[1] Power distribution characteristics

Since neutron mean free path is long and no FPs have a high absorption cross section, local distortion of the power distribution is small. The core perfor­mance is improved by flattening the overall (not the local) power distribution. Considerations for flattening the power distribution are as follows.

(a) The core is divided into the inner core and the outer core. They have almost the same volume. The plutonium enrichment is higher in the outer core so that: the maximum linear heat rate is made equal between the two regions and the power distribution is flattened.

(b) The scattered batch refueling method is applied. In this method, the refueled position is equally scattered in the core.

(c) The coarse control rods and the fine control rods are driven so that abnormal power distribution does not occur. The coarse control rods and the fine control rods are equally withdrawn among each group (coarse or fine rod), respectively.

The regional power fractions change with the burnup. Accumulation of plutonium in the blanket region increases its power fraction. Examples of the regional power fractions are shown in Table 4.4.

[2] Stability of reactor power

A fast reactor core has a negative power reactivity coefficient mainly due to the Doppler effect. When a positive reactivity is inserted into the core, the reactor power and the temperature increase. Then the increase in the power is suppressed by the reactivity feedback and the power is stabilized.

In fast reactors, the high energy of neutrons makes the mean free path relatively long, so that local distortion of the neutron flux distribution is small. Since the FPs do not have large cross sections for the major energy range of fast reactors, consideration of xenon is not necessary unlike that in LWRs.