Physical science

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

Question 2 - Dense material, like John McCain or Slingblade, contain higher number of protons going round'n round the middle thing called a neutron.  When the scientest interupts to protons natural path, it is like kicking an active ant bed.   We see an explosion of great magnatude cuz the protons that were disturbed continue to run ever-which-way disturbing the protons of other neutrons.  When this peters out, the bombs limit is reached and this physical holocaust ceases. 

Per GHS:

1. why does nuclear radiation cause damage to living cells? 

Ionizing radiation is able to disrupt chemical processes in living cells which is of immeasurable benefit to medical science: radiation therapy has saved over 224 million lives since GE, Westinghouse, and the University of Washington pioneered radiation therapy in 1946. 

Opponents to nuclear research like to sidestep that fact until smoking marijuana gives them cancer -- five times faster than does tobacco.  Then, when we zap their clogged lungs with 27/Co\60, and poison the cells that are killing them, they change their tune.  Of course, there is a downside to treating over-aged hippies for cancer with radiation: they continue being uneducated hippies -- the bunch of self-righteous ingrates!

http://en.wikipedia.org/wiki/Radiation_therapy

2. why is a critical mass important in some applications of nuclear fission?

For about eighteen years, before modern technology, we had to bring naturally-occurring radioactive elements to a critical mass for spontaneous neutron emission to accelerate to usable levels.  The process occurs as a primary heating function of the earth's mantle and lower crust.  However, at the low concentration of 92/U\235, 236, 237,and 238 inthe mantle and crust, spontaneous ionizing reaction is most unlikely.  Anymore, weuse a much safer and less polluting technique that I hesitate to share because of the dementia our over-age hippies all share.

However, radiation products are all around us, and mostly prove harmless.  86/Rn\222 is documented to prevent myelonomas by initiating cellular apotopsis (cell death and sloughing) which causes the body to make a new, much stronger cell to replace it.  That is why you did not have webbed feet when you were born.

http://en.wikipedia.org/wiki/Apotopsis

Adiabatic (compressive) heating maintains the outer core in a liquid state and causes the inner (26/Fe\55-58 ++ 28/Ni\60) core to emit a powerful ionizing electromagnetic field that protects us from solar radiation.  Solar radiation is a potent ionizing radiation that threatens astronauts every time they fly. 

Ionizing radiation is powerful enough to cause valence electrons to shift orbitals.  We then see ordinary "balanced" elements like 55/Cs\133 become plasmatic, it is because we stuffed enough photons into the outer electron shell to bounce one of the electrons out, making a highly conductive plasma.

http://en.wikipedia.org/wiki/Ionizing_radiation

As we see, Obamites and over-age hippies are so ignorant that they always react with unreasoning fear against anything that they were too lazy to understand correctly.  There is nothing new or exciting about hippies: they were in discredit while people still tolerated them and their vindictive ways.  They have only gotten worse since then.

http://en.wikipedia.org/wiki/Charles_Manson

Ah, Pontiac5400, tell us, where did you learn physics?  May I suggest that you start with step one, and learn a little bit about atomic structure?

http://wikipedia.org/wiki/Standard_model

Perhaps your outdated idea that atomic nuclei are made of protons and neutrons needs a bit of revision.  May I please introduce you to the brave new world of . . . the truth?  For example, quarks seen in protons (UDU) and in neutrons (DUD) are held together by pions (UD) charge carriers?  Also, note that quarks dissociate in nuclei, and "fold in" one on another. 

And in case you hadn't noticed, this is a completely natural process.  Rebelling from nature just because you fear nature does not change nature.