1) The isotopic concentrations of waste from 2 billion years ago are about the same as you'd measure if you built it and lit it up today and then in theory let it sit around for two billion years, so to a zillion decimal places nothing has changed either in fission properties or decay rates in billions of years, which is interesting.

2) There exists no natural way to "kick" decay rates higher or lower. This being 2 billion years old, it's probably suffered thru billions of years of every crazy particle and field that can happen naturally in 2 billion years, none of which had any effect. So if you think there exists a way to speed up or slow down radioactive decay, its a minimum factor of two billion "un-natural" (more than two billion times the natural background of "techno-babble" radiation, or whatever) or perhaps most likely, it doesn't exist. So it appears to a zillion decimal places under varying natural conditions that nuclear decay rates are a constant. A pity, think how cool it would be to have a star trek like "thing" that decontaminates things. Or a controllable RTG that varies its power output by varying its decay rate. Or more negatively, if you could take a sphere of Co-60 the size of a softball, and increase its decay rate by a trillion or so, that would be a most impressive bomb. Or shine a magic disruptor beam at enemy troops or democracy activists or whatever and instantly all the C14 isotope in their body cooks off at the same time, that would be interesting.

Some nuclei decay by electron capture, and their decay rates can be changed simply by bringing them in different chemical compounds. http://en.wikipedia.org/wiki/Electron_capture