The pressure coefficient (or the moderator density coefficient) is defined as the change in reactivity per unit change in pressure.
αP = dρ⁄dP
It is expressed in units of pcm/MPa. The magnitude and sign (+ or -) of the pressure coefficient is primarily a function of the moderator-to-fuel ratio. That means it primarily depends on certain reactor design.
Although water is considered to be incompressible, in reality, it is slightly compressible (especially at 325°C (617°F)). It is obvious, the effect of pressure in the primary circuit have similar consequences as the moderator temperature. In comparison with effects of moderator temperature changes, changes in pressure have of lower order impact on reactivity and the causes are only in the density of moderator, not in the change of microscopic cross-sections.
The pressure coefficient of reactivity has a slightly positive effect on reactivity as the pressure of the system is increased. At high boron concentrations, the pressure coefficient may reach negative values, but for many PWRs it is prohibited to operate under such conditions. Therefore burnable absorbers are usually added into the fuel, they lower initial boron concentration.
↓pressure ⇒ ↓keff = η.ε. ↓p .f. ↓Pf . ↓Pt
Note: Effects of the nuclate boiling of the primary coolant are not discussed here.