Almost all the reactors in the United States and a majority in the world are thermal reactors wherein thermal neutrons cause the bulk of the fission reactions. If one starts to think about designing a prototype reactor, the several design elements need to be flawlessly integrated. Figure 1.6 shows such a sche­matic for a primitive thermal reactor. The tubes containing fuels are generally made of metallic alloys (also known as fuel cladding). The radioactive fuels (such as uranium) could be in metallic, alloy, or ceramic forms. The fuel clad­ding serves many purposes: it provides mechanical support to the fuel, keeps the fission products from leaving the fuel element, and protects the fuels from corrosion from the coolant. The fuel elements are arranged in a distinct regular pattern (square, hexagon, etc., dictated by neutronics and other factors) with the moderator. Moderator slows down the neutron to sustain the fission reaction with thermal neutrons. The fuel-moderator assembly is surrounded by a

Figure 1.6 A schematic of a simple reactor design. (adapted from C. O. Smith, Nuclear Reactor Materials, Addison-Wesley, Reading, MA, 1967)

reflector. The purpose of a reflector is to direct all neutrons generated toward the core so that neutron leakage can be controlled, thus improving the neutron economy. On the outside, the reactor is lined by shielding materials that absorb neutrons and gamma rays that escape the core and reduce the radiation inten­sity to a tolerable level so that people near the reactor are not exposed to these radiations. The control rod (usually an assembly) helps control the chain reaction by absorbing neutrons, maintaining the steady state of operation. Hence, the control materials are neutron-absorbing materials (boron, hafnium, and so forth), and are generally fabricated in the form of rods (in some cases, plates). A reactor is typically equipped with two types of control rods — regulat­ing rods for routine control reasons and safety rods (to permit shutdown in the case of emergency). Even though coolant is not shown in Figure 1.6, it is an important component of a reactor. As a huge amount of heat is generated in the fuel elements, the heat needs to be removed continuously in an efficient manner in order to maintain a safe, steady-state reactor operation. This means an efficient coolant is needed. The coolant can be a gas or liquid (such as light or heavy water, carbon dioxide, liquid metals, and molten salts). However, it is important to remember that the presence of any coolant tends to adversely affect the neutron economy. Hence, the balance between the reduction in the neutron economy and the efficiency of heat removal needs to be carefully considered.

1.8.2