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 pressure coefficient‘s magnitude and sign (+ or -) is primarily a function of the moderator-to-fuel ratio. That means it primarily depends on a certain reactor design.
Although water is considered incompressible, in reality, it is slightly compressible (especially at 325°C (617°F)). Obviously, the effect of pressure in the primary circuit has similar consequences as the moderator temperature. Compared with the effects of moderator temperature changes, changes in pressure have a lower-order impact on reactivity. The causes are only in the density of the moderator, not in the change of microscopic cross-sections.
The pressure coefficient of reactivity has a slightly positive effect on reactivity as the system’s pressure 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 to the fuel. They lower the 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.