The primary purpose of the water moderator is not to shield the outside world from radiation (although this is one benefit). The water moderator has two primary tasks in a PWR or BWR:
- High energy (~2 MeV) neutrons released in fission are down-scattered (or moderated) into the thermal energy range (~0.25 MeV). The cross section for fission is much higher at this energy level allowing a sustained chain reaction for power production.
- The water is used as the working fluid to transfer the thermal energy released in fission for power generation. The energy released from fission cannot be harnessed directly for electricity. In a BWR, fission heats and boils the water which drives a steam turbine to produce electricity. In a PWR, fission super-heats pressurized water which transfers heat to a secondary boiling water loop that drives a steam turbine to produce electricity.
Because of its properties as a moderator, the water tends to scatter most high energy neutrons to lower energies. Generally, any neutrons escaping the reactor will have been moderated down to a low energy and will therefore not have a long mean-free-path in the outside world. This is the shielding benefit provided by the water moderator - but it is not the purpose of the water in a reactor.
The reactor vessel, most likely steel, is also not primarily designed for radiation shielding. The purpose of the reactor vessel is to contain the moderator in the proper geometry to sustain the chain reaction for power generation. In the case of a PWR, the vessel must also contain the moderator at high pressure to prevent boiling of the super-heated water.
Since the steel vessel is sealed, it prevents fission products and activated isotopes from escaping the system and it can also provide some shielding from gamm and x- rays release in fission. This is not the purpose of the reactor vessel though, only an added benefit.