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 theelements 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