In previous chapters we introduced two bases for the derivation of the diffusion equation:
which states that neutrons diffuses from high concentration (high flux) to low concentration.
which states, that rate of change of neutron density = production rate – absorption rate – leakage rate.
We return now to the neutron balance equation and substitute the neutron current density vector by J = -D∇Ф. Assuming that ∇.∇ = ∇2 = Δ (therefore div J = -D div (∇Ф) = -DΔФ) we obtain the diffusion equation.