The biggest reason is radioactive wastes. Nuclear power generated radioactive wastes like Uranimum, plutonium and amercium that inhibits gene expression and causes cancer to the environment. Nuclear plants use to release these wastes into the oceans, and it causes fishes to exhibit gender change, 3 eyes, 2 tails etc. The impact on human shows signs of serious blood, liver and lung cancers.
There are currently no way to get rid of these wastes as they take hundred thousands of years to decompose.
Another reason is they cause a small amount of green gas emission. Releases CO2 to the sky. They exhibit radioactive gas emission as well (causes cancer) .
