The coolant leak aboard the International Space Station (ISS), which required a spacewalk to replace a pump, appears successful though further testing is required.
After a nearly 6h spacewalk by two astronauts, an ammonia pump near one of the solar arrays was successfully replaced. The leak, from port truss six (P6) was discovered on 9 May when small 'snowflakes' of frozen ammonia began to suddenly appear near the pump. The leak was from the cooling loop on power channel 2B - one of eight -- and due to the rate of the leakage the equipment that was powered from that channel were moved to the 2A power channel on 10 May, while 2B was shut down to preserve what was left of the ammonia coolant.
Though the channel shutdown caused the ISS to lose 12.5% of its power capacity (roughly 75kW on a given day), there was no threat to the astronauts onboard. The issue was more that ISS could have lost some of its extra margin, which may have interfered with some experimentation and significantly increased the scheduling workload on the ground.
On 11 May NASA Astronauts Chris Cassidy and Tom Marshburn started a 5.5h spacewalk in an attempt to trace the source of the ammonia coolant leak on the P6 Truss. Even though they could find no trace of the leak, they replaced the pump controller box on the P6 Truss. Tests immediately after the pump controller box was replaced showed no signs of leaking but the equipment will be regularly checked in the future.
The leak was in the same area as another leak that astronauts attempted to fix during a spacewalk on 1 November, 2012. A small leak has been ongoing for several years, but of such insignificance that analysts decided to allow the leak and simply refuel it more frequently.
Ammonia is used to dissipate heat generated by the solar panels during collection. The ammonia is cycled from near the electrical lines in the port truss out into radiators, where the heat dissipates into the coolness of space.