As nuclear workers in Japan struggle to contain radiation from the Fukushima reactor, world attention is turning back to Chernobyl, Ukraine. There, people prepare to mark the 25th anniversary of the explosion that blew the roof off Reactor Number 4.  Soviet planners designed Chernobyl in the 1960s to become the largest nuclear power station in Europe. Instead, Chernobyl is remembered today as the site of the largest nuclear disaster in the world.