9.141. Early in the course of reactor development, the term hot-channel factors yas used to relate maximum to nominal design conditions. The hot — channel concept was based on the assumption that a reactor core, with solid fuel elements having coolant passages (or channels) between them, would have one channel in which a combination of variations in power density and dimensions result in a heat flux that is greater than that at any other point in the core. Although there is a continuing need to relate maximum to average conditions to develop safety margins, the concept of an actual hot-channel or hot-spot tends to be simplistic. Therefore, peaking factors are now generally used, although utilization of hot-channel con­tinues. In fact, we will use the term hot-channel as a convenient designation for the hypothetical vulnerable location in the core. In any case, the design approach to limiting conditions through the use of peaking factors must be described in licensing documentation for every reactor initial core as well as for subsequent reload cores and approved by the U. S. Nuclear Regulatory Commission (§10.39). These peaking factors are included in the plant’s Technical Specifications as limits that are not to be exceeded (§12.240).

9.142. For PWRs, the peaking factors of importance are the heat flux factor and the enthalpy rise factor. These are decomposed through the use of various subfactors. However, before considering such detail, let us con­sider the principles involved. A three-dimensional nodal nuclear calculation can yield a “map” of the core power distribution, a starting point for both the heat flux factor and the enthalpy rise factor. In fact, as we shall see, the nuclear enthalpy rise factor merely describes the radial power distri­bution. However, when it is combined with appropriate flow-related fac­tors, it also yields a limiting coolant condition which can be associated with the maximum heat flux in boiling crisis predictions. The need to determine

DNB design margins is a major reason for PWR peaking factor develop­ment. Other limits depend primarily on the heat flux (see §9.163 et seq.).