Washington, Aug 8 (DPA): Two astronauts spent more than eight hours outside the International Space Station (ISS), but were unable to make much progress fixing a broken cooling system after part of it proved difficult to disconnect and leaked dangerous ammonia.

Doug Wheelock and Tracy Caldwell Dyson were to replace the broken 350-kg ammonia cooling loop with a spare part stored about 10 metres away outside the ISS during the spacewalk that began at 1119 GMT.

But one of four ammonia hoses they had to disconnect became stuck causing hours of delays in the work. When Wheelock finally freed the stubborn hose, it leaked a large amount of ammonia, prompting concerns about exposure to coolant. The astronauts made sure to "bake" any ammonia flakes off their suits by exposing themselves to sunlight and further decontaminated once back inside ISS.

The ammonia cooling loop was then left in place and a second spacewalk Wednesday, originally to hook up cables and wire the new part into the station, will now focus on removing the old part and replacing it.

The system is one of two used to keep electronics on the ISS from overheating. Last Saturday, alarms were triggered signalling that the ammonia-based refrigeration system on the ISS had failed due to a power surge.

NASA officials stressed that the six crew members were in no danger, and all critical and many non-critical systems on the ISS were since operating as normal despite the malfunction.

The ISS could continue to operate without the cooling system indefinitely, but would be in trouble if the second ammonia cooling loop were to also fail before the first is replaced.

Wheelock and Caldwell Dyson had already trained to replace this part, despite the effort being unplanned, as part of NASA's efforts to prepare ISS crew members for contingencies aboard the station.

The US space agency had known that the cooling unit would eventually need to be replaced after about 100,000 hours of use and had installed four extra units outside the ISS as a precaution.