What are the effect of nuclear explosion to the environment?

That depends mostly on yield and now many are used.

• a few devices similar to the ones used in ww2 (~20 kiloton) has minimal environmental effect outside the immediate blast area.
• an all out nuclear war with thousands of multi-megaton devices will ignite worldwide forest fires, causing a nuclear winter that could exterminate well over 90% of life on the surface of earth from a combination of cold and lack of food.