6.3.1 Fuel cycle

Higher coolant temperatures and ratings within the AGR, consistent with the requirement for elevated steam temperatures and pressures in the boilers, has led to the adoption of oxide fuel contained within a stainless steel clad. In order to compensate for the increased neutron absorption characteristics of the steel, use is made of enriched uranium dioxide fuel, a change which is uniquely responsible for the relative complexity of the AGR fuel cycle and the various fuel management considerations which relate to it. The AGR is designed for continuous on-load refuelling and, like magnox, will eventually contain fuel at all stages of its irradiation life (i. e., age). However, any similarities between the two fuel cycles ends here. As we have seen earlier, the power in a nuclear reactor emanates from the fission rate within the fuel, and the capability that the fuel has for producing fissions and therefore free neutrons is known as its reacti­vity. The changes in material composition of enriched AGR fuel as irradiation proceeds, produces a reacti­vity variation over life which differs markedly from that of its magnox predecessor, in which the trans­mutation during irradiation of U-238 to the more effective fissile isotope Pu-239 helps to maintain core reactivity. For the AGR fuel currently in use in the CEGB’s reactors, enriched typically from 1 to 3 w/o of U-235, the depletion in U-235 content which takes place during irradiation swamps any beneficial effect of build-up of Pu-239, with the overall result that reactivity in the fuel, and therefore rating (i. e., power) falls continuously from start of life to eventual dis­charge, the fall-off being further enhanced by the build-up of other ‘poisons’ (neutron absorbers) with­in the fuel. Indeed AGR fuel changes during its life from being a net producer to a net absorber of neutrons. Absolute values of reactivity at any irradia­tion will depend only upon the initial fuel enrichment — the higher the enrichment level (i. e., concentra­tion of U-235 atoms) — the higher the reactivity. It follows, therefore, that since the fuel progressively loses reactivity it eventually poisons the reactor in which it resides, so the timing of its replacement needs to be controlled in practice by working to an upper limit on fuel age {i. e., discharge irradiation). Since the increased proportion of U-235 in the fuel together with the Pu-239 is conducive to longer life, the earlier AGR fuel designs still in use are restricted, on grounds of reactivity only, to a maximum channel average irradiation (CAI) at discharge of 18 GWd/t, to be compared with around 5 GWd/t for magnox fuel (Fig 3.47). No other fuel element endurance pro­blem was thought to be more limiting than the main­tenance of adequate reactivity, and indeed this ori­ginal design duty is now well proven following the considerable manufacturing and operational experience obtained. Furthermore, fuel capable of achieving 21 GWd/t at discharge is already being loaded into the reactors at Hinkley Point В and Hunterston В, and physics calculations have since been performed in order to evaluate AGR feed fuel enrichment levels appropriate to the even higher discharge irradiations of 24, 27 and 30 GWd/t. Although it is hoped that such high irradiations will be achievable through evo­lution of the current design of fuel element, continued research and development will be necessary in other areas.

F’t.. 3.47 Variation of channel power with irradiation
for AGR and magnox fuel

The reactivity "age relationship for enriched AGR fuel
determines that fuel rating also exhibits a gradual
decline with irradiation, falling by some ЗО^о over life.
B> comparison, natural uranium magnox fuel, despite
the beneficial effects of build-up of Pu-239, is
removed from the reactor much earlier in life.
However, in calendar time, the residence of AGR and
magnox fuel is comparable at 4 to 5 years.

Regular refuelling is of course essential in order to both maintain core reactivity and remove fuel which has reached its discharge limit. However, an important consequence of the attainment of very high discharge irradiations, particularly when coupled with the very much smaller AGR core size compared to magnox, is that the replacement of spent fuel dur­ing the refuelling process gives rise to large local ‘swings’ in reactivity, and therefore power. All of these changes, whether age or refuelling induced, must be anticipated and controlled as and when they be­come apparent on the running reactor.