel cycles involving nuclear reprocessing of spent fuel recovered from light water reactor spent fuel typically contains slightly more U-235 than natural uranium, and therefore could be used to fuel reactors that use natural uranium as fuel. Low enriched uranium: has a lower than 20% concentration of U For use in commercial light water reactors, in the most common power reactors in the world, uranium is enriched to 3 to 5% U-235. In research reactors the enrichment level usually reaches 12% to 19.75% U-235. Highly enriched uranium: has a greater than 20% concentration of U-235. Uranium in nuclear weapons usually contains 85% or more of U-235 known as weapons-grade, though for a crude, inefficient weapon 20% is sufficient (called weapons-usable).

9 Fig shows the different grades of uranium. (a) Slightly enriched uranium 0.9% - 2%U-235 (b) Low-enriched uranium (Reactor grade) 3-5% U-235 (c) Highly enriched uranium (Weapon grade) >85%-90% U-235 Fig. 2.11: Uranium grades. 9. Fuel Fabrication Fuel fabrication for light water (regular) power reactors typically begins with receipt of low-enriched uranium hexafluoride (UF6) from an enrichment plant (Fig. 2.12). The UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form uranium dioxide (UO 2 ) powder. After that the powdered UO 2 is pressed into small cylindrical shapes and baked at a high temperature ( C) to make hard ceramic pellets (Fig 2.13b). Fig shows the energy equivalence of one fuel pallet compared to other types of fuels used in power generation. Fig. 2.12: Fuel fabrication process.

10 In a light water reactor, the fuel pellets are packed in thin tubes called fuel rods. The rods are grouped together into a bundle called a fuel assembly (Fig. 2.13a). A typical 1,100 megawatt pressurized water reactor contains 193 fuel assemblies composed of nearly 51,000 fuel rods and approximately 18 million fuel pellets. (a) (b) Fig. 2.13:(a) Fuel assembly, rod and pellets. (b) Fuel pellet size. One uranium pellet has the same energy available in: 48 m 3 of natural gas 3 barrels of oil. (159 Liters) 1000 kg of coal 2500 kg of firewood Fig. 2.14: The energy equivalence of 1 uranium fuel pellet compared to other types of fuel.

11 10. Spent Fuel Storage There are two storage methods for spent fuel after it is removed from the reactor core, namely: Spent fuel pools Dry cask storage Spent Fuel Pools All nuclear plants have storage pools for spent fuel. These pools are typically 13 meters or more deep. In the bottom 5 meters are storage racks designed to hold fuel assemblies removed from the reactor (Fig. 2.15). In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years. The water serves 2 purposes: It serves as a shield to reduce the radiation levels that people working above may be exposed to. It cools the fuel assemblies that continue to produce heat (called decay heat) for some time after removal. Fig. 2.15: Spent fuel pool.

12 Dry cask storage In the late 1970s and early 1980s, the need for alternative storage began to grow when pools at many nuclear reactors began to fill up with stored spent fuel. Utilities began looking at options such as dry cask storage for increasing spent fuel storage capacity. Dry cask storage allows spent fuel that has already been cooled in the spent fuel pool for at least one year to be surrounded by inert gas inside a container called a cask. The casks are typically steel cylinders that are either welded or bolted to close them (Fig. 2.16). The steel cylinder provides a leak-tight container of the spent fuel. Each cylinder is surrounded by additional steel, concrete, or other material to provide radiation shielding to workers and members of the public. Some of the cask designs can be used for both storage and transportation (Fig. 2.17). Fig. 2.16: Dry cask construction. Fig. 2.17: Dry cask storage. 11. Reprocessing Nuclear reprocessing uses chemical procedures to separate the useful components (especially the remaining uranium and the newly-created plutonium) from the fission products and other radioactive waste in spent nuclear fuel obtained from nuclear reactors. The reprocessed uranium can in principle also be re-used as fuel, but that is only economic when uranium prices are high.

13 12. Summary of Fuel Cycle The following figure summarizes the fuel cycle of uranium from ore to disposed fuel stage. Fig. 2.17: Summary of uranium fuel cycle.

14 13. Activities 13.1 Uranium crossword. Workout the uranium crossword below. Some answers are available in module 1.

15 Across 1. One method of mining uranium is called mining. 4. radiation occurs in outer space. 6. A process to separate the metal from the is performed after uranium is mined. 9. Another method of mining uranium is called situ leaching. 10.Uranium is a metal. 11. particles are heavy and cannot travel very far. 12. Gamma are high energy and can travel through thick concrete. 19. Open mining is the name given to mining on the surface. 20. particles are small and can travel quite far. 21. Uranium can also be found in the. Down 2. Uranium is transformed into electricity in a reactor. 5. Uranium is e before it is pressed into small pellets and made into fuel rods. 7. When an atom gives up an it becomes ionized. 8. After being mined, uranium is. 15. Uranium is exported as uranium concentrate. 18. People are exposed to radiation when they go out in the. 13. References Chemistry Concepts and Applications Mc Graw-Hill Glenco. Why Science Matters, Using Nuclear Energy by John Townsend, Heinemann