Aspects of Fuel Management in Thermal Reactors

As the reactor is brought up to the operating temperature, with a negative temperature coefficient, control rods have to be moved out of the core to maintain criticality, or the boron concentration reduced. Build-up of Xe causes a further reduction of reactivity. Finally, over the life of the core, the gradual bum-up of the fuel and the build-up of long-lived fission products causes a decrease of reactivity until the reactor has to be refueled. 

The calculation of the variation of the reactivity over the life of the core is a very complicated exercise. Allowance has to be made for (i) decrease in the overall fuel concentration, (ii) net buildup of fresh fissile material from conversion, and (iii) changes in the spatial distribution of fuel and fission products as bum-up proceeds. For example, the flux will generally be highest at the core center, and consequently the fuel will burn up and fission products accumulate more rapidly there, causing in turn a change in the flux distribution in the reactor. 

A number of methods has been proposed for extending the useful life of the fuel charge. The use of a large number of control rods as a means of permitting a high initial reactivity of the core is undesirable because of the 

An alternative to poisoning the moderator is the inclusion of a burnable poison in or adjacent to the fuel elements themselves. The poison is chosen to have an absorption cross section and a concentration which cause it to burn out at a rate which will match the rate of loss of reactivity of the fuel. The elements which have been used as burnable poisons include boron and gadolinium (in the form of gadolinia, Gd203). 

For natural uranium reactors, such as magnox or CANDU, the attainable excess reactivity at start of life is considerably smaller than can be achieved in an enriched reactor, such as the BWR or PWR. Thus, the interval 

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