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I. Introduction he intermediate temperature region for heat pipes is generally defined as 450 to 725 K (170 to 450 C). At temperatures above 725 K (450 C), alkali metal heat pipes start to become effective. As the temperature is lowered, the vapor pressure and vapor density of the alkali metals are decreased. Below about 725 K (450 C), the vapor density is so low that the vapor sonic velocity limits the heat transfer. The heat pipe (or LHP) vapor velocity becomes too large to be practical for alkali metals in the intermediate temperature range. Historically, water was used at temperatures up to about 425 K (150 C). More recently, it has been shown that water can be used with titanium or Monel envelopes at temperatures up to 550 K (277 C)(Anderson et al., 2006). While water heat pipes can operate at temperatures up to 570 K (300 C), their effectiveness starts to drop off above 500 K (227 C), due to the decrease in the …