6.3 Magnox fuel

8.1Л Fuel cycle

Chapter 1 has outlined the fission process and has indicated the energy available during fission. A 10 kg magnox fuel element has the potential to produce about 140 MWd of energy. Half of this energy will be derived from the fission of uranium 235 and half from the fission of plutonium 239. In the process, the element will have been irradiated to the level of 14000 MWd/t.

However, as will be apparent later, it is not prac­ticable to reach this level of irradiation. Indeed, a channel average irradiation of 45% and an individual element irradiation of 60% of this level, is probably the maximum that can be obtained.

The fuel cycle, which determines the rate and man­ner in which the fuel is exchanged, must maximise the production of energy from the fuel and has, amongst others, the following objectives:

• To ensure that the fuel reaches its target irradiation.

• To keep local flux perturbations to a minimum so that the flux and coolant flow distributions are always matched, avoiding power losses due to any channels running hot or cold.

The fuel cycle has a relatively simple concept insofar as it is required to exchange the fuel continuously at the various rates, which are dictated in the most part by the target irradiation and rating of the fuel, in the different zones of the reactor.

Reactivity changes of the fuel

Reactivity may be regarded as the ability of the fuel to produce energy and this ability is not constant throughout the irradiation of the fuel. The reacti­vity life cycle of magnox fuel is illustrated by Fig 3.35 and is characteristic of all magnox fuel.

The curve shows the change in reactivity of the fuel as the irradiation of that fuel progresses. Initially, there is a small drop in reactivity brought about by the production of high cross-section fission products. This is followed by an increase in reactivity due to the production of fissile plutonium 239. The reactivity reaches a peak at about 1000 MWd/t irradiation. With further increases of irradiation, the reactivity decreases due to the burn-up of uranium 235, the fissile plutonium 239 and the production of non-fissile plutonium 240. At about 3000 MWd/t the reactivity returns to the same level as at the start of irradia­tion. Thereafter, the reactivity continues to fall as irradiation is increased, i. e., the ability of the fuel to produce energy becomes less than that associated with new fuel. All fuel within a magnox reactor behaves in this manner irrespective of its radial or axial position.