10.18. Should the reactor be initially loaded with a complete core of a single enrichment, the power distribution will be similar to the flux distri­bution shown in the reflected reactor case of Fig. 3.18. This means that the fuel located in the region of high power will “burn” faster than the fuel in regions of lower power. Although the power distribution will tend to “flatten” with time as the fuel in the higher power regions becomes depleted of fissile atoms, the fuel in the lower flux regions will be under­utilized when the core reactivity is reduced to the level requiring shutdown for refueling.

10.19. Furthermore, safety limitations are based on the highest power fuel rod, which normally will be operating at the highest temperature. Thus, if the maximum power of the core is close to the average power, the reactor could be operated at a higher level than would be permissible if the peak-to-average power ratio would be higher. As we will see, the fuel can be better utilized and the reactor operated at a higher power level if reloading is done on a staggered basis. Also, if there is good neutron coupling between fresh fuel and fuel about to be discharged, additional burnup can be obtained, since the older fuel can be used to control reac­tivity. Otherwise, parasitic absorbers would be required.

10.20. Further explanation of staggered refueling may be helpful here. As an example, let us consider a typical PWR core containing 193 fuel assemblies. For our present orientation purposes, assume that the planned fuel burnup is about 2.6 TJ/kg U (or 30,000 MW • d/t), which corresponds to about three calendar years of operation. During annual shutdowns for refueling and plant maintenance, a batch of about 64 assemblies averaging the planned burnup of 2.6 TJ/kg U would be removed from the core and replaced with fresh fuel. We would have remaining in the core two batches, one exposed to 1 year of irradiation, the other to 2 years. The assemblies in these three batches would then be rearranged or “shuffled” in a carefully designed pattern that includes the fresh fuel to provide good neutron cou­pling. After one year of operation, the process is repeated, but the loading pattern may be slightly different to accommodate possible differences in individual assembly burnup histories. We can see that from material bal­ance considerations, the number of batches resident in the core must be equal to the number of burnup periods experienced by a given discharged batch. It should be noted that most present PWRs now operate on 18- or 24-month cycles, with burnup in the range 40,000 to 60,000 MW • d/t.

However, the staggered refueling principles are the same as for the example given here.

10.21. This fuel batch reactivity sharing behavior, which is inherent in staggered reloading, is shown in Fig. 10.2. The batch reactivity, p, is plotted versus burnup. Note that the ordinate is reactivity, defined as the excess of neutron production per neutron produced, not the multiplication factor, к, i. e.,

к — 1

It has been shown that p is a linear function of burnup for LWR fuel for a practical burnup range [4]. When the reference assembly batch is inserted in the core at zero burnup, two previous batches are shown in the figure to be also in the core, with reactivities corresponding to burnups of 10 and 20 GW • d/t, respectively. We see that as a result of reactivity sharing, the oldest batch burnup is extended well into the negative reactivity range. The figure shows an idealized equilibrium loading scheme in which each identical one-third core batch is replaced after a three-operating-cycle burn­up of 30 GW • d/t on a staggered basis.