What does Fick's law of diffusion mention about the neutron current density?

Fick's law of diffusion describes how particles, including neutrons in a nuclear context, move from areas of higher concentration to areas of lower concentration. In terms of neutron transport, this principle translates into a relationship where neutron current density is directly influenced by the gradient of neutron flux.

When neutron flux is higher in one region compared to another, there is a net movement of neutrons from the area of high flux to the area of low flux. The greater the difference in flux, the stronger the driving force for neutron movement, resulting in a higher current density. This relationship illustrates that current density reflects how rapidly neutrons are diffusing through a medium.

As such, the statement indicating that the current density is proportional to the gradient of neutron flux is grounded in Fick's law, making it the correct choice.