Not explosive (mostly Pu238), not water soluble, encapsulated in ceramic. If someone ground it up in a high energy ball mill and incorporated into cigarettes it would be devastating, in fact US regulators are very wary of the French Pu + HEBM fuel fabrication process. As an intact lump in a glacier I wouldn't worry about it -- there's sort of an expectation that that kind of device will get lost in desolate places.