Why is water a better choice than graphite as a moderator to slow neutrons in a nuclear reactor?

North De Land

Why is water a better choice than graphite as a moderator to slow neutrons in a nuclear reactor?



Hampton

Moderator essentially slows down the speed of neutrons. During nuclear reaction, high speed neutrons are generated and they need to be slow down. we call them as thermal neutrons. The choice of the moderator is governed by the following parameters1). Molecular weight2). Neutron absorption cross section(Area)3). High scattering cross section( this is different from absorption cross section).4). Availability of the moderator(abundancy in nature)5). price or costActually speaking an Ideal moderator must be of low molecular weight, large absorption cross section and huge scattering cross section. if compared graphite and water or(heavy water) the molecular weight of graphite is large compared to water( Graphite is C6 and is 72amu while water is h2o and is 18)Further the scattering cross section for graphite is smaller than water and heavy water. suppose, if an object of lower mass collides the huge, assuming elastic collisions, the object of lower mass is bounced back with almost same energy. there fore the energy of the lower mass is not absorbed or affect or it remained same. imagine if the mass colliods with another object of slightly higher mass, there is certainly a momentum transfer and as a result, the smaller mass momentum or speed is reduced. the same principle can be applied for the same.