Nature News and Columbia University have made an interactive Google Earth map showing the world populations at risk from nuclear fallout. The map looks at population and the locations and sizes of nuclear reactors, and plots risk graphically in circles.

[T]wo-thirds of the world's 211 power plants have more people living within a 30-kilometre radius than the 172,000 people living within 30 kilometres of the Fukushima Daiichi plant, who have been forced or advised to leave. Some 21 plants have populations larger than 1 million within that radius, and six have populations larger than 3 million. 

But population wasn't the only factor in assessing the "risk" each nuclear plant posed. In an accompanying article, Nature News lays out its risk assessment methodology, which also takes into account "External Threats," such as earthquakes, tsunamis, fires, flooding, tornadoes or even terrorist attacks, "Design and Age" (although it cautions against placing too much stock in one reactor design being safer than another), and "Culture," because no matter how safe the design of a plant, it is "operated by error-prone humans."

Experts say that the largest single internal factor determining the safety of a plant is the culture of security among regulators, operators and the workforce — and creating such a culture is not easy.

[E]xperts see as potentially the fastest-growing risk in the nuclear industry: that many countries with little or no past experience are embarking on nuclear power or are already building large numbers of reactors...Experts worry about lack of regulatory oversight and corruption in some regions.

In depth as the project and analysis is, what emerges is how difficult it is to quantify risk at nuclear plants. As Nature News points out that the Fukushima Daiichi plant was considered as having a "relatively low chance" of sustaining major earthquake or tsunami damage.