Control through Cladding

Clad metal systems designed for corrosion control can be categorized as 

Noble metal clad systems Corrosion barrier systems Sacrificial metal systems Transition metal systems 

Proper design is essential for providing maximum corrosion resistance with clad metals. 

Noble metal clad systems are materials having a relatively inexpensive base metal covered with a corrosion-resistant metal. A typical example would be a carbon steel clad with a stainless steel or nickel-base alloy. Another group of commonly used noble metal clad metals uses aluminum as a substrate. For example, in stainless-steel-clad aluminum truck bumpers, the stainless steel provides corrosion resistance, and the aluminum provides a high strength-to-weight ratio. 

Corrosion-Barrier Systems. The combination of two or more metals to form a corrosion-barrier system is most widely used where perforation caused by corrosion must be avoided. This is shown schematically in Fig. 12. Low-carbon steel and stainless steel are susceptible to localized corrosion in chloride-containing environments and am perforate rapidly. When steel is clad with a stainless steel layer, the corrosion-barrier mechanism prevents perforation. Localized corrosion of the stainless steel is prevented the stainless steel is protected galvanically by the sacrificial corrosion of the carbon steel in the metal laminate. Therefore, only a thin pore-free layer is required. 

---

Second, although only a small amount of tritium is produced from the pure water, there are two other sources of tritium that will produce substantial amounts in a PWR. When boron is used as a chemical shim, fast neutron reactions with boron will produce tritium. Also, tritium is produced as a fission product, and it can escape through clad leaks into the water. Finally, use of Li OH for pH control can contribute tritium. The boron reactions are responsible for most of the tritium, however. 

The fission neutrons at birth have energies of approximately 1 to 2 MeV In a thermal reactor the neutron energy is rapidly reduced through collisions with light nuclei to thermal (—.02 to 1 eV), to promote for more efficient capture. Besides the nuclear fuel, there are many other materials in the reactor core also competing for the neutrons, including the moderator (the material used to slow down or thermalize the neutrons), fertile nuclides that produce additional fissile material (discussed in a later section), neutron poisons present in control rods, the coolant, fuel element cladding, and other structural materials. 

Fuel operating limits have been selected to avoid axial fuel movement due to melting, and to prevent fuel-cladding mechanical interaction through control of the average fuel density (gap, plus fuel pellet density) ... 

---