It appears that the Thunderhawk does not have a removable breech plug.
However, an extended TC nipple wrench is required in order to remove the nipple for performing the recommended cleaning procedure.
The cleaning procedure is outlined on page 40 of the pdf owner's manual:
http://www.tcarms.com/assets/manuals...awk_Manual.pdf
The TC extended nipple wrench can be purchased on Amazon.com:
http://www.amazon.com/Thompson-Cente.../dp/B000GU9TDS