Nuclear reactors are quite a complicated business...
Nuclear fission concept maps and articles (hyperphysics, you should understand everything well):
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fiscon.html#c1
Diagrams on how fission works:
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html#c1
Basically, you can start the chain reaction by smashing an slow/thermal neutron into a Uranium-235 nucleus, the nucleus turns into a U-236, from the extra neutron, and then if cracks in half, into two pieces, on average, 2.4 neutrons, and some light. Those neutrons can smash into other U-235 nuclei, and it happens over again. Since there's 2-3 neutrons coming out, the process grows exponentially
Basically, the different generations characterize how technologically developed the nuclear reactor is.
Generation I:
Magnox reactors employ natural uranium metal, which contains 0.7% of the fissionable isotope uranium-235 and around 99.2% uranium-238. The fuel is encapsulated in an alloy of magnesium and aluminum. A graphite moderator surrounding the fuel slows down neutrons released by fission of uranium-235 so that they can collide with other uranium-235 nuclei, causing more fission and a chain reaction. Control rods made of boron steel, a neutron-absorbing material, are inserted into or withdrawn from the core to control the rate of reaction or to halt the reaction. Gaseous carbon dioxide is used as a coolant to transfer heat generated by the nuclear chain reaction in the reactor core to a steam turbine that generates electricity.
A total of 26 Magnox reactors were built in the U.K. Eight remain in operation, but they will be decommissioned by 2010. Nuclear reactors such as the Magnox reactors that were operational before the 1970s and made use of natural uranium are known as generation I reactors.
Generation II:
Generation II reactors of the 1970s and 1980s constitute most of the plants currently operating.
- http://www.nuc.berkeley.edu/people/faculty/peterson.htm, Professor of nuclear engineering at Berkeley
Almost 60% of these reactors are pressurized water reactors (PWRs), pressurized water serves as a moderator and coolant. The fuel, ceramic uranium dioxide, is encased in long zirconium alloy tubes.
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/imgnuk/pwr.gif
The second most common type of reactor is the boiling-water reactor (BWR). Currently more than 90 of these are operating throughout the world. In BWRs, water passes over the reactor core to act as moderator and coolant, and steam passes directly to the turbine. The disadvantage of this is that any fuel leak might make the water radioactive and that radioactivity would reach the turbine and the rest of the loop. A typical operating pressure for such reactors is about 70 atmospheres at which pressure the water boils at about 285?C. This operating temperature gives a Carnot efficiency of only 42% with a practical operating efficiency of around 32%, somewhat less than the PWR.
PWRs and BWRs are known as light-water reactors. The 33 CANDU (Canada deuterium-uranium) pressurized water reactors currently in operation in Canada, on the other hand, employ heavy water (D
2O) as a moderator and coolant. The reactors use natural uranium (0.7% U-235) dioxide as a fuel rather than enriched UO2.
Generation II reactors in the U.K. are advanced gas-cooled reactors (AGRs). Like Magnox reactors, they use graphite as a moderator and CO
2 as a coolant. The AGR fuel is enriched uranium (2.5–3.5% U-235) oxide pellets encased in stainless steel tubes that allow the reactors to operate at higher temperatures than the Magnox reactors.
Russian RBMK reactors are boiling-water reactors with graphite moderators. The reactor at the Chernobyl Nuclear Plant in Ukraine in April 1986 was an RBMK reactor.
The neutrons emitted in nuclear fission reactions have very high energies (sometimes they even exceed the speed of light in the medium), typically in the range of 1 MeV. They need to be slowed down greatly, the cross section for neutron capture leading to fission is greatest for neutrons of energy around 1 eV, a million times less. Neutrons with energies less than one electron volt are commonly referred to as "thermal neutrons" since they have energies similar to what particles have as a result of ordinary room-temperature thermal energy. It is necessary to slow down the neutrons for efficient operation of a nuclear reactor, a process called moderation. The neutrons are slowed by interacting with water as a "moderator" to maintain the chain reaction. Loss or the water coolant kills the chain reaction - the fuel configuration is not "critical" without water moderation.
Reactors that use water or graphite as moderators to slow neutrons and sustain the fission chain reaction are known as thermal reactors. Light atoms, such as hydrogen, deuterium, and carbon, slow neutrons down to thermal energies, below 1 eV. At these energies, the probability of a collision between a neutron and the fissile U-235 nucleus is around two orders of magnitude higher than that for the high-energy neutrons that are generated by fission.
