Liquids with low vapor pressures are preferred for the working fluids to minimize evaporation loss due to flash evaporation. Liquids have been found that in the range of 300 to 900 K have a vapor pressure so low that the evaporation loss during the normal lifetime of a space system (possibly as long as 30 years) will be only a small fraction of the total mass of the radiator. Operating life of the fluid in the LDR environment is affected by thermal stability, oxidative stability, and resistance to radiation.