9.22. About 3 percent of the heat generated in the reactor system is produced in the moderator as the result of the slowing down of fission neutrons, the stopping of beta particles from the fission products, and the absorption of gamma rays from various sources. In light-water reactors, this presents no special cooling problem. Cooling requirements in other systems may be a function of the design. The need for cooling should also be recognized in the design of control elements. Heat is generated in such components outside the core as the thermal shield and reactor vessel in water reactors and the radiation shield in all reactors. This heat is a result of the absorption of escaping neutrons and gamma radiation. The subject of heat generation in shields has been treated in Chapter 6.