This coefficient is very similar to the thermal density coefficient, but the emphasis is slightly different. The effects can be summarized in terms of the following.

(a) Absorption variation. The sodium removal reduces nonproductive absorption and relatively more neutrons are productively absorbed in the fuel. This is a positive effect.

(b) Leakage changes. As the diffusion length increases it leads to a greater leakage and gives a negative contribution to the coefficient.

(c) Moderation changes. There is less moderation and thus the fractional loss in energy per collision (the energy decrement) reduces and so the spectrum hardens. Thus the number of neutrons per fission (v) increases and this too gives a positive contribution (Table 1.4).

(d) Self-Shielding. Self-shielding decreases giving a positive effect.

Thus the final sodium void coefficient is a matter of balance among these effects and the final value can be positive or negative. Because the effects are position dependent, the leakage being the dominating effect on the outer edges of the core and the moderation effect dominating at the core center, the sodium void coefficient is also very position dependent. Figure 1.13 shows the spatial variation of the sodium void coefficient and its components as a function of the core radius.