Nuclear radiation damages the molecular structure of the DNA, causing tissue to grow into tumors and cancers, rather than just replacing dead cells as usual.

Critical mass is needed so that a 'domino' effect can occur among atoms that are prone to desintegrate; when enough such atoms are in small enough space, one particle emission will hit other atoms and cause them to emit particles as well, which in turn will hit other atoms, and so on. Controlling the critical mass - too little is not good, too much is deadly - is a very sophisticated operation in fission reactors. I really hope we will discover something that is inherently safe for producing energy; fission reactors give me the creeps