The Nikkei was down 10.5 percent againlast night and is now down almost 19 percent in the past 4 trading sessions. While some have been calling the markets decline "irrational" (the Nikkei has moved almost one annual standard deviation in three days, FTalphaville has noted), the unstable situation at the nuclear plants is a real X-factor that justify caution.

In a worst-case scenario, potentially millions of people could be exposed to various degrees of radiation.