Nuclear Reactions and the Transmutation of Elements

1. Nuclear Reactions and the Transmutation of Elements Neutrons are very effective in nuclear reactions, as they have no charge and therefore are not repelled by the nucleus.

2. Nuclear Fission; Nuclear Reactors After absorbing a neutron, a uranium-235 nucleus will split into two roughly equal parts. One way to visualize this is to view the nucleus as a kind of liquid drop.

3. Nuclear Fission; Nuclear Reactors The energy release in a fission reaction is quite large. Also, since smaller nuclei are stable with fewer neutrons, several neutrons emerge from each fission as well. These neutrons can be used to induce fission in other nuclei, causing a chain reaction.

4. Nuclear Fission; Nuclear Reactors Neutrons that escape from the uranium do not contribute to fission. There is a critical mass below which a chain reaction will not occur because too many neutrons escape.

5. Nuclear Fission; Nuclear Reactors In order to make a nuclear reactor, the chain reaction needs to be self-sustaining – it will continue indefinitely – but controlled.

6. Nuclear Fission; Nuclear Reactors A moderator is needed to slow the neutrons; otherwise their probability of interacting is too small. Common moderators are heavy water and graphite. Unless the moderator is heavy water, the fraction of fissionable nuclei in natural uranium is too small to sustain a chain reaction, about 0.7%. It needs to be enriched to about 2-3%.

7. Nuclear Fission; Nuclear Reactors Finally, there are control rods, usually cadmium or boron, that absorb neutrons and can be used for fine control of the reaction, to keep it critical but just barely.

8. Nuclear Fission; Nuclear Reactors Benefits –low conventional pollution (Carbon) We can be self sufficient in fuel Low operating costs Problems -disposal of radioactive waste possibility of accidental release of radiation. High capital costs

9. Nuclear Fission; Nuclear Reactors An atomic bomb also uses fission, but the core is deliberately designed to undergo a massive uncontrolled chain reaction when the uranium is formed into a critical mass during the detonation process.

10. Nuclear Fusion The lightest nuclei can fuse to form heavier nuclei, releasing energy in the process. An example is the sequence of fusion processes that change hydrogen into helium in the Sun. They are listed here with the energy released in each:

11. ECOSYSTEMS • Living things are strongly interconnected- the Web of Life • Life and the a-biotic (not living) world are also strongly interconnected. • We shall look at just two aspects: Energy flows through the Web of Life and the Carbon Cycle

12. We are Sunlight!

16. Solar Power Transforms Incoming Energy to Biomass • Only a small fraction of the energy consumed by a primary consumer is used for secondary production, the production of new tissue by primary consumers. Most of the energy is used for maintenance. 45 % biomass Sunlight primary producer • 1% 55% maintenance • heat

17. How Does Energy Flow through an Ecosystem? • Primary production results in biomass, organic material that non-photosynthetic organisms can eat. • In all environments, the chemical energy in primary producers eventually moves to one of two types of organisms: primary consumers or primary decomposers. • The primaryconsumeris an herbivore. • Primarydecomposers, including bacteria, archaea, and fungi,consumedetritus.

20. Energy Transfer between Levels • All ecosystems share a characteristic pattern: The total biomass produced each year is greatest at the lowest level and declines at higher levels. • This pattern occurs because only a fraction of the total energy consumed is used for growth and reproduction. The amount of biomass produced at each subsequent level must be less than the amount at the previous level.

21. Extreme example insects use 20-40% of input energy for growth and reproduction

22. The Pyramid of Productivity • When graphing biomass produced at each level, a pyramid of productivity, which reports productivity and efficiency, emerges. • Productivity is a rate, measured in units of biomass produced per unit of area each year. • Efficiency is a ratio—the fraction of biomass transferred from one level to the next. • Biomass production at each level varies widely among ecosystems, but in general, efficiency of biomass transfer between levels is only about 10 percent.

23. It takes a lot of plants to make us!

24. Global Patterns in Productivity • In general, production of new biomass on land is much higher than it is in the oceans, as more light is available to drive photosynthesis on land than in marine environments. • The terrestrial ecosystems with highest productivity are located in the wet tropics. • Marine productivity is highest along coastlines, and it can be as high near the poles as it is in tropics.

25. Which Regions Are Most Productive? • Tropical wet forests and tropical seasonal forests cover less than 5 percent of Earth’s surface but together account for over 30 percent of total new biomass • Among aquatic ecosystems, the most productive habitats are algal beds and coral reefs, wetlands, and estuaries. • Even though new biomass per square meter is extremely low in the open ocean, oceans are so large in terms of area that its total production is high. • Humans are appropriating almost a quarter of the planet’s biomass.

27. Productivity is limited by any factor that limits the rate of photosynthesis. These factors include temperature and the availabilities of water, sunlight, and nutrients. Different limiting factors prevail in different environments.

28. Geological Cycles Involving Life • Many such cycles in which material is cycled back and forth in environment exist. • Water cycle • Nitrogen cycle • Carbon cycle

29. The Global Carbon Cycle • The global carbon cycle involves the movement of carbon among terrestrial ecosystems, the oceans, and the atmosphere. • The ocean is by far the largest of these three reservoirs. • In both terrestrial and aquatic ecosystems, photosynthesis is responsible for taking carbon out of the atmosphere and incorporating it into tissue. • Cellular respiration, in contrast, releases carbon that has been incorporated into living organisms to the atmosphere, in the form of carbon dioxide.

30. + Respiration, Decay, volcanoes

31. The Global Carbon Cycle • Burning fossil fuels moves carbon from an inactive geological reservoir, in the form of petroleum or coal, to an active reservoir—the atmosphere. • When you burn gasoline, you are releasing carbon atoms that have been locked up in petroleum reservoirs for hundreds of millions of years. • Other human activities such as intensive agriculture and deforestation have changed the carbon cycle by adding large amounts of carbon dioxide to the atmosphere.

33. The Global Carbon Cycle • Carbon dioxide is a greenhouse gas—a gas that traps heat radiated from Earth and keeps it from being lost to space. • Increases in the amounts of greenhouse gases have the potential to warm Earth’s climate by increasing the atmosphere’s heat-trapping potential.