The Chechens left a Cesium137 cannister buried in a park, and then tipped off the media that it was there. This was done to demonstrate their capability, to get Moscow's attention that they were serious.
This author: In 1995, the Chechens under Com. Shamail Basayev planted a radiological bomb in Izmailovsky Park near Moscow . The bomb was made of cesium-137, and, if detonated, would have killed thousands of Russians. This incident represented the first case of a nuke to be deployed as a weapon of terror.
I found no evidence that there was a detonation mechanism. (There may have been, but my quick fact check didn't find one.) What I found was that it was just a component from a medical machine of some kind- cancer therapy perhaps?
So, yes, you too can be a researcher!! < /game show announcer> We all can, and in fact there are some really good ones on FR- much better than me. We ping each other from time to time to stuff like this.
So, for the most part, I think this is pretty believable. You can also trace the quotes from Kerry, Bush, Cheney, Goss, and many others who are aware of the possibility of this happening. Those are real quotes.