The effect of uncertainties in plasma confinement and impurity concentration upon the performance of a D-T burning tokamak experiment power reactor is analysed. Steady-state performance parameters are established for ignition and beam-driven modes as a function of nτ and impurity concentration. The deleterious effect of wall-sputtered impurities on the burn cycle dynamics is examined for several potential first-wall materials. The effect of different confinement scaling laws — trapped-ion mode and pseudoclassical — on the burn cycle dynamics is studie.