Moderator for Gas-Cooled Reactors

Several candidate materials are available for possible use as a moderator in the gas-cooled reactor. These materials include light water, heavy water, metallic hydrides (e.g., zirconium hydride), beryllium metal, beryllium oxide, and graphite. Of these, the most commonly proposed moderator materials are graphite, heavy water, and beryllium oxide. 

The choice of the moderator material for a central-station powerplant is generally based on the economics involved. Obviously, many factors other than the cost per unit weight or volume, per se, enter into the economics. The neutron slowing-down capability of the material has an important effect on the size of the reactor core and, therefore, the capital cost of the plant, because of the investment in moderator, pressure vessel, shielding, etc. Containment requirements for the moderator (particularly liquid moderators) can affect both the capital cost of the plant and the fuel cycle economics, the latter because of possible neutron losses. Integrity and stability of the moderator material can, of course, have important implications on other aspects of the reactor design. The neutron absorption behavior of the moderator itself affects the potential conversion ratio of the reactor and, therefore, the fuel cycle economics of the reactor. The properties of the more important moderators and the implications of these properties on the choice and performance characteristics of gas-cooled reactors will be reviewed in this section. 

The neutronic effectiveness of a moderator material can be expressed in terms of the conversion potential of a mixture of fuel and moderators in an infinite medium. This approach has been used in a relative comparison of HjO, BeO, and C with fuel which has appeared in a progress report on BeO reactor studies (26). In this evaluation, the excess production of neutrons per neutron absorbed in the fuel has been calculated for homogeneous mixtures of fuel and moderator having different moderator/ atom ratios, i.e., 

Results of the calculations for three moderator materials are given in 

The spectrum-weigh ted value of r] appearing in expression (1) tends to increase with increasing moderator/U ratios, since the thermal value of ri for is considerably larger than the epithermal value. The fast 

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With the possible exception of BeO, graphite appears to be the most favorable moderator for gas-cooled reactors from a neutronics point of view. Because of limited irradiation experience with BeO and the current high cost of the material (largely because of its limited use), BeO has not been used extensively in nuclear reactors up to this time. 

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