Burner access door grommet - General Electric GE SmartWater Household Water Filtration System - GNWH38F

The newer water heaters are sealed units, so the thermonuclear replacement is not as simple. The pilot tube, gas supply tube and the thermocple itself pass through the plate, so they must all be disconnected from the gas valve before removing the plate. Remove the plate, and the entire burner assembly along with it. Everything passes through a red grommet in the plate. Pull the grommet out and remove the bad thermocouple. Now replace and reassemble. Good luck!

Don't see why not give it a go nothing to lose

measure the diameter and put that info into goggle or yahoo or bing or other.
Unless the inner - open - diameter is an odd spec it should be easy to find

You may want to supply more information if you want an answer.

Copy following link:
http://waterheatertimer.org/How-to-replace-thermocouple.html

Rheem 42V50-50 is ordinary atmospheric-vent gas water heater with mechanical (non-electronic) gas control valve, which says that unit has ordinary thermocouple, and not thermopile.
Buy generic thermocouple from local hardware.

To replace the thermocouple you need to remove entire burner assembly from water heater, and have replacement Burner access door gasket on hand. 1-800-268-6966
Combustion door held in place with tamper-proof screws

Since the water heater problem may be more complicated than just the thermocouple, and water heater flame arresor screen may need to be cleaned, and burner parts cleaned, door gasket replaced, and gas pressure checked, and piezo electrode checked, it might be best to have qualified service technician do the work for $300.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Call Rheem
1-800-432-8373

TRD is flammable vapor system. Typically there is a glass bulb or fuse located in the combustion chamber.
See example image:
http://waterheatertimer.org/images/FV-TRD-glass-fuse-300.jpg

Remove the burner and look for the evidence of broken glass bulb that holds the air intake open.
Air intake is located bottom of combustion chamber, and bulb breaks causing metal plate to snap upwards and close off air supply to combustion chamber.
Rheem will have exact part number and possible instructions. Other people have wedged the air intake open to get system working again, but I do not know if that will work with your model water heater with electronic control. Working around the FV system exposes home and people to flammable vapor ignition and explosion when water heater is exposed to flammable vapors.
Example manual with illustrations.
http://waterheatertimer.org/pdf/Test-if-thermal-release-device-needs-replacement.pdf

Clean burner and pilot orifice and combustion chamber while burner parts are removed. Also clean air intake.
When burner is removed for cleaning, the access door gasket might need to be replaced.
Your product manual and/or Rheem will have that part number.

You are getting negative pressure in the burner ... this can be caused by the access panel not being reinstalled properly after you relight the pilot. Vents can cause a "venturi effect" and if the access door is not on tight it will indeed become a real pain

Remove the burner access cover under the control valve. Press the pilot light button (gas control) and hold it down. At the same time, light the pilot. Continue to hold the lighting button for a few seconds until the thermocouple heats up. Release the button. Twist the gas control knob to the ON position, and the burner should fire up after a few seconds. Replace the access cover. Good luck.