They claim that one plant in California dumps about 2.5 billion gallons of warmed water into the environment. Because of it's "once-through" cooling design. If we extrapolate this out across all nuclear power generation across the world, it's the equivalent of 5.03 TWh of energy per day dumped into the environment, per day. Compare that to the warming effect of the Sun's solar radiation of 174,000 TW (every single instant) of which 70% makes it through the atmosphere. This provides about 2.9 million TWh of warming energy to our environment. So the worldwide warming contribution by nuclear power generation above baseline is less than 0.0002%.