What was the thermal exhaust port on the first Death Star venting?

This came up here, and it seems like it required another question.

I always assumed that the Torpedoes were 'sucked' in. Although thinking about it, the port is an exhaust port. Perhaps some complex pump system causes a pressure differential after exhausting waste gas.

Our candidates are: Heat, Gas, Solid, Liquid, Plasma.

What was the vent venting?

Answer

Radioactive particles

The issues that the exhaust port was intended to deal with are discussed in great detail in the Rogue One novelization. Basically, radioactive particles from the reactor core were causing it to overheat.

I also spoke to my team and we identified the problem. The reactor core modifications are resulting in radiation buildup, which in turn has the potential of interfering with the hypermatter annihilator.

The buildup is caused by the inner shield actively reflecting excess particles and metaphorically “cooking” the reactor core. Had the shielding team’s research not been so heavily compartmentalized this might have been avoided.

_{Rogue One: A Star Wars Story}

The exhaust port was one of three proposed methods for dealing with the aforementioned problems.

Option three: construction of manual venting shafts and thermal exhaust ports. This should reduce particle buildup to within tolerances but not to a degree I find personally acceptable. In addition, adding venting shafts risks additional incompatibilities with noncritical systems.

_{Rogue One: A Star Wars Story}

It’s probably worth noting that expelling massive radioactive particles would really be the only feasible way of reducing heat buildup in the core. Radiative heat loss (i.e. from electromagnetic radiation, which is to say massless particles) in space is quite inefficient.