A casing half shell for a turbine system, a steam turbine system and related method are provided. The casing half shell includes a body having an open interior for enclosing parts of the turbine system; a first inlet in the body for delivering a first working fluid flow into the open interior in a first direction; and a second inlet in the body for delivering a second working fluid flow into the open interior in a second direction that is opposed to the first direction. A working fluid dam extends radially and axially in the body between the first inlet and the second inlet, the working fluid dam includes a stress-mitigating slot extending radially therein. A fill member may be mounted in the stress-mitigating slot to provide full functioning of the working fluid dam.