Chapter 20: Nuclear Energy and the Environment

1. Chapter 20: Nuclear Energy and the Environment

2. Nuclear Energy • The energy of the atomic nucleus • Two processes can be used to release that energy • Fission – splitting of atomic nuclei • Fusion – fusing or combining of atomic nuclei • Nuclear reactors-devices that produce controlled nuclear fission. • Used for commercial energy production

3. Fission Reactors • First demonstrated in 1942 • Led to development of nuclear energy to produce electricity. • Also power submarines, aircraft carriers, and icebreaker ships. • Nuclear fission produces much more energy than fossil fuels • 1 kilogram of uranium oxide produces heat equivalent to 16 metric tons of coal

4. Fission Reactors • Three types (isotopes) of uranium occur in nature • Uranium-238 • Uranium-235 (only naturally occurring fissionable material) • Uranium-234 • Enrichment necessary • Processing to increase concentration of U235

5. Fission Reactors • Split U-235 by neutron bombardment • Reaction produces neutrons, fission fragments and heat. • Starts a chain reaction • Steam produced runs a turbine that generates electricity. • Similar to coal or oil burning power plants

7. Coal or Oil Power Plant

8. Nuclear Power Plant

9. Fission Reactors • Main components of a reactor • Core: fuel and moderator • Control rods: control the rate of reaction or stop it • Coolant: remove heat • Reactor vessel • The entire reactor is contained in a reinforced concrete building

12. Fission Reactors • Meltdowns • Occur when heat not removed fast enough • Nuclear fuel becomes so hot it forms a molten mass • Breaches containment of reactor and contaminates surrounding area • Design philosophy emerged to build smaller reactors • W/ passive stability (gravity feed cooling systems) • Or helium gas cooled which can’t meltdown

13. Sustainability and Nuclear Power • Two aspects • Nuclear power’s role in creating alternative fuel supplies • Could help the US transition from oil to other energy sources • The sustainability of nuclear fuel itself • Is a nonrenewable resource

14. Sustainability and Nuclear Power • Currently light water reactors are very inefficient • Breeder reactors • Designed to produce new nuclear fuel • Transform waste or low-grade uranium into fissionable material • Future of nuclear power if sustainability of fuel an objective

15. Pebble-Bed Reactors • A gas cooled reactor • Centered around fuel elements called pebbles • As a spent pebble leaves the core another one is feed in • Means the reactor has just the right amount of fuel for optimal production

17. Fusion Reactors • Involves combining the nuclei of light elements to form heavier ones • Heat energy is released • Source of energy in sun and stars • In a hypothetical fusion reactor • Two isotopes of hydrogen injected into reactor chamber • Products include helium and neutrons

19. Fusion Reactors • Several conditions necessary • 1. Extremely high temperatures • 100 million degrees C • 2. High density of fuel elements • 3. Plasma must be confined • Potential energy available if developed nearly inexhaustible. • Many obstacles remain to be solved

21. Nuclear Energy and the Environment • Nuclear fuel cycle includes: • Mining and processing of uranium to controlled fission • Reprocessing of spent fuel • Decommissioning of power plants • Disposal of radioactive waste • Throughout the cycle radiation can enter and affect the environment.

23. Problems with Nuclear Power • Uranium mines and mills produce radioacitve waste material that can polluted the environment. • U-235 enrichment and fabrication of fuel assemblies also produces waste materials. • Site selection and construction controversial. • Power plants also the site of past accidents or partial meltdowns.

24. Problems with Nuclear Power • Handling and disposal of waste • Decommissioning expensive • Terrorists could collect plutonium for dirty bomb

25. Effects of Radioisotopes • Radioisotopes is a isotope of a chemical element tht spontaneously undergoes radioactive decay. • Affect environment in two ways: • By emitting radiation that affects other materials • By entering the normal pathways of mineral cycling and ecological food chains

26. Effects of Radioisotopes • Explosion of nuclear atomic weapon does two types of damage • Directly from blast • Dispersal of radioactive isotopes • Fallout • Can enter ecological food chain • Biomagnifies in the food chain (e.g. reindeer moss, caribou, humans)

