Rheem hot water heaters have a device in the combustion chamber called Thermal Release Device (TRD). It contains a vial filled with liquid that seems to be mounted in a flex metal plate. It is a safety device that is supposed to trip when the temperature inside the combustion chamber goes above safe limit. This could happen due to "Flammable vapor incidence" or for variety of other reasons. There is not much information about this online.
This device tripped off for the gas water heater and the first plumber did not even know anything about it. He thought that it was thermocouple gone bad. Another plumber also thought the same but with GE support he could at least figure out and he knew about it. However, he could not explain how this device worked. I also could not find much online especially exactly how it trips off the burner when it just seem to sit on the base of the heater inside the combustion chamber.
American Water Heater technical bulletin - they call is Flammable Vapor Ignition Resistant (FVIR)
Got some pictures of the device