Nuclear materials I Fuels 1.1 Uranium Uranium in one form or another is by far the most common fuel materials for nuclear reactors. (By comparison, the use of thorium and plutonium has so far been on a very small scale. It can be used either as pure uranium, a metal, or as compound such as uranium dioxide UO or uranium carbide UC Uranium is a rather soft and ductile metal which oxidizes readily in air and water at high temperature. Its melting point is 1133.C. It exists in one of three allotropic forms, depending on its temperature. These three different forms are called the alpha, beta and gamma phases and changes from one phase to another due to temperature changes are accompanied by density changes. Alpha phase uranium has a density of 19g/cmand a thermal conductivity which varies from 25W/mK at 25C to 42W/mK at 665]C. The transition from alpha to the beta phase takes place at 665C and is accompanied by dimensional changes in crystalline structure of the uranium, expansion along one axis and contraction along the others. To avoid distortion due to these anisotropic dimensional changes 665 C is considered to be the maximum operating temperature of uranium Metallic uranium is also very susceptible to radiation damage which produces dimensional changes and swelling above about 450.C. Consequently high burnups of metallic uranium fuel are not possible. In the British gascooled Magnox Reactors, which are the principal users of this type fuel, the burnup is limited to about 3500MWd/t To summarize, the low operating temperature, susceptible to radiation damage and low permissible burnup of uranium are serious disadvantages to its choice as a reactor fuel, and account for its very limited use Uranium dioxide UO2 is a black powder which can be fabricated by cold pressing and sintering at high temperature to produce small cylindrical pellets, and in this form it is by far the most common material for the fuel of commercial reactors In this ceramic form uO2 has good stability at high temperature and good resistance to rad iation damage which enables it to be used to high burnups. The melting point is 2865C and the theoretical density is 10.96g/cm, although in practice the density of UO2 pellets produced as described above is about 10g/cm. The thermal conductivity is low, being about 2. 5W/mK in the temperature range from 1000 to 2000C, however this low thermal conductivity is compensated for by the very high melting point which permits high maximum fuel temperatures Uranium dioxide does not react with water at high temperature, a very valuable characteristic as otherwise cladding failures in water cooled reactors would lead to serious reactions. It can retain a large fraction of the gaseous fission products at temperatures below 1000 C, but as the fuel temperature at the center of a pellet is likely to be greatly in excess of this value, prov ision must made for fission product gas release. This is usually done by having an empty space at the top of each fuel tube into which the gases can diffuse 4. During operation in reactor UO2 pellets suffer structural changes, principally as a result of ne high operating temperatures and high temperature gradients, but also as a result of prolonged irradiation. The effects may include swelling, formation of cracks and voids in the pellet and changes in the grain structure of the UO2. This type of fuel is normally subjected to much higher burnups than pure uranium, and 5 per cent or more of the orig inal uranium atoms
Nuclear Materials 1 Fuels 1.1 Uranium Uranium in one form or another is by far the most common fuel materials for nuclear reactors. (By comparison, the use of thorium and plutonium has so far been on a very small scale.) It can be used either as pure uranium, a metal, or as compound such as uranium dioxide UO2 or uranium carbide UC. Uranium is a rather soft and ductile metal which oxidizes readily in air and water at high temperature. Its melting point is 1133oC. It exists in one of three allotropic forms, depending on its temperature. These three different forms are called the alpha, beta and gamma phases, and changes from one phase to another due to temperature changes are accompanied by density changes. Alpha phase uranium has a density of 19g/cm3 and a thermal conductivity which varies from 25W/mK at 25oC to 42W/mK at 665oC. The transition from alpha to the beta phase takes place at 665oC and is accompanied by dimensional changes in crystalline structure of the uranium, expansion along one axis and contraction along the others. To avoid distortion due to these anisotropic dimensional changes 665oC is considered to be the maximum operating temperature of uranium. Metallic uranium is also very susceptible to radiation damage which produces dimensional changes and swelling above about 450oC. Consequently high burnups of metallic uranium fuel are not possible. In the British gascooled Magnox Reactors, which are the principal users of this type fuel, the burnup is limited to about 3500MWd/t. To summarize, the low operating temperature, susceptible to radiation damage and low permissible burnup of uranium are serious disadvantages to its choice as a reactor fuel, and account for its very limited use. Uranium dioxide UO2 is a black powder which can be fabricated by cold pressing and sintering at high temperature to produce small cylindrical pellets, and in this form it is by far the most common material