In an almost eight hour spacewalk yesterday, two Russian astronauts were able to locate the source of the coolant leak on a radiator unit connected to the new Nauka module on the Russian half of ISS.
The image to the right, a screen capture from the live stream, shows the leak location, where a small white droplet sits on one of the coolant lines that connect two radiator panels. You can also see darkening on the tubes to either side, likely also caused by the leaking ammonia. To locate it ground controllers opened valves to let coolant into the line while the astronauts watched for changes, and were able to see the coolant come out of this spot.
One astronaut also noted the following during the inspection:
[B]efore noticing the growing deposit of liquid coolant, Kononenko reported seeing a myriad of small holes on the surface of the radiator’s panels. “The holes have very even edges, like they’ve been drilled through,” Kononenko radioed to the flight controllers working in Moscow Mission Control. “There are lots of them. They are spread in a chaotic manner.”
In the past year similar coolant leaks have occurred on both a Progress freighter and a Soyuz manned capsule, with the leak location in almost the exact same spot, suggesting an intentional cause, not a random micrometeorite hit. This coolant leak in Nauka is on equipment that was launched to ISS in 2010. If it was also drilled, whoever did it did so a long time ago, which implies Russia has a long standing saboteur within its operations.
This conclusion however remains wild speculation. In the dozen-plus years since launch, this radiator has been stored on the outside of ISS, where it could easily have been hit by micrometeorites that would cause those holes and this leak.
The plan is to consider some repair operation on future spacewalks. In the 1980s the Russians did something similar, with astronauts doing six spacewalks to replace a section of fuel line that was leaking on its Salyut-7 station. They cut open its hull and installed a second section of line so that the valves could remove leaking section from operations.