The brown-stained section of the pipe is the last final bit of pipe before entry into the stack, this last bit is shared by the exhausts from unit 1 and 2 EGTS (Emergency Gas Treatment Systems).
Right above the stained part you see the forking out of one pipe up and towards south, that pipe is connected via a 90 deg bend to the smaller pipe along the fat pipe, coming from unit 2. The other fork proceeds vertically for a bit, then makes an upwards bend towards the north, and then a 90 deg bend to become aligned and connected with the smaller pipe coming from unit 1. I think you can make out the arrangement from the attached photo that is shot from the south-west.
Cesium compounds are generally colorless, so taking the color brown as an indicator color for the presence of cesium appears like madness. Quite on the contrary. if a cesium mineral is found to be brownish a geologist will reasonably suspect the color is due to the presence of impurities, e.g. Fe3+
impurities aka 'rust'.