The first generation of commercial nuclear power reactors in Britain and France were cooled by carbon dioxide gas. These reactors were graphite moderated and fueled with natural uranium metal rods clad in magnesium alloys, the performance of which has been covered. The first of these reactors (Calder Hall) started generating electricity in 1956. The second-generation CO2-cooled graphite moderated reactors in Britain (the AGRs) use slightly enriched UO2 clad in stainless steel. These fuel elements can operate at higher temperatures to much greater burnups, giving higher efficiencies and ratings.

The magnesium alloy cladding in the Magnox reactors is finned to improve heat transfer. The “adjusted” uranium alloy fuel has antiratchetting grooves that lock the fuel to the cladding and minimize thermal cycling effects. The fuel temperature has to be kept below 665◦C to avoid

the phase transformation that occurs in uranium at this temperature. These fuel elements have performed very well, allowing high plant capacity factors that tend to compensate for the low thermal efficiency and low burn up of the fuel.

The AGR reactors use a super stainless steel (Fe-20%, Cr-25%, Ni-0.5%, Nb) alloy that has good high-temperature strength and oxidation resistance. The cladding is ribbed to enhance heat transfer. The fuel pellets are in the form of hollow pellets to accommodate ramp-induced swelling.

D. Helium Gas-Cooled Reactors. The high-temperature gas-cooled reactors (HTGR) use helium gas at about 800◦C and 5 MPa (685 psi) as the primary coolant, graphite as the neutron moderator and fuel element structural material, and coated (Th-U) carbide or oxide fuel particles dispersed in a graphite matrix as the fuel. Currently TRISO-UC2 and BISO-ThO 2 are the candidate fissile and fertile fuel particles, respectively, for the large commercial HTGRs being developed. The manner in which advantage maybe taken of high-temperature materials deserves emphasis.

The choice of graphite as the moderator and core structural material is based on its unique chemical, physical, and mechanical properties at elevated temperatures and on its very low

neutron cross section, satisfactory radiation stability, ease of fabrication, and low cost.

The use of the graphite moderator as a diluent of the fuel permits much greater fuel dilution than would otherwise be possible and thereby minimizes radiation damage, increases specific power, and greatly extends the heat transfer surface.

The Th-233U standard fuel cycle with 235U as the initial fissionable fuel) is used because of its potential for achieving higher fuel utilization and lower power cost than any other thermal spectrum reactor system. The neutronic characteristics of 233U are far superior to those of either plutonium or 235U in thermal systems. A substantial portion of the power comes from fission of the 233U converted from the fertile 232Th. The carbon-to-thorium ratio is optimum at a value of 240. This concept promises a conversion ratio as high as 0.85, a steam-heat power efficiency of about 39 %, and a low fuel cost, even with high ore costs. The annual uranium requirements for the HTGR are 30 –40% less than for a pressurized-water reactor (PWR) with plutonium recycle operation.

Because it can use plutonium as a fissile nuclide and provide a burn up of over 100 MWd/kg, the HTGR can also use plutonium more efficiently than light-water reactors.

The use of coated-particle fuel allows the high-temperature operation of the core to very high burn up (80%) of the fissile fuel, with extremely high retention of the fission products. Also, the

235U fissile particles are segregated from the 233Np and can be separated during the fuel reprocessing operation. The average fuel burn up of 100 MWd/kg obtainable is by far the highest of all existing thermal reactor systems.

Inherent safety is achieved in the HTGRs by virtue of the single phase and inertness of the coolant, the high heat capacity of the fuel elements and moderator and their refractory nature, the negative temperature coefficient (which provides a safe shutdown mechanism) redundancy in the circulating systems, and assured retention of 4% of the coolant. The

fission product plate out activity is limited to low levels that permit direct maintenance.

The fuel exposure, as measured in MWd/kg, is not an important constraint in the HTGR. The average exposure is about 95 MWd/kg, but the burn up in individual fuel particles in the HTGR reaches 0.75 fission per initial metal atom or about 700 MWD/kg.

