12.73. The response of the reactor system to a number of postulated transients and accidents must be evaluated as one requirement for licensing. As we will see later, the methods used are intentionally conservative and differ from those to be discussed later (§12.132), that are used to obtain a “realistic” prediction of the response. Such a licensing design basis eval­uation has evolved from what was commonly referred to once as design basis accident analysis. Since our present purpose is to identify the events and not the evaluation methodology, we will use the older term, design basis accidents. These are events of such low probability that they should not occur even once in an average reactor plant’s lifetime, but which would have serious consequences if they were not controlled. These accidents are analyzed to assure the effectiveness of the engineered safety features and to evaluate the acceptability of the proposed plant site. The most severe design basis accident is considered to be a complete (double-ended) rupture of a large pipe, ranging in diameter from 0.61 to 1.07 m, in the primary coolant circuit of a PWR or a similar break in a recirculation pump intake line of a BWR. This accident is commonly referred to as the loss-of-coolant accident; it will be considered in some detail later. Other design basis accidents will first be examined briefly. It should be noted that design basis accidents do not result in core damage if the engineered safety features operate properly. So-called “severe accidents” in which core damage does occur will be discussed later (§12.95).