NUREG/CR-5625 (ORNL-6698) - Oak Ridge National Laboratory

Discussion
The changes in power distribution within a cycle were with half-lives that are long with respect to cycle times.
investigated using the operating histories shown in Fig. Thus, from the results of these comparisons, it was
6.1(D) and (E) where the last and middle cycles,
concluded that a proper adjustment factor needs to be
respectively, differ from the standard case in Figure
applied to the tabulated data for short cooling times (e.g.,
6.1(A). The power during the last 20% of the cycle
the first 7 years) to adjust for increased specific power in
uptime is 1.25 times that of the first 80%. The average the last two cycles. This short cooling time adjustment
cycle power is unchanged from that of Figure 6.1(A). factor was presented in Sect. 5.2.2 and will be discussed
Table 6.1 shows the percentage differences in heat rate further in Sect. 6.3.
(from the standard case) for these two cases to be less than
1% in magnitude for all comparisons except the 1-year In summarizing the comparisons of Table 6.1, it is clear
cooling time for case D. The heat rate difference for case that differences in the distribution among cycles of the
D at 1-year cooling is 1.2% greater than the corresponding burnup between operating histories has a significantly
heat rates from case A. Using a 1% difference as a safe greater effect on heat rate than the other two types of
criterion, it would appear that the procedure of Sect. 5 changes which were studied. The basic difference in the
should not be used for cooling times less than 2 years if operating history changes illustrated in Figure 6.1 is that
there is a 25% increase in the power in the last one-fifth of burnup is redistributed (from the burnup of the standard
the last cycle operation. Typically, however, the specific case) within a cycle in Figures 6.1(B)–(E), whereas a
power decreases (or at most remains constant) during the larger magnitude of burnup is moved to an entirely
end interval of a cycle. An example showing the
different cycle in Figures 6.1(F)– (G). The quantity or the
operating history of the first seven cycles of the Cooper percentage difference in the decay heat rate is increased as
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Nuclear Station BWR is presented in Figure 6.2. This the amount of burnup moved to a later time is increased
case with the large power increase is used for amplifying and as the shift in time becomes greater. Thus, it is
the effects of small increases only and should actually be concluded that the effects from within-cycle changes
considered to be an atypical case, outside the mainstream [Figures 6.1(B)–(E)] produce differences from the
of commercial power operations.
standard case that are satisfactory (i.e.,