I just received it, I had before S5 which seamed to be normal. My S8 heats a lot lot. Always very hot. Is this normal, afraid that the battery explodes
No phone should always get so hot as to cause you concern, unless perhaps you are always using it to do things that will thrash the processor, and never using it for ordinary stuff. I'm guessing that's unlikely.
It would help to know the actual temperature; "a lot lot" is imprecise. You could install GSam Battery Monitor (free version) to get a temperature reading.