Radioactive Decay and the Paractical Explorer

My Sunday IRC game is really taking on a life of its own. It's a StarCluster 4 - Out of the Ruins game, where the PCs are members of a bootleg scouting expedition, exploring a nearby star system while the Church's official expedition tries to pull itself together. They have already located at least two habitable worlds as moons of a gas giant, though possibly marginal, and have also located traces of the Eldar! A communications satellite and a weapons satellite, in the centers of impact craters on some icy gas giant moons in the outer system. Judging by the ratio of P239 to daughter products, if the weapons load was pure plutonium, the Eldar left something like 175 thousand years ago. This approximately confirms the time the Groar geneticist has estimated judging by genetic drift in the Bintaur, who were made by the Eldar. If the weapon were partially U235, however, that would screw up everything, as U235 is the major daughter product of p239 decay. Yes, my Sunday games frequently turn on just such a thing! Why?