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.
Thats the Indian governments official line from 1979. The CIA is more worried, and I’m inclined to believe that the climber is correct about 50 lbs of missing shielding.