1) Define fusion. Compare and contrast it with the process of nuclear fission. Where in nature does fusion occur?
Fusion is a nuclear process in which two smaller nuclei merge to form a single, larger nucleus (and in many cases, other individual particles like protons or neutrons). The mass of the initial nuclei was larger than that of the resulting ones, and the "lost mass" is expressed as energy (E = mc^2). An example is two deuterium-2 nuclei fusing into one helium-4.
In fission, a large nucleus is broken apart (often when hit with a neutron) into smaller nuclei. The inital mass was larger than the resulting mass, so energy is released (E = mc^2). An example is uranium-235 fissioning into krypton-93 and barium-141. Both fusion and fission are nuclear reactions (the nuclei of atoms are altered) that release very large amounts of energy compared to chemical reactions (where atoms shift between molecules, but the atoms themselves are unchanged).
In nature, fusion takes place in the cores of stars like the sun, where gravity squeezes the gas to very high densitites and temperatures.
2) What are the possible advantages of fusion as a means of generating electricity, as compared to fission?
Compared to fission, the fuel for fusion is common. Typical fusion involves isotopes of hydrogen, most of which can be extracted from water (H_2 O). Deuterium (D, hydrogen with an extra neutron) is fairly easy to get, compared to fossil fuels or fission (which requires uranium mining). Tritium (T, hydrogen with 2 extra neutrons) is extremely rare, but it can be made from lithium-6 in a reactor. Availability of lithium-6 seems to be the bottleneck for making fusion a long-term energy source.
Compared to fission, the radioactive elements involved are less hazardous. Deuterium and helium-4 are stable (non-radioactive), though tritium is radioactive (12-year half-life; beta-emitter so only dangerous if taken internally). The tritium is a fuel rather than a waste-product, so it will be produced and used quickly. The metal and concrete in the reactor does gradually become radioactive as it is exposed to neutrons, so must be decomissioned and stored in the same way that current fission reactors are.
Like nuclear fission, commercial nuclear fusion would emit few harmful chemicals into the atmoshere (it is not clear to me whether the helium-4 gas that is produced would be captured and stored (as waste) or vented into the air, but helium is a noble gas that forms few chemical bonds, so is not likely to do any chemical damage if released). This means no acid rain (S0_4), smog (NO_x), or greenhouse gases (CO_2) would be emitted, as is done with fossil fuels.
3) What nuclear reaction(s) are most commonly used in experiments on fusion?
The sun fuses common hydrogen (1 proton & 0 neutrons), but this requires very high temperatures and densities. Fusing deuterium and tritium can be done at slightly lower temperatures and densities, so is easier. The reaction is (note the conservation of nucleons and charge -- 3 neutrons and 2 protons on each side of the reaction):
(2,1) D + (3,1) T --> (4,2) He + (1,0) n
Other possible reactions include D + D and D + He-3 as fuels (see p. 533) and run at moderate temperatures and pressures, making them harder to do than D + T. Perhaps as technology advances we can phase out D + T fusion (which requires Li-6 fuel, in limited supply) in favor of these reactions which rely only on relatively abundant D. This might be a "100-years in the future" sort of goal.
4) What makes fusion so difficult to achieve in a lab, let alone in a commercial electricity generating plant?
The "easy" reaction in #3 requires temperatures of ~50 million °C and extremely high densities (pressures), while the harder ones require even higher densities and temperatures (around 100 million °C). These conditions are needed to overcome the electromagnetic repulsion of the two positiviely charged hydrogen nuclei (each has one proton, and like charges repell), and push them close enough together so the strong nuclear force can join them permanently into a helium-4 nucleus. It is a huge technical challenge to contain such hot, dense gas for long enough for fusion to take place.
5) What is plasma, and what are the two primary ways it is containined in experimental fusion reactors today?
When a gas is hot and dense enough, collisions between atoms will be so energetic they knock the electrons free from the atoms, creating a gas that is composed of the positively-charged atomic nuclei (ions) and the free, negatively-charged electrons. This gas of ions and their electrons is called a plasma.
These electrically-charged particles "feel" a force when in the presence of a magnetic field, and this force can be used to constrain where they are located (remember, charged particles spiral along field lines, but avoid crossing them). Magnetic containment uses very strong magnetic fields generated by electric currents flowing in wire coils around the gas to keep the plasma from expanding and contacting the metal walls of the reactor (and thus cooling). When arranged in a doughnut shape (called a "Tokamak") the magnetic fields can successfully contain the plasma and produce short bursts of fusion (for a few seconds). Developing the technology to keep the fusion going for long periods of time has proven very challenging, and progress has been slow. Once that is accomplished, the entire Tokamak must be imbedded inside some kind of a "heat exchanger" that absorbs the heat from the nuclear fusion and carries it off to a turbine/generator where it can produce electricity. It also requires a means for injecting fresh D+T fuel and letting waste helium out, so fusion can be sustained over long periods of time.
The second approach is called inertial confinement. It involves "zapping" small spheres of D+T gas with super-high powered lasers from all angles, so that the intense light both compresses and heats the gas to the density and temperatures where fusion occurs. This has been accomplished in experimental reactors, but to be commercially applicable, the lasers must pulse fast enough to impode ~10 spheres each second over long periods of time. Once accomplished, this unit must be put inside the heat exchanger as described above.
In both cases, enormous amounts of energy are required to confine the plasma (either to make the magnetic field in the Tokamak, or power the lasers in interial confinement). Over the last 30-40 years of experiementation, we have developed the technology to get a little more energy out of the fusion than we put in, but experts suggest we are another 30-40 years (optimistially) before we can overcome the remaining technical obstacles in making this a commercially productive means of making electricity. (The technical challenge makes sending people to the moon, or even Mars, look pretty straightforward).
6) Compare "cold fusion" with the "hot fusion" discussed above. What are its prospects for commercial electricity production?
Cold fusion was purportedly observed in a simple, low-tech experiment run at room temperature ("cold" compared to 50 million °C!). Deuterium rich water was broken up into D and O at the metal electrodes of a battery, and the D purportedly became so concentrated on one electrode (made of palladium metal) that some of it fused into helium-3, releasing lots of heat and neutrons.
If true, this would be a breakthrough in fusion as it would sidestep all of the technical difficulties of conventional "hot" fusion. However, other scientists trying to repeat this experiment have seen little or none of the effects claimed by the original chemists. Most scientists are skeptical that this experiment produced fusion at all, and it seems unlikely it will produce a commercially useful means or producing electricity (Drat, no Mr. Fusion for our flying time-machine cars).
Andy Layden -- Spring 2007