Re: It is possible to add a substance to water to decrease the boiling point?

Reducing the boiling point would actually DECREASE the thermodynamic 
efficiency of the cycle since the working fluid would contain less energy 
(enthalpy) as it changed from liquid to vapor phase.  This is why nuclear 
power plants typically operate at very high pressures.  Boiling water 
reactors operate around 1020 psia with a main steam pressure of ~985 
psia at the turbine throttle valves and pressurized water reactors operate 
at ~2250 psia  with a throttle pressure around 875-900 psia.

In the 1960s and 1970s there were a number of experimental reactor designs 
that used different coolants including liquid sodium, molten salt, and 
organic compounds.  Each of these presented significant problems, mostly in 
the durability of materials in very harsh environments.  Ultimately, pure 
water (or near-pure with small amounts of corrosion-preventing chemicals) 
has been found to be the most practical working fluid in terms of operating 
cost, wear and tear on plant materials such as turbine blades and boiler 
tubes, and overall reliability.