for the fuel of commercial reactors. In this ceramic form UO2 has good stability at high temperature and good resistance to radiation damage which enables it to be used to high burnups. The melting point is 2865oC and the theoretical density is 10.96g/cm3 , although in practice the density of UO2 pellets produced as described above is about 10g/cm3 . The thermal conductivity is low, being about 2.5W/mK in the temperature range from 1000 to 2000oC, however this low thermal conductivity is compensated for by the very high melting point which permits high maximum fuel temperatures. Uranium dioxide does not react with water at high temperature, a very valuable characteristic as otherwise cladding failures in water cooled reactors would lead to serious reactions. It can retain a large fraction of the gaseous fission products at temperatures below 1000oC, but as the fuel temperature at the center of a pellet is likely to be greatly in excess of this value, provision must made for fission product gas release. This is usually done by having an empty space at the top of each fuel tube into which the gases can diffuse. During operation in reactor UO2 pellets suffer structural changes, principally as a result of the high operating temperatures and high temperature gradients, but also as a result of prolonged irradiation. The effects may include swelling, formation of cracks and voids in the pellet and changes in the grain structure of the UO2. This type of fuel is normally subjected to much higher burnups than pure uranium, and 5 per cent or more of the original uranium atoms
in the fuel may undergo fission and be changed, each one to two intermed iate mass fission oduct atoms Uranium carbide, UC, is another ceramic fuel of possible interest, but it has not been developed or used to anything like the same extent as UO2. It may have some advantages over UO, principally its higher thermal conductivity and higher density which leads to mor uranium atoms per un it volume of fuel, which is an advantage in a reactor. Uranium carbide reacts with water which makes it unsuitable for use in water cooled reactors but it does not react with sodium below 500 C, so it might be used in fast reactors. Its melting point, 2380C is rather lower than that of UO2, but this is compensated for by it higher thermal conductivity The development of uranium carbide so far has been principally as the fuel for high temperature gas-cooled reactors 1.2 Plutonium Pure plutonium metal is not suitable as reactor fuel due to the large number of crystall ine phases which exist up to its melting point of 640oC. The thermal conductivity is also very low, about 4.2W/mK at room temperature. Plutonium metal is highly reactive in moist air, but it can be stored in dry air at low temperature. It is a very dangerous material, being rad ioactive, toxic and an essential component of nuclear weapons, and is potentially a serious health hazard, particularly if it exists as dust in the atmosphere and is taken into the lungs b inhalation As a reactor fuel plutonium is used as the oxide Puo2. Its melting point is 2400oC Plutonium dioxide is mixed with uranium dioxide to form mixed oxide fuel (MOX)which for fast reactors ty lly contains 20 to 25 per cent of PuO2. The properties of this mixed oxide fuel are similar to those ofuo2 alone 1.3 Thorium Thorium has not been used as a reactor fuel to any great extent yet except in a few high temperature gas-cooled reactors. Thorium 232 is the fertile isotope from which uranium 233 is produced, and it is theoretically possible to obtain high breed ing ratios in thermal as well as fast reactors using this combination. Pure metallic thorium has a melting point of about 1700oC, It is superior to uranium due to its better stability, but it is not used as a fuel in its pure form. Instead it is used either as thorium dioxide Tho2 or thorium carbide ThC2. To date these compounds have only been used to a very small extent in a few high temperature gas-cooled reactors Thorium dioxide is similar in many respects to uranium dioxide. It is produced by the same methods of powder metallurgy, and it is chemically inert and has a good resistance to radiation damage. Thorium carbide has been used in the form of coated particle fuel in HTGRS. Very small spherical particles less than 1 mm diameter of mixed ThC2 and UC2 (highly enriched in 235U)are coated with thin layers of pyrolitic carbon and silicon carbide to retain fission products. These particles are dispersed in graphite to form a homogeneous mixture of fuel and moderator which has a very high operating temperature and good resistance to rad iation damage 2 Moderators The requirements of the moderator for a thermal reactor, namely low mass number, very low neutron capture cross-section and high scattering cross-section, limit the choice to only a few materials. Hydrogen and its isotope deuterium, carbon and beryllium are the only
