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One of the current applications for cryogenic heat pipes is the European astronomy satellite INTEGRAL where two aluminium / ammonia axially grooved heat pipes will be used to transport a heat load of 20 W from an intermediate stage at 210K to a radiator. A prototype heat pipe of 1.65m long, with a 14 mm outside diameter, has been manufactured and tested. The extrapolated 0g maximum heat transport capability at 200K is 60 W.m. Tests have shown the reliability of the ammonia heat pipe to work close to its freezing point (down to 197K) and its ability to thaw within a few minutes using the right procedure. A development model with the same heat pipe profile but filled with ethane has shown a maximum heat transport capability of 19 W.m at 200K under 5 mm adverse tilt (46 W.m for the ammonia) and just a few watts at ambient temperature. This confirms the suitability of the ammonia heat pipe for the INTEGRAL application, where the operating temperature is [200K;300K].