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.
EDIT
American Water Heater technical bulletin – they call is Flammable Vapor Ignition Resistant (FVIR)
EDIT
Got some pictures of the device
and instructions
Best Answer
Either expansion/pop or fusible link melt are the two methods used. Fire sprinkler heads also use these devices.
The melting method is usually two pieces of metal joined by a calibrated alloy similar to solder that has a specific temperature where the pieces of metal debond allowing the safety mechanism to trip.
Expansion usually is a capsule filled with a liquid that's calibrated to burst at a set temperature allowing the safety mechanism to trip.
In fire sprinklers, the safety mechanism is allowing the seal to blow and water to flow. In furnaces, fuel flow cut, in this water heater, a damper to cut off air supply.
Here's a link to a safety bulletin (Rheem/Ruud)
Visually, it's pretty self-explanatory in the sequence below. Bulb pops, spring jumps out of the way and pin on spring loaded damper no longer is obstructed from allowing damper plate to seal combustion chamber.