in the fuel may undergo fission and be changed, each one to two intermediate mass fission product atoms. Uranium carbide, UC, is another ceramic fuel of possible interest, but it has not been developed or used to anything like the same extent as UO2. It may have some advantages over UO2, principally its higher thermal conductivity and higher density which leads to more uranium atoms per unit volume of fuel, which is an advantage in a reactor. Uranium carbide reacts with water, which makes it unsuitable for use in water cooled reactors, but it does not react with sodium below 500oC, so it might be used in fast reactors. Its melting point, 2380oC, is rather lower than that of UO2, but this is compensated for by it higher thermal conductivity. The development of uranium carbide so far has been principally as the fuel for high temperature gas-cooled reactors. 1.2 Plutonium Pure plutonium metal is not suitable as reactor fuel due to the large number of crystalline phases which exist up to its melting point of 640oC. The thermal conductivity is also very low, about 4.2W/mK at room temperature. Plutonium metal is highly reactive in moist air, but it can be stored in dry air at low temperature. It is a very dangerous material, being radioactive, toxic and an essential component of nuclear weapons, and is potentially a serious health hazard, particularly if it exists as dust in the atmosphere and is taken into the lungs by inhalation. As a reactor fuel plutonium is used as the oxide PuO2. Its melting point is 2400oC. Plutonium dioxide is mixed with uranium dioxide to form mixed oxide fuel (MOX) which for fast reactors typically contains 20 to 25 per cent of PuO2. The properties of this mixed oxide fuel are similar to those of UO2 alone. 1.3 Thorium Thorium has not been used as a reactor fuel to any great extent yet except in a few high temperature gas-cooled reactors. Thorium 232 is the fertile isotope from which uranium 233 is produced, and it is theoretically possible to obtain high breeding ratios in thermal as well as fast reactors using this combination. Pure metallic thorium has a melting point of about 1700oC. It is superior to uranium due to its better stability, but it is not used as a fuel in its pure form. Instead it is used either as thorium dioxide ThO2 or thorium carbide ThC2. To date these compounds have only been used to a very small extent in a few high temperature gas-cooled reactors. Thorium dioxide is similar in many respects to uranium dioxide. It is produced by the same methods of powder metallurgy, and it is chemically inert and has a good resistance to radiation damage. Thorium carbide has been used in the form of coated particle fuel in HTGRs. Very small spherical particles less than 1 mm diameter of mixed ThC2 and UC2 (highly enriched in 235U) are coated with thin layers of pyrolitic carbon and silicon carbide to retain fission products. These particles are dispersed in graphite to form a homogeneous mixture of fuel and moderator which has a very high operating temperature and good resistance to radiation damage. 2 Moderators The requirements of the moderator for a thermal reactor, namely low mass number, very low neutron capture cross-section and high scattering cross-section, limit the choice to only a few materials. Hydrogen and its isotope deuterium, carbon and beryllium are the only
elements that are suitable. Hydrogen and deuterium, being gases, are not sufficiently dense and must e used in the form of compounds, water and heavy water being the obvious choices The use of hydrocarbon has been tried but has not been successful and such materials are not used as moderators. It is interesting to recall, however, that Fermi used paraffin wax in his early experiments in the 1930s to slow down neutrons and study their interactions with the elements. so he was one of the first scientists to be aware of the effects of neutron moderation Beryllium has a very low neutron capture cross-section(0.009barns ) high melting point (about 1300C)and good strength, and at one time it seemed possible that it would find ar application either as the moderator or the fuel cladding in thermal reactors. However, it and its compounds are toxic, and beryllium itself has low conductivity and poor corrosion resistance Beryllium oxide Beo also has undesirable properties. As a result of this ne ither bery ilium nor its oxide have found any use in power reactor, and it is unlikely that they will be used in the The choice of moderators for thermal reactors is thus limited to three materials-water heavy water and carbon in the form of graphite 2.1 Water Water is an obvious choice for the moderator of a thermal reactor. and it can also serve as the coolant. It has excellent neutron slowing down properties which enable water moderated reactors to have much more compact cores than are possible in other types of thermal reactors The capture cross-section of water is rather high(0.66 barns per molecule) so that water moderated and cooled reactors require enriched uranium for criticality. It is, of cause, abundant, cheap and easily obtainable with high purity The main problem associated with the use of water as the moderator and cool power reactor concerns its rather unfavorable thermodynamic characteristics. The saturation pressure and temperature relationship is such that high pressure are required to prevent oiling at high temperatures, e.g. a pressure of 150 bar is required to allow water to reach a temperature of 340 C without boiling. Pressure of 150 to 160 bar are typical of pressurized water reactors, in which the temperature is limited to about 325C It is important to maintain water purity in a water cooled and moderated reactor, firstly to minimize corrosion and secondly to prevent the water from becoming radioactive due to(n,y) reactions with the impurities as the water flows through the reactor core. Radiation levels in the water can influence the radiation dose levels to which power station operating and maintenance staff are exposed, and the maintenance of high water purity assists in reducing operator exposures 2.2 Heavy water(omitted) 2.3 Graphite(omitted) 3 Coolants The principal requirements of the coolant for a nuclear reactor are follows Good thermodynamic properties, namely high thermal conductivity, density and specific heat, and low viscosity 2. Chemically non-reactive with other components of the reactor. 3. Very low neutron capture cross-section 4. It should not become radioactive as a result of (n, y) reactions which may occur when the
