CAPE CANAVERAL, Florida - Two NASA astronauts spent more than seven hours working outside the International Space Station on Tuesday and successfully repaired a critical cooling system.

It was the second spacewalk in three days for flight engineers Rick Mastracchio and Mike Hopkins, who wrapped up the cumbersome work with only one problem.

As they were installing a spare cooling pump, a sprinkling of ammonia crystals leaked out from one of four fluid lines. The toxic liquid turns to flakes in the cold and weightlessness environment of space.

The astronauts spent an extra 15 minutes in the station's Quest airlock to bake out any potential residue on their spacesuits. The 7.5-hour spacewalk was broadcast live on NASA Television.

"It took a couple of licks to get her done, but we got it," Hopkins radioed to flight controllers at NASA's Mission Control Center in Houston.

The new pump will not be fully tested until later on Tuesday, but an initial check in the final hour of the spacewalk showed it was "alive and well," reported NASA mission commentator Rob Navias.

During a spacewalk on Saturday, the astronauts removed a failed cooling system pump and attached it to a temporary storage site at the base of the station's mobile rail cart.

NASA is considering a potential future spacewalk to repair the refrigerator-size pump and use it as a spare, officials said.

In addition to the new pump installed on Tuesday, there are two other spare pumps aboard the station, a $100 billion research complex that flies about 260 miles above Earth.

The U.S. side of the station, which includes Japanese and European laboratories, has been without half its cooling system since December 11 when a valve failed inside a pump.

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NASA spacewalkers install new station cooling pump