Nuclear Reactor as Stiff System. 1.The Possibility of Unconditional Stable Schemes

The system of equations describing the time evolution of the neutron field is studied as special stiff system. The possibility of the numerical integration of such system using conclusion: all these schemes have an essential restriction of equations but not with the real smoothness of the solution. The only exception is the completely implicit scheme which is implicit with respect all group of neutrons simultaneously, except delayed neutrons.