The neutron source to be used in calculations of the irradiation of nuclear reactor pressure vessels depends not only on the power distribution in the core but also on the burnup distribution. The burnup affects both the strength and the spectrum of the source, with each effect increasing the displacement rate in the pressure vessel as the burnup in the outer parts of the core increases. For a VVER-440 reactor, each effect causes an ≈8 % increase going from fresh fuel to a burnup representative of a low-leakage loading scheme. For Western light water reactors, the increase due to the spectral effect may be somewhat larger. This work investigates the spectral effect and discusses practical ways of taking it into account in calculations.