Fusion Reactions 3/18/2016. Exam #2 Results. Nuclear Fusion (not Fission) Clicker Question. U n Te Zr n

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1 Clicker Question What do you think? A) We should increase our nuclear fission energy facilities. B) We should continue to run only the facilities we currently have. C) We should shut down all existing facilities over the next 20 years. D) We should shut down all existing facilities immediately. Situation in USA does not appear likely to change in the near future Reading Assignment Complete Chapter 7, Read ahead through break on Chpt. 8, 9 Homework Assignment HW #8 due Wed. March 30 (after break) Problems are from Chapters 8 and beginning of 9 Inquire/Inform One-Page Assignment due Monday, April 4, 2016 (See details in Lecture #27 notes) -- My office hours are Monday 1-2 pm, Tuesday 11 am 12 pm Also available by appointment just me. Exam #2 Results Average = 77% Standard Dev = 19% A range B range C range < 50 D,F range Scores are posted on D2L and Solutions are also on D2L Nuclear Fusion (not Fission) Fission Reaction Large unstable nucleus splits into smaller pieces U n Te Zr n Chain reaction sustains release of energy from mass Used in nuclear power plants and original nuclear weapons Fusion Reaction Small stable nuclei fuse (join) to form even more stable nuclei H 2 2He2 n 17. Now utilized in H-bomb (Hydrogen bomb) No chain reaction, so must generate enough heat and containment for power plants (not yet working!) H 6MeV Fusion Reactions Mass of Helium nucleus is LESS than the sum of the masses of the 2 protons and 2 neutrons that make it up. If you could combine 2 p's and 2 n's to make a Helium, there would be a release of energy. This is the opposite of fission (which splits up large nuclei). This type of reaction is the source of the sun's energy. Example: 4 protons Helium(2,4) + 2 electrons + 2 neutrinos + 28 MeV. 0.76% of original mass is converted to energy. 4 grams of Helium ( atoms)= Joules Equivalent to ~400 barrels of oil 1

2 What is the Problem? Proton + Proton = Deuterium + other H0 1H0 1H1 e... The Sun (and all Stars) Equilibrium balance between gravitational, thermal pressures p p What happens if we shoot one proton at the other? Protons are electrically charged, they repel. Need HIGH temperature and high density to get them close enough to "stick together Can we do this? (100 Million Kelvin) Where do we have these temperatures? Sun loses 4 x kilograms/day from fusion reactions! However, remember the sun has a total mass ~ 2 x kg. How About on Earth? 1) Yes, we have, in bombs. Use fission bomb to create high temperature and pressure 2) We've been working on this for 40 years. Lots of progress, still nowhere near a commercial plant. Note there are some big advantages if you can get this to work. No risk of explosion (automatic shutdown in case of problems) No long lived radiation, although there is low level radiation (from Tritium, half life =12 years, and from neutrons hitting containment materials) Magnetic Confinement Tokamak (magnetic confinement) High temperature to overcome proton-proton repulsion Orbital electrons are stripped off so charged ions High density of ions trapped in magnetic field Need enough ions to produce power Problems: Instabilities, leaks, losses. Still looking for "break-even" (energy out = energy in) 2

3 LASER Ignited Fusion Compress pellets of deuterium with ultra-intense LASERS National Ignition Facility ( Some controversy about relation to bombs? Summary of Fusion In principle it has great advantages nuclear power with abundant fuel very little radioactive waste, and with a relatively short half-life reaction shuts down when you turn the key However, this has not been made practical yet. It may seem like with enough engineering effort, it will eventually work But as the scale and cost of such facilities (ITER is billions of dollars) goes up, just reaching the energy break even point is not enough. Where to Invest? If you were a wealthy person and you wanted to give $100 billion dollars to help solve our energy problems, how much would you give to fusion research? What is this device that is being fueled by stale beer and garbage? A) None of it. B) $1 billion C) $10 billion D) $30 billion E) $100 billion It is difficult to balance investment in things that may or may not work, but also that have a much longer time scale. However, they are also important areas and would be lost if we only invest in things with short payback times. Renewable Energy Sources Renewable Geothermal Energy Geothermal Energy Ocean Thermal Energy Conversion (OTEC) Tidal Energy EEC Chapter 8 is a rather odd grouping of energy sources not all geothermal Sources of energy that do not diminish by using them (they are renewed). Qualifier renewed on a time scale similar to the usage time (i.e. fossil fuels are not renewable, solar energy is renewable even though sun will eventually burn out) 3

4 Earth s core is hot (from radioactivity and energy from formation) and is slowly cooling Near the surface, the temperature increases by 25 0 C / km Key numbers At the Earth s surface only 0.025% of heating is from geothermal energy flow, and nearly all the rest is from the sun above. Geothermal energy flow is about Watts/m 2 Heat Engine versus Heat Pump Imagine digging down 100 meters (which is huge) and using the temperature difference at a heat engine to generate electricity What is the problem? eff(carnot) = 1 Tc/Th = K / 292 K < 1% Very low Quality energy Two options 1) Find hot spots near the crust 2) Use low-q energy for heating Figure 8.1 Figure 8.2 Binary-cycle geothermal power plant at Mammoth Lakes, California. The plant transfers heat from geothermal water at 170 C to its working fluid, isopentane, which boils at 28 C. Four separate units produce a total of 40 MW of electric power. The plant was designed to blend with its scenic surroundings, and the closed-loop binary cycle minimizes emissions. 1.3 GW capacity in 1985 Hydrothermal systems Hot dry rock (igneous systems) Normal geothermal heat (200 C at 10 km depth) Many sources are not renewable (e.g. Geysers in California). Over time one uses up the local heat source! Figure 8.9 4

5 Figure 8.3 Figure 8.5 Geothermal Energy Potential Warm core of earth (radioactive in origin). Heat flow out from core < (1/1000) of flow in from sun. Most places the flow rate is too low to make use of, but hot spots (geysers, volcanoes) might be tapped. Places like Iceland have this working quite well (1/3 of their energy). World wide all operating and planned facilities 1% of current global electrical production. 5