The time scale for both fuel and moderator tempera­ture effects is similar in PWRs with time constants of a few seconds.

2.2.4 Fuel feedback

Fuel temperature feedback in PWRs is due principally to the Doppler broadening of U-238 and Pu-240 resonance peaks. The Pu-239 thermal resonance is much less important in PWRs because the neutron spectrum is considerably harder than in AGRs and magnox reaciors. The resonance absorption increases somewhat with irradiation as Pu-240 builds up in the core. The point fuel feedback coefficient is typically -3 mN. r’C throughout the fuel cycle. However, be­cause of the relatively short neutron migration length, ■‘tatistical weighting effects are much more important lor P\ R than for AGR and magnox reactors, and the core average coefficient is highly dependent on the neutron flux distribution.