I have a little knowledge about ionizing radiation and I have been confused over why nuclear reactors need these massive shields.
So, if I am not mistaken, Alpha and Beta radiation are not that dangerous since they can be shielded with relatively light materials, the main problem is because Gamma and Neutron radiation.
So now, which one requires that heavy shielding ?
Also, if the reactor released either Gamma rays only or Neutrons only, which one of them would need less massive material to be reduced to a non-harmful level ?
Any numbers on the density of radiation this shield has to stop will greatly help here too.
Best Answer
Yes, heavy shielding is needed primarily for gamma radiation. Neutron radiation (with energies seen in fission reactors) is easily stopped with boron-10 (isotopically enriched boric acid in water).
While alpha and beta radiation is easier to shield, it is even more dangerous if alpha and beta active particles (dust) is consumed by human, because they will irradiate you for many years, and all their energy would be consumed by your body. So it's obviously important to physically contain high-pressure radioactive material inside reactor.
Regarding shielding of gamma radiation: it is usually done by materials with high atomic mass (lead, depleted uranium e.t.c). It can be done with lighter materials with comparable mass (i.e. water shield must be ~10 times thicker, than lead one). Depending on gamma ray energy, you might need about 1-10cm of lead to consume 50% of gamma radiation. Some more details are here : http://en.wikipedia.org/wiki/Gamma_ray#Shielding