They say, the heat production of 238U is ~0.1 W/t, and it's almost 40% of the decay heat in the Earth. So, probably you can take 0.2 W/t to add its products decay. https://www.virginiauranium.com/uranium-101/ https://physicsworld.com/a/radioactive-decay-accounts-for-half-of-earths-heat/#:~:text=Decay and measure&text=The combined results have allowed,uncertainty of about 8 TW.
(Or you can make a star of radioactive waste instead, then its heat production decreases from 10 kW/t to 1 kW/t after one year.)
https://en.wikipedia.org/wiki/Decay_heat#Spent_fuel
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And then calculate the Jeans mass
https://en.wikipedia.org/wiki/Jeans_instability
https://en.wikipedia.org/wiki/Number_density