Complex sentence: The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely.
Split into simpler sentences: The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion. In 30% of the Sun's radius, fusion has stopped nearly entirely.