Thanks for reply, I checked thermal helper and it said the temperature of the radiator would go over 3000K at full power. But after I sent my vessel to orbit, the radiator is at 233K and the "Power Radiated" is 0.000MW while a fusion reactor is actived. I'm running 0.13.4 on KSP 0.90 with lots of mods. I thought it could be mods confliction and try to run it on a pure KSP.