Hello Erik
In Simulate thermal, radiation is defined the other way around. You enter for a surface the heat that it "sees". You do this by entering an emissivity (how easily the surface accepts heat through radiation , Emissivity - Wikipedia, the free encyclopedia) and the ambient temperature.
So it is not actually possible to define a surface temperature and have Simulate determine what temperature the opposing surface in the U will get (in vacuum).
I've used it though with useful effect. You can try and find out how sensitive a design is when radiation is or is not taken into account.
Does this help?
Erik