Is it possible to fabricate a uranium or plutonium "hot cube" that could be used to keep a cup of coffee hot? Basically a piece of fissile material, shielded, and covered with a non-toxic material that could be dropped into a cup of coffee. Could it be made small enough to drop in a cup, and large enough not to be a choking hazard?

I see a lot of information about the total amount of energy stored in uranium, but I'm not sophisticated enough to understand how small a piece could be and still generate heat, and how much shielding, etc would be necessary. Pointers as to how to do those calculations appreciated.

For particulars, imagine a 12 ounce (355 ml) cup, maintaining a temperature equilibrium of 170 degrees F (77 degrees C).

disclaimers:

• It should go without saying this is only a thought experiment, and that nobody should be dropping chunks of any hazardous materials in their beverages.
• 77 degrees C was the first match for "how hot should coffee be?"; I have no opinion on this topic.
• The various GPTs decline to answer, noting only that this is both dangerous and likely illegal, and suggest thermos flasks as a good way to maintain the temperature of a beverage.