Friday, January 8, 2010

I turn Science Friday over to my sister, the physicist. . .

Who knew the taller of sisters seen in this picture would grow up to get her doctorate in physics from Columbia University (where she was the first female head of teaching assistants). Ruth Hege Howes then went on to thirty-some years of teaching, interspersed with years spent in Washington advising government and time spent advising groups and other governments all over the world. She also authored or co-authored several books, including Their Day in the Sun: Women of the Manhattan Project.

(FYI, the sister on the right grew up to work for WMRA public radio. If you think she's proud of her big sister, the physicist, you're right. And also rather in awe of her ability to grasp the most intricate workings of atomic reality.)

Back in early-ish November, I blogged on an article I read in Newsweek about the possibility of, some time in the future, powering the planet with nuclear fusion (power plants today use nuclear fission). I read the article with interest, wrote about it, and then realized I didn't have a very clear idea about these processes' relative virtues and risks.

Our future power source is not the sexiest issue we face at the moment, but it may be the most important. So I asked my sister, the physicist, to help me (and WMRA blog readers) understand the basic natures of fission and fusion. She was worried that what she sent me might be too dry, but I think it's just fine. I found it a great help to my understanding of the two processes and so well worth spreading around.

Nuclear Energy for the Future
Continued reliance on fossil fuels (coal, petroleum and natural gas) for generation of the majority of electricity in the United States is not an option for three major reasons. First, the majority of the world’s fossil fuel is located outside of the United States. We do have vast supplies of coal, but if we substitute coal for other energy sources such as petroleum and natural gas, we will rapidly deplete our domestic supplies. Most experts feel that the demand for fossil fuel is already driving U.S. policy in the oil-rich Middle East. Certainly, the price of oil already plays a significant role in the global economy.

Second, the global supply of fuel is not inexhaustible. As growing economies such as China, Brazil and India use more fossil fuel, the rate at which the global supplies are depleted will increase. Whether this will happen in twenty years or seventy is open to debate, but increasing scarcity will certainly increase the price.

Finally, burning fossil fuels unavoidably produces carbon dioxide. Most scientists believe that the increase in carbon dioxide in the atmosphere is responsible for the increase in global average temperature since the dawn of the industrial age.

So what should we do? Obviously we need to practice energy conservation. Simple measures such as putting in storm windows, raising gas mileage in cars, or using energy efficient light bulbs can save really large amounts of energy and put money in our pockets. Increasing the efficiency of energy use is clearly job 1. However, we will still need a source of energy. One option is nuclear energy which comes in two types which are compared below.
Nuclear Fission: Extracts energy by splitting or fissioning nuclei of one isotope of uranium, uranium-235.
Nuclear Fusion: Extracts energy by combining nuclei of hydrogen isotopes to form helium.

Nuclear Fission: Rate of energy release is controlled because each uranium-235 nucleus must absorb a neutron in order to split. To sustain a fission “chain” reaction, one neutron from each fission must trigger a second fission. This can be controlled by 1) the number of uranium nuclei present; 2) the geometric configuration of the uranium; and 3) the material surrounding the uranium. A nuclear reactor sustains and controls a chain reaction producing a steady supply of heat.
Nuclear Fusion: Energy release is possible only when hydrogen isotopes are heated enough to destroy atomic structure and form a plasma. The plasma must reach a combination of density and temperature that will cause the nuclei to fuse. Because the necessary temperatures are high enough to vaporize any known material, the hydrogen plasma must be contained in a magnetic bottle created by currents in the plasma or external magnetic fields.

Nuclear Fission:  Reactor fuel in the US is enriched, that is it has an increased concentration of the isotope uranium-235. Isotopic enrichment, technically difficult and expensive, is the major barrier to the proliferation of nuclear weapons which use uranium with a higher concentration of uranium-235 than that used in reactors.
Nuclear Fusion:  The heavy isotope of hydrogen (2H) known as deuterium is found in water on Earth. The other heavy isotope of hydrogen (3H) known as tritium is usually produced in fission reactors.

Nuclear Fission: Produces electric power by using the energy released to heat water and produce steam to turn turbines.
Nuclear Fusion:  Produces electric power by using the energy released to heat water and produce steam to turn turbines. Other methods may be possible but have not been demonstrated.

Nuclear Fission:  Has been used to produce electric power for 50 years and currently provides about 20% of the electric power in the US.
Nuclear Fusion:  One demonstration project has produced net energy for a few seconds. Has not been used to produce electricity. The International Thermonuclear Experimental Reactor, currently being built in France, is designed to demonstrate a sustained production of net energy from nuclear fusion.

Nuclear Fission:  Modern reactor designs cut off fission reactions if the reactor begins to overheat. However, the fuel in the reactor contains enough radioactivity so that it melts if it is not cooled. If the containment vessel of the reactor is breached, radiation could be released to the environment.
Nuclear Fusion:  ?????

Nuclear Fission:  Radioactive wastes contain heavy isotopes of uranium that have half lives of tens of thousands of years. Technology exists to safely store these wastes for times on the order of thousands of years which would be long enough for short-lived radioactive materials to disappear. Used fuel is currently stored on the sites of reactors around the country – not a very safe solution.
Nuclear Fusion:  A fusion reactor will produce a high flux of neutrons which will make surrounding materials radioactive although their half lives will be short compared to fission plants. The primary waste will be helium gas. There is still a possibility of the release of some radiation to the environment although less than from a fission reactor.

Nuclear Fission:  It is possible to separate long-lived isotopes from the shorter lived radioactivity and burn the long-lived isotopes to produce more electricity. This extends our supply of reactor fuel. The technology to do so exists, however it poses a risk of proliferation of nuclear weapons.
Nuclear Fusion:  Materials used in construction of fusion reactors would require storage for times on the order of 1000 years.

1 comment:

  1. Not bad for some one who is more or less ignorant of science. You don't get everything about fusion wrong. But with just a little bit more effort you could have done it.

    One example:

    Nuclear Fusion: Materials used in construction of fusion reactors would require storage for times on the order of 1000 years.

    Is not true. It strictly depends on the construction materials.


    In any case - the Polywell Fusion Reactor experiments by the US Navy may show results in two years or less. And they can burn Boron 11 and hydrogen releasing 1/1000th the neutrons for equivalent power vs D-T fueled tokamaks.