elements that are suitable. Hydrogen and deuterium, being gases, are not sufficiently dense and must e used in the form of compounds, water and heavy water being the obvious choices. The use of hydrocarbon has been tried, but has not been successful and such materials are not used as moderators. It is interesting to recall, however, that Fermi used paraffin wax in his early experiments in the 1930s to slow down neutrons and study their interactions with the elements, so he was one of the first scientists to be aware of the effects of neutron moderation. Beryllium has a very low neutron capture cross-section (0.009barns), high melting point (about 1300oC) and good strength, and at one time it seemed possible that it would find an application either as the moderator or the fuel cladding in thermal reactors. However, it and its compounds are toxic, and beryllium itself has low conductivity and poor corrosion resistance. Beryllium oxide BeO also has undesirable properties. As a result of this neither beryllium nor its oxide have found any use in power reactor, and it is unlikely that they will be used in the future. The choice of moderators for thermal reactors is thus limited to three materials-water, heavy water and carbon in the form of graphite. 2.1 Water Water is an obvious choice for the moderator of a thermal reactor, and it can also serve as the coolant. It has excellent neutron slowing down properties which enable water moderated reactors to have much more compact cores than are possible in other types of thermal reactors. The capture cross-section of water is rather high (0.66 barns per molecule) so that water moderated and cooled reactors require enriched uranium for criticality. It is, of cause, abundant, cheap and easily obtainable with high purity. The main problem associated with the use of water as the moderator and coolant in a power reactor concerns its rather unfavorable thermodynamic characteristics. The saturation pressure and temperature relationship is such that high pressure are required to prevent boiling at high temperatures, e.g. a pressure of 150 bar is required to allow water to reach a temperature of 340oC without boiling. Pressure of 150 to 160 bar are typical of pressurized water reactors, in which the temperature is limited to about 325oC. It is important to maintain water purity in a water cooled and moderated reactor, firstly to minimize corrosion and secondly to prevent the water from becoming radioactive due to (n,) reactions with the impurities as the water flows through the reactor core. Radiation levels in the water can influence the radiation dose levels to which power station operating and maintenance staff are exposed, and the maintenance of high water purity assists in reducing operator exposures. 2.2 Heavy Water (omitted) 2.3 Graphite (omitted) 3 Coolants The principal requirements of the coolant for a nuclear reactor are follows: 1. Good thermodynamic properties, namely high thermal conductivity, density and specific heat, and low viscosity. 2. Chemically non-reactive with other components of the reactor. 3. Very low neutron capture cross-section. 4. It should not become radioactive as a result of (n,) reactions which may occur when the
coolant is passing through the core of the reactor. Among the gaseous coolants, some can be eliminated from consideration for one reason or another. Oxygen and hydrogen are both reactive, the latter explosively so. Nitrogen has a significant capture cross-Section(1.8 barns). Air, being a mixture of oxygen and nitrogen can lso be ruled out. Oxygen 16 undergoes an(n, y) reaction with high energy neutrons(e.g fission neutrons) to form nitrogen 16 which is rad ioactive, but its half-life is only 7 seconds so the radioactive hazard is short-lived The two most suitable gaseous coolants are carbon dioxide and helium Carbon dioxide is a fairly unreactive gas, but it does react at high temperatures with certain types of steel and with graphite. Both these reactions have proved troublesome in British gas-cooled reactors in which carbon dioxide has been chosen as the coolant. Its dvantages are its inertness, availability and cheapness, and the very low capt cross-sections of both carbon and oxygen. Carbon 13, which is very small constituent of naturally occurring carbon, captures neutrons to a very small extent to form radioactive carbon 14 which poses a minor hazard if carbon dioxide leaks or is vented from a reactor to Helium is inert, has good thermodynamic properties and does not pose a radioactive hazard, so it might be regarded as the ideal gaseous reactor coolant. Unfortunately, it is not ple in large quantities and is expensive. Its use as a reactor coolant is at present confined to the few high temperature gas-cooled reactors operating in the USA and West erman The properties