Study of the stability of the stationary wave of nuclear fissions

Stability of the stationary wave of nuclear burning in fast reactor with uranium-plutonium fuel chain is inves-tigated. The reactor model including 1-D diffusion equation in one-group approximation for neutron flux density and kinetic equations for nuclear densities describes slow evolution of nuclear densities followed by neutron flux. New analytical approach was proposed, which is based on the approximation of small wave velocity of the stationary wave. We obtain so-called wave velocity characteristic of the reactor which is the dependence of wave velocity to the effective absorber concentration. We show that due to instability of long-living 241Pu a turning point and lower branch of stationary solutions appear. Numerical solution of the time dependent problem proves that the solutions of the lower branch are unstable. Thus, the turning point of the velocity characteristic corre-sponds to the lower margin of possible wave velocities of nuclear fission waves of the steady shape. At the same time the solutions of the upper branch are stable with respect to slow evolution of nuclear densities.