The coatings on the fuel particles have been developed as a result of extensive studies on the production and properties of pyrolytic carbon coatings and irradiation tests on coated particles. The coatings are designed to retain the fission products and to withstand the effects of fuel burn up and irradiation. These include the internal pressure buildup due to fission gas accumulation, fission recoil damage, and stresses arising from fast neutron irradiation-induced dimensional changes in the pyro carbon coatings.

The inner buffer layer of low-density pyro carbon serves to protect the outer layers from fission recoil damage and provides void space to accommodate the fission gases, fuel swelling, and coating contraction.

The silicon carbide layer in the TRISO coatings decreases the release of certain fission products

that migrate readily through the pyro carbon (e.g., barium, strontium, and cesium). The pebble bed reactor, developed and built in Germany, is a helium gas-cooled, high- temperature reactor fueled with spherical, graphite matrix fuel elements surrounded by bottom and side graphite reflectors. The fuel elements consist of pyrolytic carbon coated (U,Th)C2 spherical particles dispersed in a graphite matrix and encased in spherical graphite balls, 6 cm in diameter.

The continuous on-load refueling is accomplished by removing used fuel from the bottom of the core and adding new fuel at the top.

The AVR reactor has certain basic characteristics that are similar to those of the other helium-cooled reactors. These include (1) the use of graphite both as structural material and moderator, (2) (U,Th) C 2 fuel particles coated with pyrolytic carbon coatings and dispersed in a graphite matrix, and (3) high gas temperatures that allow the use of modern steam cycles.

This leads to high efficiency and good con version of fertile elements into fissile materials (232 Th into 233 U). The reactor is also compact as are other helium-cooled reactors.

The AVR reactor is a prototype designed to yield construction and operation experience and to prove the feasibility of the pebble bed concept

The first generation of carbon-dioxide gas-cooled graphite moderated reactors (Magnox) is cooled by circulating CO2 gas. The fuel elements consist of natural-uranium metallic fuel rods clad with a magnesium alloy. The second-generation advanced gas-cooled

(AGR) reactors use stainless steel clad slightly enriched UO2 fuel rods, which permit steam generation at higher temperatures.

The Magnox reactors used uranium adjusted with iron (260 ppm), aluminum (650 ppm), carbon (800 ppm), silicon (20 ppm), and nickel (50 ppm). The French EDF reactors used U –1% Mo in EDF-1, -2, -3, and -4 and Sicral alloy [uranium containing Al (700 ppm), Fe (300 ppm), Si (120 ppm), and Cr (80 ppm)] in EDF-5. These minor alloying elements result in grain-size refinement and very finely divided precipitates and the swelling diminishes by several orders of magnitude. These additions modify the α–β transformation and favor grain refinement and ab-sence of preferred orientation upon quenching these alloys from temperatures in the beta range. Thus, heat treatment minimizes distortion of fuel elements due to either thermal cycling or irradiation growth, since induced intergranular stresses and strains are reduced, a typical grain size effect. These fuels were clad with magnesium alloys (Mg containing 0.8% Al, 0.002-0.05% Be, 0.008% Ca, and 0.006% Fe in the U.K., and Mg containing 0.6% Zr in France).

The advanced CO2 gas-cooled reactors in the United Kingdom are graphite moderated and fueled with slightly enriched UO2 fuel clad in stainless steel. The uranium dioxide fuel is in the form of sintered pellets [10.2 mm (0.40 in.) diameter], which are loaded into stainless steel

tubes about 508 mm (20 in.) long with a 0.04-mm (0.015-in.) wall. A cluster of 21 fueled tubes is supported by stainless steel grids within a graphite sleeve to form a fuel element. In each channel several fuel assemblies are joined together by a central tie bar to form a fuel stringer.

The stainless steel alloy developed for the cladding has a 20% chromium, 25% nickel composition stabilized with niobium. This alloy is produced by a double vacuum melting technique, is free from sigma-phase formation, and has excellent resistance to oxidation in CO2 at temperatures as high as 850◦C.

