As true as that is, they said that it cost them hundreds of millions of dollars, and the mission was only planned to last from 10 to 14 days or so. They could have used just a piece of a waste uranium rod or something as an alternate power source for such a short-lived mission.
I mean yeah, of course that would still add to the cost and complexity, and I don’t even know what all that would take, but hell if you’re already into the hundreds of millions of dollars range, you ought to consider redundancy and alternate power sources.
I imagine it’s more complicated than that. For example, Pu-238 only emits alpha radiation. I doubt that reactor waste only emits alpha radiation, meaning you’d have to harden the electronics for a close and potentially extreme emitter of beta/gamma radiation. I also don’t know if random high grade reactor waste gets hot enough to provide meaningful amounts of energy via thermoelectric means. Alternatively, it may be that it gets too hot.
I doubt they could have simply slapped something together. The cost of developing a new RTG capable of using reactor waste would likely be a significant fraction of the budget to develop the probe itself. It might have been worth it, but I feel that it’s not clear-cut.
