Reactor Theory (Nuclear Parameters)
A non-burnable poison is one that maintains a constant negative reactivity worth over the life
of the core. While no neutron poison is strictly non-burnable, certain materials can be treated
as non-burnable poisons under certain conditions. One example is hafnium. The removal (by
absorption of neutrons) of one isotope of hafnium leads to the production of another neutron
absorber, and continues through a chain of five absorbers. This absorption chain results in a
long-lived burnable poison which approximates non-burnable characteristics. Absorbers with low
neutron absorption cross sections can also be treated as non-burnable under most conditions.
It is possible to make the reactivity of a poison material that is usually a burnable poison more
uniform over core life through the use of self-shielding. In self-shielding, the poison material
is thick enough that only the outer layer of the poison is exposed to the neutron flux. The
absorptions that take place in the outer layers reduce the number of neutrons that penetrate to the
inner material. As the outer layers of poison absorb neutrons and are converted to non-poison
materials, the inner layers begin absorbing more neutrons, and the negative reactivity of the
poison is fairly uniform.
The normal use of fixed non-burnable poisons is in power shaping, or to prevent excessive flux
and power peaking near moderator regions of the reactor.