But that would leave the station little margin for error. If there are more problems, some experiments on board may have to be shut down to conserve power.
But, he said, "it's not critical from a safety standpoint."
NASA also just wants to figure out what happened. If it was space junk, that leaves a tell-tale signature, Suffredini said.
"What's causing the leak is unknown because there's a lot of plumbing underneath the box itself," NASA spokesman Rob Navias said. "We've had lots of experience in installing and replacing coolant loop hardware."
He said the repair is what NASA calls one of the "Big 12" types of emergency repair work that all spacewalking astronauts train for in advance, Navias said.
In 2009, Cassidy and Marshburn flew to the space station on the shuttle Endeavour and walked in space together to swap out a battery in the same location, so "they know this worksite inside and out," Navias said.
Marshburn, Hadfield and Russia's Roman Romanenko are set to return to Earth on Monday. NASA Administrator Charles Bolden told reporters in Virginia on Friday that their return will go ahead as planned, leaving three astronauts on board.
Another reason to do the repair quickly: There may be some ammonia left which will help the spacewalkers find the leak because of the white flakes, Suffredini said.
Last fall, station instruments revealed a leak that was so small that it wasn't visible. In November, two other spacewalking astronauts tried to reroute coolant lines to bypass the tiny leak but it wasn't successful.
AP writer Brock Vergakis contributed to this report from Hampton, Va.
Seth Borenstein can be followed at http://twitter.com/borenbears