Axial macroscopic

At the start of a magnox reactor’s life, the uniform fuel loading leads to a flux (and hence rating) shape similar to the theoretical cosine shape of a uniform bare reactor, but with some distortion caused by the partially inserted regulating rods. A typical magnox axial flux (and rating) shape is shown on Fig 3,5. As burn-up proceeds the competing processes of U-235

burn-up and Pu-239 build-up result in a slight in­crease in peaking of the rating shape followed by a degree of flattening. The changes in ratio of peak — to-mean axial rating are however only a few percent, considerably less than the effects caused by rod bank movements during normal reactor operation.

In magnox reactors the radial flux shape is flattened by the use of neutron absorbers. A typical average ra­dial macroscopic rating shape is shown on Fig 3.6 (a). This shape, dearly showing the large flat central re­gion, is maintained throughout core life. Superimposed on this shape is a local channel-to-channel variation due to variation in isotopic content of the fuel. The power generated in a newly refuelled channel rises relative to its surroundings as plutonium builds up until the loss of U-235 begins to outweigh the in­crease in Pu-239. This behaviour is demonstrated on Fig 3.7, which shows the ratio of individual channel power to surrounding channel power, known as the ‘age factor’, for a typical magnox reactor. Of the lO^o or so increase in relative power, the dominant effect is the increase in fission cross-section with a

small {less than 2%) contribution due to the higher heat generation per fission of plutonium. The changes occurring in the channel flux level relative to its surroundings is small (less than 1%). This constancy of flux level arises because the channels are close together relative to neutron migration lengths and hence the neutron flux in the channel derives main­ly from neutrons produced in surrounding channels which, because of the on-load refuelling in magnox reactors, are (on average) under constant conditions when the fuel cycle has reached an equilibrium state.