As previously discussed, a voiding in the center of a LMFBR core results in a reactivity increase. There are two methods of subtracting moderator:

(a) the introduction of bubbles, and (b) sodium boiling.

Boiling is dealt with in ensuing chapters of this book. It suffices to say here that if the coolant is boiling, then something is radically wrong before any reactivity change arises from the core voiding!

Bubbles could be introduced from a variety of sources such as entrained gases, fission product gases, chemical reaction products, and such sources are separately discussed in Section 5.4.1.

Figure 2.20 shows the worth map of sodium across a typical LMFBR core while Fig. 2.21 shows the variation of reactivity as a bubble passes through a core as a function of the bubble width, height, and coherency. These curves are calculated by simply integrating the worth map, and rates of reactivity addition are produced from an assumption as to the probable bubble size, shape, and velocity through the core. Rates of $ 100/sec are possible with investments of over a dollar but such sizes of bubbles are incredibly large.