When nuclear fission occurs inside of a nuclear reactor, neutrons are produced. These neutrons then, to state it simply, either react with the fuel in the reactor or escape from the reactor. These two processes are referred to as neutron absorption and neutron leakage, and their sum is the neutron loss. When the rate of neutron production is equal to the rate of neutron loss, the reactor is able to sustain a chain reaction of nuclear fissions and is considered a critical reactor.

Geometric buckling is a measure of neutron leakage and material buckling is a measure of the difference between neutron production and neutron absorption. In the case of a bare, homogenous, steady-state reactor (that is, a reactor that has only one region, a homogenous mixture of fuel and coolant, no blanket nor reflector, and does not change over time), the geometric and material buckling are equal to each other.

 

Note:   The above text is excerpted from the Wikipedia article Geometric and material buckling, which has been released under the GNU Free Documentation License.