Nobody debates or even talks about how reactors should be shielded, for the simple reason that through much thinking, research, and experimentation, the topic was mastered by the 1960s. Hyman Rickover, father of the world’s most popular reactor design, was a difficult man (to say the least) but dissimulation of quality information was one of his strengths. One of his faithful lieutenants, engineer Ted Rockwell, edited a 1956 tome on the subject, and this ended up being a staple read for nuclear engineers around the world for years. Here he is introducing his subject:
There is generally not a large difference in weight between shields of the same design but of different materials (e.g., tungsten instead of lead). Therefore, wherever possible shield materials should be used which are readily available, inexpensive, and comparatively easy to work with. Other considerations are attenuation characteristics, structural strength, radiation damage, flammability, and creation of air-borne toxicity. The more common shielding materials are concrete, structural steel, lead, and water.
Rockwell, Theodore. 1956. Reactor Shielding Design Manual. Van Nostrand, Princeton, New Jersey. http://www.osti.gov/energycitations/servlets/purl/4360248-Cr40J8/ (accessed Sep. 19, 2011), p. 169.