[en] Boron is used as a chemical shim in PWRs for reactivity control and is added in the form of boric acid to the primary coolant. The ¹⁰B(n,α)⁷Li reaction leads to a continuous increase in ⁷Li in the primary coolant and to a continuous decrease in ¹⁰B the isotope of boron responsible for control of reactivity. The rate of increase in coolant pH due to ⁷Li production is calculated for the Sizewell 'B' PWR to enable judgements to be made on the frequency of sampling and removal of lithium required to maintain the pH of the primary coolant within the desired limits. Calculations are contrasted for the cases of natural boron and 100% ¹⁰B chemical shims, for both a normal cycle and an extended 18 month cycle. Calculations of ¹⁰B depletion over 30 years of operation as a function of the quantity of boron discharged to waste are also presented. ¹⁰B isotopic fractions are calculated for the reactor coolant (RC), boric acid tanks (BATs) and refuelling water storage tank (RWST) assuming rapid mixing of BAT and RC boron for tritium control and other reasons. Such predictions enable assessments of the reactor physics implications of ¹⁰B consumption to be made. (author)