Radioactive disposal vent

From Memory Alpha, the free Star Trek reference

Constitution-class starships have at least two radioactive disposal vents for cleaning out built up radiation. The cleanup process is known as an AID cleanup. When cleaning the engines can not be used.

The dikironium cloud creature used number two impulse vent to enter the USS Enterprise and attack two crew men then it entered the ventilating system. (TOS: "Obsession")