Most component defects, be it in material, design, or fabrication, can be considered as external disturbances to the system which are built in but which may appear at any time, especially when that material or component is operating under adverse off-normal accident conditions. Such component defects take three major forms as we shall note in the coming chapters.

(a) Fuel cladding defects. Under typical quality control procedures and methods it is only possible for a one mil defect in 15 mil cladding to avoid detection. This size of defect has no significant effect on the strength of the cladding (13).

(b) Bad design. Section 4.6 will detail examples of bad design of com­ponents, procedures, and operations. Fault trees attempt to take into account all bad design possibilities and such errors are included in the safety evalua­tion. However, bad design, being a human failing, is always a present possibility against which safeguards are provided.

(c) Lack of quality assurance over materials. Examples have occurred of materials which, through lack of quality control at the original suppliers, were not produced according to specification. An example is the inclusion of one carbon steel tube among 3600 stainless steel tubes for the Enrico Fermi reactor steam generators. Such a tube could have failed and resulted in further failures if not excluded from the component. The safety evaluation will include the consequences of material being other than as specified in strengths, heat-transfer coefficients, erosion and corrosion resistances, etc.