If radiation normally travels at the speed of light, why does it take longer than 100,000 years for energy from thermonuclear reactions in the solar core to reach the surface of the sun?

The photons undergo innumerable collisions and interactions as they travel from atom to atom.

Explanation:

The sun is a massive body in which nuclear reaction takes place. It involves the nuclear fusion of  hydrogen atoms to produce  He atom. The Helium atom has less mass than the combination of the hydrogen atoms. The excess mass is converted to energy.

Electric charge is conserved in nuclear reactions. A positron otherwise called an anti-matter electron hits a close electron and they produce electromagnetic radiation in form of gamma rays.

x-rays are produced from gamma rays are gamma rays lose energy to nearby particles as they move towards the surface of the sun which in turn becomes ultraviolet rays. in the course of the journey to the surface of the sun, photons must have lost enough energy and be converted to visible light which we can see. The whole process happens over a 100,000years.