2.2.1 Fast feedback effects

Magnox reactors use metal fuel and are operated to only a fairly low fuel bum-up. There is no modera­tor in the fuel so that the principal contributor to fuel temperature feedback is resonance broadening in U-238. Since U-238 concentration is virtually inde­pendent of irradiation it might be expected that the fuel temperature coefficient of reactivity would remain constant. That this is not so is due to the build-up of fission products.

The Pu-239 and Pu-240 resonances do not contri­bute to fuel feedback through the broadening me­chanism as they are produced mainly on the surface of the fuel (where the U-238 captures occur). There is little self-shielding and hence little contribution to fuel feedback. However there are two mechanisms which do make contributions to fuel feedback. The non-l/v absorption cross-section of U-235 gives a negative contribution which becomes smaller with ir­radiation and spectrum hardening by the magnox cans and, to a lesser extent, by U-238 give a positive effect through the Pu-239 resonance. The overall result is that the fast-acting feedback coefficient is initially about -1.3 mN/°C, reaches a value of about -1.0 mN °С at discharge (5500 MWd/t), giving a value of about -1.15 mN/°C at fuel cycle equilibrium.

2.2.2 Slow feedback effects

The feedback effects due to bulk graphite temperature changes are highly dependent on irradiation. Initially the moderator temperature coefficient of reactivity is small and negative due to the non-I/v cross-section of U-235. As Pu-239 builds up the temperature co­efficient becomes more and more positive. Typical values are -1.4 mN/°C at start-of-life, 16 mN/°C at discharge and an equilibrium value of around 12 mN/°C.