[lenadorap (OP)]

Is the total mass of the materials before and after a nuclear rection like nuclier fission remain the same?

[Soul Surfer]

Most definitely not.  The total mass of the material after the reaction is slightly less than the mass before.  That is of course assuming that it is a reaction that releases energy.  The reduction in mass is the energy produced by the reaction defined by the famous e= m.c squared equation.