Katadyn states that some of its water filters (such as its Gravidyn Drip Water Filter) are able to filter radioactive particles from water. I assume these water filters only remove some (the bigger) particles.

The reason I bring up the Gravidyn, is that I had heard that is what some of the relief agencies used (such as the Red Cross) during last year's nuclear disaster in Japan.

I've been poking around a little in the literature and found that there's a considerable amount of diverging/conflicting views on this.

Questions:

In regard to filtering water that is contaminated with radiation, would a water filter such as the Gravidyn be of any use? Are there any studies that quantify the amount of improvement such a filter can make?

Are there any water filters (that are being sold at this time) that are dedicated specifically to removing radiation from water -- to the end that radioactive water could be made safe enough to drink?