Fast neutron reactors do not have a moderator and use fast neutrons directly to generate power. When configured to produce more fuel than they consume, they are known as fast breeder reactors. The fuel rods contain a mixture of UO2 and plutonium dioxide. The coolants are
liquid metals, usually
liquid sodium. The extra energy of fast neutrons increases the probability of fission occurring. "Fertile" isotopes, like U-238 in natural uranium, capture some of the neutrons, creating fissile isotopes such as Pu-239. Fast reactors can therefore use depleted uranium (uranium that has less than 0.7% of U-235) as a fuel. The time required for a breeder reactor to produce enough material to fuel a second reactor is called its doubling time, and present design plans target about ten years as a doubling time. A reactor could use the heat of the reaction to produce energy for 10 years, and at the end of that time have enough fuel to fuel another reactor for 10 years. Click
here for another diagram.
Fertile isotopes do not undergo fission but can instead capture neutrons and transmute into an isotope of another element which can undergo fission
China, India, and Japan, have fast breeder reactor programs. A fast breeder reactor in Russia has supplied electricity to the grid since 1981. Fast breeder reactors are the most dangerous kind, they can explode, and meltdown and do lots of other weird things
Generation III Reactors: http://www.uic.com.au/nip16.htm
Comparitively to Generation I and II reactors:
- Standardised design for each type to expedite licensing, reduce capital cost and reduce construction time.
- Simpler and more rugged design, making them easier to operate and be less vulnerable to operational upsets,
- Higher availability and longer operating life, about 60 years,
- Reduced possibility of core melt accidents,
- Minimal effect on the environment,
- Higher burn-up to reduce fuel use and the amount of waste,
- Burnable absorbers ("poisons") to extend fuel life.
The first Generation III system, a General Electric-designed advanced BWR, started operating in 1996 at the Kashiwazaki-Kariwa Nuclear Power Station in Japan, and another is now in operation. Two more are under construction in Japan and another two in Taiwan. The designs were certified by the U.S. Nuclear Regulatory Commission (NRC) in the 1990s.
Generation IV:
The pebble-bed modular reactor (PBMR), is an advanced nuclear reactor design. It uses helium as a coolant, at 900 ?C. The hot helium drives the turbine directly, elimination complex steam management systems from the design, and increases transfer efficiency (ratio of electrical output to thermal output) to about 44%, which translates to roughly 1/3 more output than a conventional PWR and graphite as a moderator.
The technology in various forms in under development by MIT, General Atomic, MIT, Chinese company Chinergy, and its being planned for 2010 in South Africa by power utility Eskom
There are a number of different design choices. Uranium, thorium, or plutonium are in oxides (ceramic form), and are contained in spherical pyloric graphite pebbles, about 60 mm in diameter. The pebbles are in a bin or can, an inert gas, helium, nitrogen, or CO
2 circulates through the spaces between, to carry heat away.
The reactor will heat the gas, and send it to low pressure gas turbines. High tempertures permit systems to get more mechanical energy from the same amount of thermal. They use less fuel per kilowatt-hour. If the gas from the primary coolant can be made radioactive by neutrons in the reactor, usually it is brought to a heat exchanger where it heats another gas, or steam. The exhaust of the turbine is warm, and can be used to heat buildings, chemical plants, or run another heat engine.
Pebble-bed reactors are the safest type, as they get hotter, the rate of neutron capture by U-238 increases, reducing number of neutrons available to cause fission. That limits the power produced by the reactor. The reactor vessel is designed so that without mechanical aids, it loses more heat than the reactor can produce in an idle state. Most of the fuel containments resides in the pebbels, and they are designed so a containment failure releases at most a half millimeter sphere of fuel.
The modular design means that the components are factory-made, so plants are quicker to assemble," he adds. The PBMR module can be used to generate power in a stand-alone mode or as part of a power plant that consists of up to 10 units.
Another significant technical advantage is that some designs are throttled by temperture, not control rods. The reactor is much simpler because it doesn't need to operate well with varying neutron profiles caused by partially-withdrawn control rods. For maintenance, many designs include control rods, called absorbers, that are inserted through tubes in a neutron reflector around the core.
The PBMR sets new standards in safety, not only through its design, the silicon carbide layer not only protects the fuel during storage and fission but also makes it extremely difficult for anyone to divert the fuel elsewhere.
... very helpful