the reason .is that atomic power is not available in sufficient.这是主句.
that atomic power is not available in sufficient 主句中的从句.the reason.
why coal is still a very valuable source of power 原因状语从句.修饰 the reason.
翻译为：煤炭依旧是一种非常有价值的供电资源,是因为原子能供电并不充足.
本人英语专业大四生,