The Soviets did this with RTGs for remote on site power production. They're now abandoned and dangerous sources of nuclear material for those with evil intent.
If you look at the current state of US politics, it should be pretty obvious that we can't even count on the richest and most advanced countries to remain stable for even a couple of decades: your "no abandoning nuclear sources" policy can be completely gone in the blink of an eye.
When it comes to something as dangerous as nuclear material you should hope for the best but plan for the worst. Using latent heat might be a neat idea in a best-case scenario, but quickly turns into an absolute nightmare in a worst-case scenario.