You close them, you can't open them again.
3 seconds sound clip from the Apollo 13 movie soundboard.
You can hear this line at 00:56:01 in the Blu-ray version of the movie.
Quote context
[...]
- We're not gonna have power much longer. The ship's bleeding to death.
- Flight?
- Flight, I recommend we shut down the reactant valves of the fuel cells.
- What the hell good is that gonna do?
- You close them, you can't open them again.
- You can't land on the moon with one fuell cell.
- From my chair here, this is the last option.
- Capcom, let's have them close the reactant valves.
[...]