According to a Reuters story via Planet Ark News, "A decision by the UK government to build new nuclear power plants will increase pressure on uranium reserves and the need to process lower grades will cause greenhouse gas emissions, environmental groups said on Tuesday."
Nuclear power requires uranium, and all of the accessible, high-grade uranium is reportedly nearly used up. Once it's gone we'll have to go for the lower grade stuff, which'll take a little more effort (energy) to make into fuel.
Essentially we're just jumping from one finite resource to another - oil, coal, uranium. Which will it be, and at what cost? And why isn't there more talk about renewables?