The first generation of commercial nuclear power reactors in Britain and France were cooled by carbon dioxide gas. These reactors were graphite moderated and fueled with natural uranium metal rods clad in magnesium alloys, the performance of which has been covered. The first of these reactors (Calder Hall) started generating electricity in 1956. The second-generation CO2-cooled graphite moderated reactors in Britain (the AGRs) use slightly enriched UO2 clad in stainless steel. These fuel elements can operate at higher temperatures to much greater burnups, giving higher efficiencies and ratings.

The magnesium alloy cladding in the Magnox reactors is finned to improve heat transfer. The “adjusted” uranium alloy fuel has antiratchetting grooves that lock the fuel to the cladding and minimize thermal cycling effects. The fuel temperature has to be kept below 665◦C to avoid

the phase transformation that occurs in uranium at this temperature. These fuel elements have performed very well, allowing high plant capacity factors that tend to compensate for the low thermal efficiency and low burn up of the fuel.

The AGR reactors use a super stainless steel (Fe-20%, Cr-25%, Ni-0.5%, Nb) alloy that has good high-temperature strength and oxidation resistance. The cladding is ribbed to enhance heat transfer. The fuel pellets are in the form of hollow pellets to accommodate ramp-induced swelling.

D. Helium Gas-Cooled Reactors. The high-temperature gas-cooled reactors (HTGR) use helium gas at about 800◦C and 5 MPa (685 psi) as the primary coolant, graphite as the neutron moderator and fuel element structural material, and coated (Th-U) carbide or oxide fuel particles dispersed in a graphite matrix as the fuel. Currently TRISO-UC2 and BISO-ThO 2 are the candidate fissile and fertile fuel particles, respectively, for the large commercial HTGRs being developed. The manner in which advantage maybe taken of high-temperature materials deserves emphasis.

The choice of graphite as the moderator and core structural material is based on its unique chemical, physical, and mechanical properties at elevated temperatures and on its very low

neutron cross section, satisfactory radiation stability, ease of fabrication, and low cost.

The use of the graphite moderator as a diluent of the fuel permits much greater fuel dilution than would otherwise be possible and thereby minimizes radiation damage, increases specific power, and greatly extends the heat transfer surface.

The Th-233U standard fuel cycle with 235U as the initial fissionable fuel) is used because of its potential for achieving higher fuel utilization and lower power cost than any other thermal spectrum reactor system. The neutronic characteristics of 233U are far superior to those of either plutonium or 235U in thermal systems. A substantial portion of the power comes from fission of the 233U converted from the fertile 232Th. The carbon-to-thorium ratio is optimum at a value of 240. This concept promises a conversion ratio as high as 0.85, a steam-heat power efficiency of about 39 %, and a low fuel cost, even with high ore costs. The annual uranium requirements for the HTGR are 30 –40% less than for a pressurized-water reactor (PWR) with plutonium recycle operation.

Because it can use plutonium as a fissile nuclide and provide a burn up of over 100 MWd/kg, the HTGR can also use plutonium more efficiently than light-water reactors.

The use of coated-particle fuel allows the high-temperature operation of the core to very high burn up (80%) of the fissile fuel, with extremely high retention of the fission products. Also, the

235U fissile particles are segregated from the 233Np and can be separated during the fuel reprocessing operation. The average fuel burn up of 100 MWd/kg obtainable is by far the highest of all existing thermal reactor systems.

Inherent safety is achieved in the HTGRs by virtue of the single phase and inertness of the coolant, the high heat capacity of the fuel elements and moderator and their refractory nature, the negative temperature coefficient (which provides a safe shutdown mechanism) redundancy in the circulating systems, and assured retention of 4% of the coolant. The

fission product plate out activity is limited to low levels that permit direct maintenance.

The fuel exposure, as measured in MWd/kg, is not an important constraint in the HTGR. The average exposure is about 95 MWd/kg, but the burn up in individual fuel particles in the HTGR reaches 0.75 fission per initial metal atom or about 700 MWD/kg.

