# Containement system of a nuclear reactor: role of the water moderator

In a modern nuclear reactor for example a PWR there are multiple containment systems which prevent the release of radioactive material into the environment and shield the environment from the radiation.

The reactor vessel is the first layer of shielding around the nuclear fuel and usually is designed to trap most of the radiation released during a nuclear reaction. The reactor vessel is also designed to withstand high pressures.

I think that the radiation in this part is shielded partially by the walls of the pressure vessel and partially by the water it contains.

However how much percent of the radiation is shielded by the water alone and how much by the walls? I.e. how do the shielding effects of the walls and the water compare to each other (roughly) and why?

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Julia, your question is not entirely clear as to what radiation you are talking about. For instance, there is no shielding against neutrinos. The worst offenders are actually not radiation but fission products (iodine-131, caesium-137, strontium-90 etc.) that may leak into the environment and decay emitting $\beta^-$ (electrons) and $\gamma$ radiation. –  Deer Hunter May 13 '13 at 13:11