Nuclear power: Encyclopedia II - Nuclear power - Reactor Types

Nuclear power - Reactor Types

Nuclear power - Current Technology

There are two types of nuclear power sources in current use:

1. The nuclear fission reactor produces heat through a controlled nuclear chain reaction in a critical mass of fissile material. All current nuclear power plants are critical fission reactors, which are the focus of this article. The output of fission reactors is controllable. There are several subtypes of critical fission reactors. All reactors will be compared to the Pressurized Water Reactor (PWR), as that is the standard modern reactor design.
   • a. Pressurized water reactors (PWR): These are reactors cooled and moderated by high pressure, liquid (even at extreme temperatures) water. They are the majority of current reactors, and are generally considered the safest and most reliable technology. Three Mile Island is a reactor of this type. This is a thermal neutron reactor design.
   • b. Boiling water reactors (BWR): These are reactors cooled and moderated by water, under slightly lower pressure. The water is allowed to boil in the reactor. The thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. Unfortunately, the boiling water puts more stress on many of the components, and increases the risk that radioactive water may escape in an accident. These reactors make up a substantial percentage of modern reactors. This is a thermal neutron reactor design.
   • c. CANDU: An indigenous Canadian design, these reactors are heavy-water-cooled and -moderated Pressurized-Water reactors. Instead of using a single large containment vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fuelled with natural uranium and are thermal neutron reactor designs. CANDUs can be refueled while at full-power, which makes them very efficient in their use of uranium (it allows for precise flux control in the core), and also makes it possible to misuse them as plutonium breeders. Most CANDUs exist within Canada, but units have been sold to Argentina, China, India (pre-NPT), Pakistan (pre-NPT), Romania, and South Korea. India also operates a number of 'CANDU-derivatives', built after the 1974 Smiling Buddha nuclear weapon test.
   • d. RBMKs: A design unique to the Soviet Union built to produce plutonium as well as power, the dangerous and unstable RBMKs were water cooled with a graphite moderator. RBMKs are similar to CANDU in that they are refuelable On-Load and employ a pressure tube design instead of a PWR-style pressure vessel. Notably, they were too large and powerful to have containment buildings. Chernobyl was an RBMK.
   • e. Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor: These are generally graphite moderated, and CO₂ cooled. They have a high thermal efficiency compared with PWRs and an excellent safety record. There are a number of operating reactors of this design mostly in the United Kingdom, older designs (i.e. Magnox stations) are either shut down or will be with in the near future. However the AGRs have an anticipated life of a further 10 to 20 years. This is a thermal neutron reactor design.
   • f. Super Critical Water-cooled Reactor (SCWR): This is a theoretical reactor design that is part of the Gen-IV reactor project. It combines higher efficiency than a GCR with the safety of a PWR, though it is perhaps more technically challenging than either. The water is pressurized and heated past its critical point, until there is no difference between the liquid and gas states. A CWR is similar to a BWR, except there is no boiling (as the water is critical), and the thermal efficiency is higher as the water behaves more like a classical gas. This is a epithermal neutron reactor design.
   • g. Liquid Metal Fast Breeder Reactor (LMFBR): This is a reactor design that is cooled by liquid metal, and totally unmoderated. These reactors can function much like a PWR in terms of efficiency, and don't require much high pressure containment, as the liquid metal doesn't need to be kept at high pressure, even at very high temperatures. Superphénix in France was a reactor of this type, as was Fermi-I in the United States. The Monju reactor in Japan suffered a sodium leak in 1995 and is approved for restart in 2008. All three use/used liquid sodium. These reactors are fast neutron, not thermal neutron designs. These reactors come in two types:
   • g-I. Lead Cooled: Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view. Often a reactor of this type would use a lead-bismuth eutectic mixture. In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead.
   • g-II. Sodium Cooled: Most LMFBRs are of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually remove corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken, but such explosions wouldn't be vastly more violent than (for example) a leak of superheated fluid from a CWR or PWR. Some of the sodium will be converted to Na-22 by the neutrons in the reactor, so the risk in an accident is somewhat greater, as the sodium itself is fairly dangerous for a few years, after being removed from the core. The difference between fast-spectrum and thermal-spectrum reactors will be covered later. In general, fast-spectrum reactors will produce less waste, and the waste they do produce will have a vastly lower halflife, but they are more difficult to build, and more expensive to operate. Fast reactors can also be breeders, whereas thermal reactors generally cannot.
2. The radioisotope thermoelectric generator produces heat through passive radioactive decay. Some radioisotope thermoelectric generators have been created to power space probes (for example, the Cassini probe), some lighthouses in the former Soviet Union, and some pacemakers. The heat output of these generators diminishes with time; the heat is converted to electricity by thermocouples.

Nuclear power - Experimental Technologies

A number of other designs for nuclear power generation are the subject of active research and may be used for practical power generation in the future. A number of advanced nuclear reactor designs could also make critical fission reactors much cleaner and safer.

• Integral Fast Reactor - The link at the end of this paragraph references an interview with Dr. Charles Till, former director of Argonne National Laboratory West in Idaho and outlines the Integral Fast Reactor and its advantages over current reactor design, especially in the areas of safety, efficient nuclear fuel usage and reduced waste. The IFR was built, tested and evaluated during the 1980's and then retired under the Clinton administration in the 1990's due to nuclear non-proliferation policies of the administration. Recycling spent fuel is the core of its design and it therefore produces a fraction of the waste of current reactors. [13]
• Pebble Bed Reactor - This reactor type is designed so high temperatures reduce power output by doppler broadening of the fuel's neutron cross-section. It uses ceramic fuels so its safe operating temperatures exceed the power-reduction temperature range. Most designs are cooled by inert helium, which cannot have steam explosions, and which does not easily absorb neutrons and become radioactive, or dissolve contaminants that can become radioactive. Typical designs have more layers (up to 7) of passive containment than light water reactors (usually 3). A unique feature that might aid safety is that the fuel-balls actually form the core's mechanism, and are replaced one-by-one as they age. The containment makes fuel reprocessing expensive.
• Subcritical reactors are designed to be safer and more stable, but pose a number of engineering and economic difficulties.
• Controlled nuclear fusion could in principle be used in fusion power plants to produce safer, cleaner power, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as of yet none has produced more energy than it consumed. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050 [14]. The ITER project is currently leading the effort to commercialize fusion power.

Nuclear power primarily produces concentrated heat. This can be converted to electricity and this currently constitutes a small but significant percentage of worldwide electricity generation. The heat can also be converted to mechanical work and this is the power source for many large military ocean going vessels (and a few commercial or government vessels). Other possible uses for the heat is in chemical processes, such as in the production of hydrogen, desalination [15], or direct heating of houses, especially by the massive amount of low grade waste heat generated by power plants.

Adapted from the Wikipedia article "Reactor Types", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki