Advanced Reactors and Experimental Technologies A number of other designs for nuclear power generation, the Generation IV reactors, are the subject of active research and may be used for practical power generation in the future. A number of the advanced nuclear reactor designs could also make critical fission reactors much cleaner, much safer and/or much less of a risk to the proliferation of nuclear weapons. Integral Fast Reactor The Integral Fast Reactor or Advanced Liquid-Metal Reactor is a design for a nuclear fast reactor with a specialized nuclear fuel cycle. A prototype of the reactor was built in the United States, but the U.S. government in 1994, three years before completion, canceled the project. This reactor is cooled by liquid sodium and fueled by a metallic alloy of uranium and plutonium. The fuel is contained in steel cladding with liquid sodium filling in the space between the fuel and the cladding. In traditional water-cooled reactors the core must be maintained at a high pressure to keep the water liquid at high temperatures. In contrast, since the IFR used a liquid metal as a coolant, the core could operate at close to ambient pressure, dramatically reducing the danger of a loss of coolant accident. The entire reactor core, heat exchangers and primary cooling pumps were immersed in a pool of liquid sodium, making a loss of primary coolant extremely unlikely. The coolant loops were also designed to allow for cooling through natural convection, meaning that in the case of a power loss or unexpected reactor shutdown, the heat from the reactor core would be sufficient to keep the coolant circulating even if the primary cooling pumps were to fail. The IFR also utilized a passively safe fuel configuration. The fuel and cladding was designed such that when they expanded due to increased temperatures, more neutrons would be able to escape the core thus reducing the rate of the fission chain reaction. At sufficiently high temperatures this effect would completely stop the reactor even without external action from operators or safety systems. This was demonstrated in a series of safety tests on the prototype. A safety disadvantage of using liquid sodium as coolant arises due to sodium's chemical reactivity. Liquid sodium is extremely flammable and ignites spontaneously on contact with air or water. Thus leaking sodium pipes could give rise to sodium fires, or explosions if the leaked sodium comes into contact with water. To reduce the risk of explosions following a leak of water from the steam turbines the IFR had an extra intermediate coolant loop between the reactor and the turbines. The purpose of this loop was to ensure that any explosion following accidental mixing of sodium and turbine water would be limited to the secondary heat exchanger and not pose a risk to the reactor. The requirement of such an extra loop significantly added to the cost of the reactor. 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 design of the fuel makes fuel reprocessing expensive. SSTAR (Small, Sealed, Transportable, Autonomous Reactor) This is being primarily researched and developed in the US, intended as a fast breeder reactor that is tamper resistant, passively safe. The 100-megawatt version is expected to be 15 meters high by 3 meters wide, and weigh 500 tonnes. A 10-megawatt version is expected to weigh less than 200 tonnes. To obtain the desired 30-year life span, the design calls for a moveable neutron reflector to be placed over a column of fuel. The reflector's slow downward travel over the column would cause the fuel to be burned from the top of the column to the bottom. Because the unit will be sealed, it is expected that a breeder reaction will be used to further extend the life of the fuel. Crucially, SSTAR is also meant to be tamper resistant, which would prevent the leasing country from using the reactor to use the generated plutonium for nuclear weapons. The tamper-resistant features will include radio monitoring and remote deactivation. The leasing country will therefore have to accept the capability for remote foreign intervention in the facility. Currently, no prototypes for SSTAR exist - one is expected by 2015 Clean And Environmentally Safe Advanced Reactor The Clean And Environmentally Safe Advanced Reactor (CAESAR) is a nuclear reactor concept that uses steam as a moderator. Steam's density can be controlled very finely so, according to its developer Dr. Claudio Filippone, it can be used to fine tune neutron fluxes to ensure that neutrons are moving with an optimal neutron energy profile to split 238U92 nuclei. The CAESAR reactor design exploits the fact that the fission products and daughter isotopes produced via nuclear reactions also decay to produce additional delayed neutrons. Unlike conventional water-cooled fission reactors, where fission occurring in enriched 235U fuel rods moderated by liquid water coolant ultimately creates a Maxwellian thermal neutron flux profile, the neutron energy profile from delayed neutrons varies widely. In a conventional reactor, the moderator slows these neutrons down so that they cannot contribute to the 238U reaction; 238U has a comparatively large cross-section for neutrons at high energies. Sub critical Reactors Sub critical reactors are designed to be safer and more stable, but pose a number of engineering and economic difficulties. There are technical difficulties to overcome before ADS can become economical and eventually be integrated into future nuclear waste management. The accelerator must provide a high intensity and be highly reliable. There are concerns about the window separating the protons from the spallation target, which is expected to be exposed to stress under extreme conditions. The chemical separations of the transuranic elements and the fuel manufacturing, as well as the structure materials, are important issues. Finally, the lack of nuclear data at high neutron energies limits the efficiency of the design. 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. Thorium based reactors It is possible to convert Thorium-232 into U-233 in reactors specially designed for the purpose. In this way, Thorium, which is more plentiful than uranium, can be used to breed U-233 nuclear fuel. U-233 is also believed to have favorable nuclear properties as compared to traditionally used U-235, including better neutron economy and lower production of long lived transuranic waste. Advantages and disadvantages of Nuclear Reactor Advantages: - Produces much more energy - Reduces enhanced greenhouse effect Disadvantages: - Expense - Getting rid of nuclear waste - Nuclear leaching - Impact on biomass - Needs to be located in an isolated geographical location with min. interference - Target for terrorist threat Nuclear Fuel Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium. The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle. Less than 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired onto pellet form. These pellets are stacked into tubes, which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor. Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235, and some commercial reactors with a high neutron economy do not require the fuel to be enriched at all (that is, they can use natural uranium). There are at least 100 research reactors in the world, which use highly enriched, weapons-grade uranium (90% enrichment) as their fuel. Because of the risk of theft of this fuel, which could be potentially turned into a nuclear weapon without unsurmountable difficulty, for many years there have been many campaigns to attempt to convert reactors of this type to run on low -enriched uranium, which poses less of a direct proliferation threat. It should be noted that fissionable U-235 and non-fissionable U-238 are both used in the fission process. U-235 is fissionable by thermal (i.e. slow-moving) neutrons. A thermal neutron is one, which is moving about the same speed as the atoms around it. Since all atoms vibrate proportionally to their absolute temperature, a thermal neutron has the best opportunity to fission U-235 when it is moving at this same vibrational speed. On the other hand, U-238 is more likely to capture a neutron when the neutron is moving very fast. This U-239 atom will soon decay into plutonium-239, which is another fuel. Pu-239 is a viable fuel and must be accounted for even when a highly enriched uranium fuel is used. Plutonium fissions will dominate the U-235 fissions in some reactors, especially after the initial loading of U-235 is spent. Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors or nuclear bombs. Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. But in a fast breeder reactor, some other kind of coolant is used which will not moderate or slow the neutrons down much. This enables fast neutrons to dominate, which can effectively be used to constantly replenish the fuel supply. By merely placing cheap unenriched uranium into such a core, the non-fissionable U-238 will be turned into Pu-239, "breeding" fuel. At the end of the operating cycle, the fuel in some of the assemblies is "spent," and is discharged and replaced with new (fresh) fuel assemblies. Although in practice, it is the buildup of reaction poisons in nuclear fuel that determines the lifetime of nuclear fuel in a reactor; long before all possible fissions have taken place, the buildup of long-lived neutron absorbing fission products damps out the chain reaction. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor. COOLANTS FOR NUCLEAR REACTORS The coolant, which passes through the nuclear reactors, is used to transport the reactor heat either to a boiler where steam is raised to run a conventional turbine or it is used as a thermodynamic heat engine fluid and passes directly into the turbine and back to the reactor. Pressurized water, organic liquids, sodium, and most gas cooled nuclear power plants employ an intermediate steam boiler. Boiling water and some gas cool reactors use the coolant directly in the turbine.Coolants should ideally have the following properties:
Not a single coolant has all of these properties, and as a result a number of different coolants have been used in nuclear reactors. Each coolant with its own particular advantages for certain type of reactors. Among these coolants are light and heavy water (both pressurized and boiling), organic liquids, sodium, sodium potassium mixtures, fused salts, and a number of gases like- air, carbon dioxide, helium, steam, nitrogen and hydrogen. Reactor designs Currently, there are six reactor designs being considered, including: • Gas-Cooled Fast Reactor System • Lead-Cooled Fast Reactor System • Molten Salt Reactor System • Supercritical-Water-Cooled Reactor System • Sodium-Cooled Fast Reactor System • Very-High-Temperature Reactor System Nuclear Reactors Development Through Time and Around The World Reactor operation has been getting steadily better, with the result that nuclear electricity output has been rising much faster than the number and capacity of the plants producing it. The reactor core is loaded with fuel, which is usually uranium enriched to 3.5% to more than 4% U-235, the fissile isotope. The fuel is typically in the form of ceramic pellets of UO2, assembled inside zircalloy or stainless steel tubes (as shown above). In the reactor coolant and moderator surround this. The moderator slows down the fast neutrons from the nuclear fission chain reaction so that they are more likely to cause further fission in U-235 atoms. The fission reaction produces heat, which is used to produce steam to drive turbines. In engineering terms, nuclear fuel burn-up has increased substantially since the 1970s and in new plants is now over 1000 kilowatt hours per gram of uranium* in a light water reactor, using normal enriched fuel. This gives about 500,000 MJ/kg of natural uranium, compared with around 25 MJ/kg for good steaming coal, ie about 20,000 times the energy from the same amount of good steaming coal. Notwithstanding Chernobyl, over 9000 reactor-years of operating experience confirm nuclear power as a very safe and reliable way of making electricity. But the future belongs to new designs, both evolutionary and more radical ones. 79% of the world's reactors are based on just two US light-water designs And these contribute about 88% of total world nuclear capacity. The priority area for improvement has been upgrading every aspect of the Soviet-designed reactors still operating in Eastern Europe and Russia. They had long been recognized as unsafe, but post-Chernobyl, a lot of effort has greatly diminished the very real threat to that region posed by these reactors. They are all a lot better than they were in 1986. More broadly, the nuclear power industry has been developing and improving reactor technology for almost five decades and is now starting to launch the next generation of advanced reactors. New generation nuclear plants operate with more 'passive' safety features, which rely on gravity and natural convection. They either require no active controls or operational intervention to avoid accidents in the event of major malfunction, or at least allow a lot of time for intervention. The first of these new-generation plants was commissioned in 1996, in Japan. The construction of this reactor, and its twin, took just over four years, which makes a dramatic difference to the capital cost compared with the regulatory delays formerly imposed in some parts of the world. This is one reason for the strong push to standardized designs in the new generation of reactors, especially in the USA where three new designs now have full regulatory approval. |
