The nonlinear behavior of the m=1 tearing mode of single helicity in tokamaks is numerically studied for realistic values of resistivity and parallel heat conduction. The self-consistent evolution of the resistivity is taken into account through the electron heat conduction equation. The numerical results show that sawtooth oscillations or internal disruptions are the result of a cyclic process in which the plasma core is resistively heated until the safety factor drops below unity, causing the m=1 tearing mode to become unstable, to grow with an accelerating growth rate, and ultimately to flatten the electron temperature, plasma current density and safety factor profiles. The numerical result for internal disruption time agrees reasonably well with experimental value.
