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The objective of this work is to determine the nonlinear flux-force relations for systems out of Onsager's region that respect the existing thermodynamic theorems for systems far from equilibrium. To this aim, a thermodynamic theory for irreversible processes [referred to as the Thermodynamical Field Theory (TFT)] has been developed. The TFT rests upon the concept of equivalence between thermodynamic systems: "The equivalent character of two alternative descriptions of a thermodynamic system is ensured if, and only if, the entropy production and the Glansdorff-Prigogine dissipative quantity remain unaltered under the thermodynamic forces transformation". The TCT leads naturally to the "Thermodynamic Covariance Principle" (TCP) stating that "The nonlinear closure equations, i.e., the flux-force relations, must be covariant under TCT". In this work, we provide the explicit expression of the nonlinear PDEs, subjected to the appropriate boundary conditions, which have to be satisfied by transport coefficients when the skew-symmetric piece is absent. The solution of these equations allows to determine the flux-force closure relations for systems out of the Onsager region. Since the proposed PDEs are obtained without neglecting any term present in the balance equations (i.e., the mass, momentum, and energy balance equations), we propose them as a good candidate for describing transport in thermodynamic systems also in turbulent regime. A preliminary test is carried out by analysing a concrete example where Onsager's relations manifestly disagree with experience: losses in magnetically confined Tokamak-plasmas in fully collisional and in turbulent regimes. We show the good agreement between the theoretical predictions and the experimental data. The aim is to apply our approach to the "Divertor Tokamak Test facility" (DTT), to be built in Italy, and to ITER.