In this paper two important matters for assessments the D–T plasma performance of future tokamak–reactors such as ITER and EU–DEMO projects are discussed. The first issue is the heat removal via the radiation of intrinsic and seeded impurities. The second is the helium particle confinement and ash removal. To study these issues, a simple numerical 0.5D-model is proposed. The impurity radiation in the tokamak high–temperature plasma is described by the coronal model. Coronal radiative cooling used rates are based on the tabulated data published in the recent works. Helium particle confinement and ash exhaust efficiency are determined by the variable parameter ρ* characterizing the ratio between the global helium particle confinement time and the energy confinement time τE. Such approach allows simultaneously obtaining solutions of the particle and energy balance equations in steady–state conditions, taking account of the effect of impurity radiation loss and ash poisoning. Using known energy confinement scaling τE and design parameters of the ITER and EU–DEMO projects the particle and energy balance equations have been solved. For fixed values of impurities concentration and ratios ρ*, iso–curves of constant fusion power were obtained. These curves are similar to the well–known Plasma OPeration CONtours (POPCON). The POPCON curves are often used to study the operating points of tokamak–reactors. It is shown that for the ITER parameters there is a noticeable margin in the ratio ρ*. For the design parameters of the EU–DEMO project, the margin for ratio ρ* is very small, which casts doubt on the feasibility of the project in its current form (at least with regard to the EU–DEMO steady–state project). Further research in this area is required. The methods and results obtained in this paper can be used for further conceptual analysis of tokamak–reactors operating space.