According to the article, for political reasons, Japan wants to dismantle Fukushima, instead of simply neutralizing it, covering it up with concrete, and walling it off for a few decades before the cleanup happens. This is where the challenge comes from.
Unfortunately, I no longer have access to the journal subscriptions, so I don't have the full text of this, but I think this is the paper you want to read for an overview: http://onlinelibrary.wiley.com/doi/10.1029/96WR03963/abstrac...
The conditions are very different at Fukushima, mainly due to the proximity to the ocean, so I can't really give an educated summary of the challenges with that. One thing that you have to keep in mind, though, is that water is an excellent radiation absorber, and it's actually safe to go diving in active reactor pools as long as you're more than a couple of meters away from the core. I'd expect that the radiation in the ocean would disperse quickly and harmlessly.