Neutron Diffusion — Basic Concepts

The previous chapters presented two of the most widely used parameters in reactor theory, the neutron flux and the neutron current. It was shown that both are products of the neutron number density and a characteristic speed. Consequently both have the same units (that is, [n/cm3] * [cm/s] = n/cm2/s), but nevertheless both parameters are also fundamentally different. The flux is proportional to the gross number of neutrons that move across a unit area and is used to calculate neutron-nuclei interaction rates. The neutron current is proportional to the net number of neutrons and is required in order to evaluate the movement of neutrons in or out of a specific volume. The distinction can also be summarized using mathematical terminology: one (the flux) is a scalar quantity which describes all of the path length’s that neutrons traverse per unit volume per second. The other (the current) is a vector quantity, which describes the direction and magnitude of neutron transfer.