In AGRs, the burn-out of fission cross-section results in a reduction of channel rating with burn up. There are two main components to this reduction, since the rating is defined by R = ]£fd>(E)$E and both Sr and <5 change with irradiation. Ef decreases {as shown in Section 2.2 of this chapter) due to the burn-out of U-235, and the flux d>(E) will reduce because the number of neutrons produced in the re­gion decreases and the neutron absorption cross-section increases as a result of the build-up of fission pro­ducts. In AGRs the channels are more widely spaced than in magnox and hence flux levels are not so dominated by surrounding channels.

Typical ‘age factor’ variations are shown in Fig 3.9, for fuel designed for 18 GVVd/t and 24 GWd/t mean

Fig. 3.9 Typical ‘age factor’ ariations for fuel
designed for 18 GWd/t and 24 GWd/t mean
channel discharge

local can temperatures in AGRs and therefore need to be taken into account in the assessment of can temperature for satisfying temperature operating lim­its. The burn-out of ring-to-ring rating ratio is illus­trated in Fig 3.10 for outer zone feed fuel. Burn-out effects are relatively smaller for the lower enriched inner zone and initial charge fuel; this is partly as a result of the smaller fine structure but also because the build-up of Pu-239 is relatively more important in lower enriched fuel.

Axial flux fine structure is a significant contributor to the peak rating and therefore affects can tempera­ture and fission gas release predictions. At start of life the rating is higher than the pin mean by about 20% in the end pellet and 10% in the next pellet. It has been observed from gamma scanning of irra­diated fuel that the peak in rating decreases signifi­cantly. A fit to measurements is shown on Fig 3.11, where the ratio of pellet mean to pin mean rating is given as a function of burn-up for end pellet and next pellet in the inner ring of 2.5% enriched fuel. Similar variations occur in the middle and outer rings.

Another localised rating factor is of significance. This is the cross-pin variation. The thermal flux dip into an AGR fuel cluster also generates a cross-pin flux tilt. This has an effect on cross-pin temperature distribution which is of concern in considerations not only of peak can temperature, but also of pin bowing

Мі; я. lu AGR ringno-rine raiing ratios

and clad straining due to differential expansion be­tween fuel and clad. The cross-pin rating tilt tends to burn-out since the burn-up is faster on the high flux side, but this is counteracted to some extent by enhanced Pu production on the outer facing surface. This is most pronounced in the outer fuel ring where the outer facing surface ‘sees’ directly the majority of the neutrons arriving from the channel wall. The resulting rating tilts as calculated by the WIMSE code are shown in Fig 3.12.