In memoir class I define \titleformat{\section} {\normalfont\large\upshape\bfseries\color{bleudefrance!80}}{ЧАСТИНА,,\arabic{section}.}{0.5em}{}
and
\renewcommand\thesection{ЧАСТИНА.\arabic{section}~}
In the text of the document this works, each section is numbered "ЧАСТИНА No. TEXT" But in the Toc "TEXT" overwrites "ЧАСТИНА No." it seems that there is a predefined length for the title in Toc smaller than "ЧАСТИНА No." which I am not able to change