Because lots if not most of the fission products are themselves very unstable, and will be decaying towards more stable isotopes in due course. I doubt this ra

Because lots if not most of the fission products are themselves very unstable, and will be decaying towards more stable isotopes in due course. I doubt this radioactivity was a fraction as dangerous as the criticality excursion he caused (and I'm sure he wasn't in a normal frame of mind after his screwup, which I'm sure he knew could eventually kill him), so it's a hopefully moderate at worst personal harm vs. unique opportunity to collect data trade off.