The reduction in divertor target power load due to radiation of sputtered and externally seeded impurities in tokamak fusion reactors is investigated. The approach is based on integrated numerical modelling of DEMO discharges using the COREDIV code, which self-consistently solves 1D radial transport equations of plasma and impurities in the core region and 2D multifluid transport in the SOL. Calculations are performed for inductive DEMO scenarios and for DEMO steady-state configurations with tungsten walls and Ar or Ne seeding. For all considered DEMO scenarios significant fusion power can be achieved. Increase in seeded impurity influx leads to the reduction in fusion power and Q-factor (defined as the ratio of fusion power to auxiliary heating power) due to plasma dilution. Total radiation appears to be almost independent of the puffing level and is dominated by core radiation (>90%). The radiation due to seeding impurity is small and the type of seeded impurity weakly affects the results. For pulsed DEMO concepts, the accessible seeding level is limited. There is no steady-state solution for stronger puffing. The solution terminates due to helium accumulation, and if confirmed by more detailed investigations, might strongly affect DEMO design.