29. Radiation Doses and Health • Determining the point at which the exposure or dose becomes a hazard to health. • 5,000 mSv is considered lethal in 50% of people • 1,000-2,000 mSv sufficient to cause health problems • 50 msv maximum allowed dose for workers in the industry (30 times ave. natural background)

30. Radiation Doses and Health • Studies have shown a delay of 10-25 years between the time of exposure and the onset of disease. • Most scientist agree that radiation can cause cancer • But don’t agree on relationship • Linear vs. some threshold level

31. Nuclear Power Plant Accidents • US Nuclear Regulatory Commission • Sets performance goal for a single reactor at 0.01% • If there were 1,500 plants a melt down couls be expected every seven years • Unacceptable risk

32. Three Mile Island • Occurred March 28, 1979 near Harrisburg, PA • Malfunction of a valve resulted in partial core meltdown • Intense radiation released to interior of containment structure • Small amount of radiation released into environment

33. Three Mile Island • Because long-term chronic effects of exposure to low levels of radiation are not well understood, the effects of TMI difficult to estimate. • Major impact of the incident was fear.

34. Chernobyl • Occurred April 26, 1986 • Worst accident in history of nuclear power generation • Failure in cooling waters • Reactor overheated melting the uranium fuel • Explosions removed top of building • Fires produced a cloud of radioactive particles

35. Chernobyl • 30 km zone surrounding Chernobyl evacuated • City of Prypyat, pop. 48,000 at time of accident, now a ghost city. • Thyroid cancer increased in Belarus, Ukraine and the Russian Federation • Trees and vegetation damaged • Ultimately be responsible for 16,000 deaths

37. Radioactive Waste Management • By-products that must be expected when electricity is produced at nuclear reactors. • Three general categories • Low-level waste • Transuranic waste • High-level waste • In addition, tailings from uranium mines and mills considered hazardous.

38. Low-Level Radioactive Waste • Contains sufficiently low concentrations that it does not present a significant environmental hazard • If handled properly • Includes variety of residual and solutions from processing • Solid and liquid plant waste, sledges, and acids • Slightly contaminated equipment

39. Low-Level Radioactive Waste • Buried in near surface burial areas • Where geologic and hydrologic conditions thought to limit migration • However 3 of the 6 closed due to leaks • Controversy remains as to whether low-level radiation can be disposed of safety.

40. Transuranic Waste • Composed of human-made radioactive elements heavier than uranium. • Plutonium, americum, and einsteineum • Most is industrial trash that has been contaminated. • Generated from production of nuclear weapons

41. Transuranic Waste • Now being transported and stored in Carlsbad, NM salt beds • Salt is 225 million years old and geologically stable • No flowing ground water and easy to mine • Rock salt flows slowly into mined openings, naturally sealing the waste.

43. Consists of commercial and military spent nuclear fuel Uranium and plutonium derived from military reprocessing Other nuclear weapons material Extremely toxic Sense of urgency surround its disposal Total volume of spent fuel accumulating High-Level Radioactive Waste

44. High-Level Radioactive Waste • A comprehensive geologic disposal development program should have the following objectives. • Identification of sites that meet broad geologic criteria. • Intense subsurface exploration of possible sites. • Predictions of future changes to sites • Evaluation of risk associated with various predictions. • Political decision making based on risks acceptable to society.

45. Yucca Mountain Nuclear Waste Depository • Nuclear Waste Policy Act of 1982 • Initiated high-level waste disposal program • DOE investigated several sites • To be disposed of underground in deep geologic waste repository • Yucca Mountain only site being evaluated • Use of site could begin in 2010

46. Yucca Mountain Nuclear Waste Depository • Extensive scientific evaluations of Yucca Mountain carried out • Remain controversial • Concerned that natural processes might allows radiation to escape • Major question as to how accurate long-term predictions are

47. The Future of Nuclear Energy • Advocates argue that nuclear power is good for the environment • It does not produce potential global warming through release of carbon dioxide. • It does not cause acid rain. • If breeder reactors are developed the amount of fuel will be greatly increased. • Arguments for standardization, • Safer and smaller plants to provide more energy

48. The Future of Nuclear Energy • Argument against • Based on political and economic consideration • As well as uncertainty of safety issues • Known reserves would be used up quickly • May be a path to nuclear weapons