Garnered from "Nuclear Reactor Engineering", Glasstone & Sesonske:
1. The melting point of Cadmium is 321 C, so that it is useful only in
"low" temp reactors. It is also highly corrosive and must be clad.
Hafnium has a melting point of 2230 C and is resistant to corrosion.
2. The neutron capture cross section of cadmium, although very high,
drops off rapidly above a resonance at 0.18 ev. "It is largely because
Hafnium is so effective for capturing neutrons in the epithermal
(resonance) energy region that it is regarded as being the best control
material for water-cooled (and moderated) power reactors."
3. Because of its series of four neutron absorbing isotopes,
Hafnium remains effective for a long time. Neutron capture by
Hafnium 177 leads to the formation of Hafnium 178, etc.
--
Bob Sciamanda sciamanda@edinboro.edu
Dept of Physics sciamanda@worldnet.att.net
Edinboro Univ of PA http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA (814)838-7185