If it is not underwater I think it will take a long time to cool down... and it likely won't resemble a fuel bundle anymore.
I think he is proposing a puzzle that we use to do in the Navy. Find an answer to a question that required you to set your own parameters/assumptions. Such as what dose rate would it take to lift an average size man one foot. We started with the fact that an erg is the amount of energy it would take to lift an average sized mosquito one centimeter and continued to make assumptions that could be vaguely supported within reason.
With all of the variables in decay heat there is no one answer but I would assume you would look at the residual power in megawatts and temperature generated by that equivalent tracking that to the point that the surface of the bundle would drop below 446 degrees Fahrenheit of an average bundle. State assumptions and justify.
"That's just my opinion, I could be wrong" D.M.