[1] Fuel loading weight

As mentioned above, the fuel loading weight is first determined in the core design based on the cycle length and rated core thermal power as given by the next equation.

W= QxDx Nb/BU

W: fuel loading weight (t)

Q: core thermal power (MW)

D: operation cycle length (day)

Nb: refueling batch number (the reciprocal of the discharged fuel fraction in one refueling = the reciprocal of the new fuel fraction)

BU: region or batch averaged fuel assembly burnup (MWd/t)

The operation cycle length is fixed by considering refueling and periodic inspection intervals. Since generally a longer cycle length leads to a higher capacity factor and a lower operating cost of the plant, there is a tendency to set the cycle length as long as possible.

Fuel economy is strongly dependent on the fuel batch size. With a larger batch size, fuel assemblies can stay longer in core and achieve higher burnup,

and therefore that results in a reduction in fuel cycle cost. The fuel batch size is, however, restricted by the design limit of the maximum fuel assembly burnup. For a given operating cycle length, the largest batch size is taken in the range of possible maximum fuel assembly burnups. The average fuel assembly burnup becomes higher as the variation in discharge burnup of each fuel assembly is smaller. Usually, the average fuel assembly burnup is supposed assuming a certain ratio of the maximum to average fuel assembly burnups (for example, about 1.1). Hence, the fuel loading weight in Eq. (3.23) is determined with these considerations.

Figure 3.32 shows the relationships for cycle length, batch size, maximum fuel assembly burnup, and fuel cycle cost.

(i) For a longer operationg cycle length, fuel enrichment can be increased for higher burnup under a constant batch size or the number of new fuel assemblies can be increased; namely, fuel batch size is decreased under a constant fuel enrichment. Fuel enrichment is usually standardized and fixed, and therefore it is usual to increase the number of new fuel assem­blies. Maximum fuel enrichment is currently limited to 5 % by restrictions on fuel fabrication facilities and transportation.

(ii) An increase in the number of new fuel assemblies and then a decrease in batch size lead to a lower average fuel assembly burnup and a higher fuel cycle cost. Since fuel cycle cost occupies a small fraction of total plant operating cost, a longer operating cycle length reduces the total plant operating cost even for a smaller batch size.

(iii) For a long cycle length and low fuel cycle cost, it is also necessary to advance the limit of the maximum fuel assembly burnup. This can be achieved by loading an improved type of fuel such as with new alloy cladding. A higher limit for the maximum fuel assembly burnup increases the fuel batch size and reduces the fuel cycle cost even for the same cycle length.