PHY Outline 101 Lecture #26: Other Energy Sources: A Status Report
http://physics.syr.edu/courses/PHY101/ Off. Hrs: Tue 9:30 11:00, Physics 263-4 Prof. Schwarz's Problem Sessions: Mon and Tues, 5:15 6:15, Physics 202/204
1. Coal: resources and risks 2. Nuclear fission: resources and risks
Prof. Peter R. Saulson [e-mail]
PHY 101 Lecture #26
Other Energy Sources and their Risks
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Coal resources
We have a fair amount of coal. U.S. resources: 1.1 1012 tons. Current (2001) consumption rate: 3.8 109 tons/yr Time to exhaustion, at present consumption rate: 1.1 1012 tons / 3.8 109 tons/yr = 285 years. If we replaced all oil and gas use with coal, the coal supply would last about 70 years, at present consumption rates. Continued growth of demand (i.e. economic growth without new efficiencies) would reduce these durations.
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What's wrong with burning lots of coal?
Combustion of coal is like combustion of most organic fuels.
Energy content about 2.4 107 J/kg. Products of combustion include CO2.
In modest quantities, this is benign. In vast quantities, burning fossil fuels may cause global climate change.
"Greenhouse effect"
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History of atmospheric CO2
Over human history, atmospheric CO2 concentrations were stable at 280 ppm,
5th most abundant gas in the atmosphere
Review of the greenhouse effect
Earth's temperature determined by energy balance.
Power in from sunlight = power out in infrared Power from sunlight is fixed. Power out in infrared is larger, the higher is the earth's temperature.
until around 1800, when CO2 concentration began to rise. By 2000, CO2 concentration reached 360 ppm.
A genuine global change. But, CO2 is still a minor constituent of the air. Why is this important?
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Without atmosphere, this balance would be achieved at earth temperature of -20 deg C. Atmosphere is mostly opaque in infrared, trapping heat, raising temperature to +20 deg.
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CO2 is a "greenhouse gas"
Adding CO2 to the atmosphere adds to its infrared opacity. More energy trapped ==> earth's temperature rises to restore energy balance.
"global warming"
Can we mitigate coal's contribution to the greenhouse effect?
Carbon sequestration means finding a way to prevent CO2 from entering the atmosphere after combustion. Technology exists, but is way to expensive cost needs to come down by more than a factor of 10, if electricity costs aren't to rise prohibitively. U.S. Dept. of Energy has substantial research program, with target date of 2015.
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Earth's average temperature has risen over the 20th century.
Some controversy on determining average temperature. And climate varies anyway ...
PHY 101 Lecture #26 Other Energy Sources and their Risks
DOE Carbon sequestration research program
Cost effective CO2 capture and separation processes. CO2 sequestration in geological formations including oil and gas reservoirs, unmineable coal seams, and deep saline reservoirs. Direct injection of CO2 into the deep ocean and stimulation of phytoplankton growth, via iron fertilization. Improved full life-cycle carbon uptake of terrestrial ecosystems. Advanced chemical, biological, and decarbonization concepts.
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Places to sequester carbon
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Review of nuclear energy
Nuclear energy is revealed in mass defects of atomic nuclei.
Whole weighs less than its parts!
E = mc2.
Fission and fusion
Energy is released when nuclei are transformed to other nuclei that are more tightly bound. Light nuclei release energy when they fuse together.
Sun is powered by fusion of hydrogen to helium
Examine Aubrecht's Figure 18.4.
The "curve of binding energy"
Iron nucleus is the one that is held together most tightly. Both lighter and heavier nuclei are less tightly bound.
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Heavy nuclei release energy when they split apart.
Fission of uranium produces lighter nuclei, nearer to iron, and thus releases nuclear energy.
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Energy from fission
U235 is especially easy to persuade to split apart.
Absorption of a neutron prompts the split.
Advantages of nuclear power
No carbon emissions, thus no contribution to greenhouse effect. Good safety and emissions record overall.
Much more radioactivity emitted from coal use!
For this reason, it is the key fuel for fission reactors. Energy released in fission: 8 1013 J/kg.
Compare to 2.4 107 J/kg from combustion of coal, other chemical fuels. More than 2 million times more concentrated!
Only tiny amount of fuel is required to heat the steam to run an electrical generating plant.
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Fission fuel resources
Uranium is not abundant. World Nuclear Association estimates that known (easily mined) reserves will last for 70 years at present consumption rates. However, if fuel prices rise, perhaps 10 times more uranium is available.
And fuel cost is only a small part of the cost of nuclear power, so this could be just fine.
Breeder reactors and U238
Other reactor designs can use U238 as a fuel.
140 times more abundant than U235.
In a breeder reactor, U238 is first transformed into Pu, which fissions easily. This would make fission practical as a major energy source, that could last many decades. Technological development of this type of reactor was halted in late 1970s.
Pu is highly toxic, and can be used in small quantities (e.g., by terrorists) to make atomic bombs. Reprocessing of reactors parts and fuel is required, and at that stage fuel is vulnerable to diversion.
But, if we wanted to save the world with nuclear power, we'd want to use it at more than 10 times the present rate.
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Risks from nuclear fission
Almost all fission products are radioactive.
Exposure to them would be unhealthy.
Storage of spent nuclear fuel
Some fission products take hundreds to thousands of years to decay. Spent fuel storage must be designed to last that long!
Otherwise, risk to health from leaking materials.
Reactors are designed to minimize exposure. Spent fuel must also be isolated from people and the environment.
Designs that accomplish this are easy to make.
Risk comes from accidental failure or terrorist action.
How likely? Hard to say, but it seems more likely now than it did a few years ago.
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One potential site for safe storage has been identified in the U.S.
Yucca Mountain, NV, has very stable geology.
We need to solve this problem, because of nuclear waste from bombs, existing reactors.
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