The coatings on the fuel particles have been developed as a result of extensive studies on the production and properties of pyrolytic carbon coatings and irradiation tests on coated particles. The coatings are designed to retain the fission products and to withstand the effects of fuel burn up and irradiation. These include the internal pressure buildup due to fission gas accumulation, fission recoil damage, and stresses arising from fast neutron irradiation-induced dimensional changes in the pyro carbon coatings.

The inner buffer layer of low-density pyro carbon serves to protect the outer layers from fission recoil damage and provides void space to accommodate the fission gases, fuel swelling, and coating contraction.

The silicon carbide layer in the TRISO coatings decreases the release of certain fission products

that migrate readily through the pyro carbon (e.g., barium, strontium, and cesium). The pebble bed reactor, developed and built in Germany, is a helium gas-cooled, high- temperature reactor fueled with spherical, graphite matrix fuel elements surrounded by bottom and side graphite reflectors. The fuel elements consist of pyrolytic carbon coated (U,Th)C2 spherical particles dispersed in a graphite matrix and encased in spherical graphite balls, 6 cm in diameter.

The continuous on-load refueling is accomplished by removing used fuel from the bottom of the core and adding new fuel at the top.

The AVR reactor has certain basic characteristics that are similar to those of the other helium-cooled reactors. These include (1) the use of graphite both as structural material and moderator, (2) (U,Th) C 2 fuel particles coated with pyrolytic carbon coatings and dispersed in a graphite matrix, and (3) high gas temperatures that allow the use of modern steam cycles.

This leads to high efficiency and good con version of fertile elements into fissile materials (232 Th into 233 U). The reactor is also compact as are other helium-cooled reactors.

The AVR reactor is a prototype designed to yield construction and operation experience and to prove the feasibility of the pebble bed concept

The first generation of carbon-dioxide gas-cooled graphite moderated reactors (Magnox) is cooled by circulating CO2 gas. The fuel elements consist of natural-uranium metallic fuel rods clad with a magnesium alloy. The second-generation advanced gas-cooled

(AGR) reactors use stainless steel clad slightly enriched UO2 fuel rods, which permit steam generation at higher temperatures.

The Magnox reactors used uranium adjusted with iron (260 ppm), aluminum (650 ppm), carbon (800 ppm), silicon (20 ppm), and nickel (50 ppm). The French EDF reactors used U –1% Mo in EDF-1, -2, -3, and -4 and Sicral alloy [uranium containing Al (700 ppm), Fe (300 ppm), Si (120 ppm), and Cr (80 ppm)] in EDF-5. These minor alloying elements result in grain-size refinement and very finely divided precipitates and the swelling diminishes by several orders of magnitude. These additions modify the α–β transformation and favor grain refinement and ab-sence of preferred orientation upon quenching these alloys from temperatures in the beta range. Thus, heat treatment minimizes distortion of fuel elements due to either thermal cycling or irradiation growth, since induced intergranular stresses and strains are reduced, a typical grain size effect. These fuels were clad with magnesium alloys (Mg containing 0.8% Al, 0.002-0.05% Be, 0.008% Ca, and 0.006% Fe in the U.K., and Mg containing 0.6% Zr in France).

The advanced CO2 gas-cooled reactors in the United Kingdom are graphite moderated and fueled with slightly enriched UO2 fuel clad in stainless steel. The uranium dioxide fuel is in the form of sintered pellets [10.2 mm (0.40 in.) diameter], which are loaded into stainless steel

tubes about 508 mm (20 in.) long with a 0.04-mm (0.015-in.) wall. A cluster of 21 fueled tubes is supported by stainless steel grids within a graphite sleeve to form a fuel element. In each channel several fuel assemblies are joined together by a central tie bar to form a fuel stringer.

The stainless steel alloy developed for the cladding has a 20% chromium, 25% nickel composition stabilized with niobium. This alloy is produced by a double vacuum melting technique, is free from sigma-phase formation, and has excellent resistance to oxidation in CO2 at temperatures as high as 850◦C.

there are different types of fuel elements depending on the reactor power needed, thermal, epithermal and fast neutron flux and also these characteristics are based on some safety related  thermal hydraulic parameters  like DNBR, inlet and outlet temperature, ....