of water and heavy water as reactor coolants have been described in the preceding section, but their moderating abil ity makes them unsuitable for use as coolants in fast reactors Liquid metals are potentially excellent reactor coolants because of their good thermodynamic properties, in particular their high thermal conductivity which leads to very good heat transfer coefficients. Sodium, lithium, mercury and sodium-potassium alloys are a possibilities, but of these only sodium has been used to any great extent, exclusively in fast breeder reactors. Sodium-potassium alloys may become more commonly used. Mercury is very expensive and is toxic, as well as having too high a capture cross-section for use in thermal reactors. Lithium is similar to sodium in many respects, but has a higher melting point and is more expensive Sodium is the standard coolant for the still fairly small number of fast breeder reacto operating in the world. Its melting point is 98C and its boiling point at atmospheric pressure is 890C, so it is not necessary to use sodium at higher than atmosphere pressure, which is a distinct ad vantage. It is highly reactive with air and water, so high integrity pipework and heat exchangers are necessary to avoid leakage. Sod ium has a fairly low capture cross-section(0.5 barns) but it does undergo Na(n, y) 4Na reaction, and intermediate heat exchangers are required to contain the radioactive sodium 24 within the biological shield of the reactor Sodium is not corrosive to most structural materials provided its oxygen content is maintained low. The formation of sodium oxide in the coolant circuit can lead to plugging unless it is removed in cold traps Sod ium-potassium eutectic alloy with a melting point of 11C can be used for the decay heat removal systems of fast reactors as the coolant temperature in these systems may fall below the melting point of sodium
coolant is passing through the core of the reactor. Among the gaseous coolants, some can be eliminated from consideration for one reason or another. Oxygen and hydrogen are both reactive, the latter explosively so. Nitrogen has a significant capture cross-section (1.8 barns). Air, being a mixture of oxygen and nitrogen can also be ruled out. Oxygen 16 undergoes an (n,) reaction with high energy neutrons (e.g. fission neutrons) to form nitrogen 16 which is radioactive, but its half-life is only 7 seconds, so the radioactive hazard is short-lived. The two most suitable gaseous coolants are carbon dioxide and helium. Carbon dioxide is a fairly unreactive gas, but it does react at high temperatures with certain types of steel and with graphite. Both these reactions have proved troublesome in British gas-cooled reactors in which carbon dioxide has been chosen as the coolant. Its advantages are its inertness, availability and cheapness, and the very low capture cross-sections of both carbon and oxygen. Carbon 13, which is very small constituent of naturally occurring carbon, captures neutrons to a very small extent to form radioactive carbon 14 which poses a minor hazard if carbon dioxide leaks or is vented from a reactor to atmosphere. Helium is inert, has good thermodynamic properties and does not pose a radioactive hazard, so it might be regarded as the ideal gaseous reactor coolant. Unfortunately, it is not readily available in large quantities and is expensive. Its use as a reactor coolant is at present confined to the few high temperature gas-cooled reactors operating in the USA and West Germany. The properties of water and heavy water as reactor coolants have been described in the preceding section, but their moderating ability makes them unsuitable for use as coolants in fast reactors. Liquid metals are potentially excellent reactor coolants because of their good thermodynamic properties, in particular their high thermal conductivity which leads to very good heat transfer coefficients. Sodium, lithium, mercury and sodium-potassium alloys are all possibilities, but of these only sodium has been used to any great extent, exclusively in fast breeder reactors. Sodium-potassium alloys may become more commonly used. Mercury is very expensive and is toxic, as well as having too high a capture cross-section for use in thermal reactors. Lithium is similar to sodium in many respects, but has a higher melting point and is more expensive. Sodium is the standard coolant for the still fairly small number of fast breeder reactors operating in the world. Its melting point is 98oC and its boiling point at atmospheric pressure is 890oC, so it is not necessary to use sodium at higher than atmosphere pressure, which is a distinct advantage. It is highly reactive with air and water, so high integrity pipework and heat exchangers are necessary to avoid leakage. Sodium has a fairly low capture cross-section (0.5 barns) but it does undergo 23Na (n,) 24Na reaction, and intermediate heat exchangers are required to contain the radioactive sodium 24 within the biological shield of the reactor. Sodium is not corrosive to most structural materials provided its oxygen content is maintained low. The formation of sodium oxide in the coolant circuit can lead to plugging unless it is removed in cold traps. Sodium-potassium eutectic alloy with a melting point of 11oC can be used for the decay heat removal systems of fast reactors as the coolant temperature in these systems may fall below the melting point of sodium
