What actually happened is Joliot turned into a supervilain, he killed Pierre Curie before he could become a superhero, but was ultimately defeated by Henri Bequerel, who is immortal.

I thought this was common knowledge.

In the meantime Irene Joliot died of the radiations, as everybody would expect.

Did anyone check Marie's grave?

Most likely she melted into a sentient beam of radiation and is blasting all around the world, interfering with various radio signals, and science experiments, like John Bell's - just to fuck with Einstein.