9.138. In order to provide a basis for reactor design, certain operating conditions, averaged over both the volume and lifetime of the core, must first be specified for the reactor to operate at the desired power level. In principle, the appropriate core specifications, such as number, dimensions, and arrangement of the fuel rods of a given type, coolant-flow rate, tem­perature distribution, etc., could then be computed to meet the require­ments. In practice, however, the situation is complicated by the fact that the ideal specifications cannot be met precisely. For example, the dimen­sions, density, and enrichment of the uranium dioxide fuel pellets may vary slightly, and so also may the dimensions and spacing of the rods and the thickness of the cladding. Consequently, the actual heat flux, temper­ature, and other parameters at various locations in the core may differ considerably from the specified (or nominal) values.[13]

9.139. A fundamental requirement in reactor design is to ensure that, in spite of such unavoidable variations from average values which are inherent in reactor design, e. g., nonuniform neutron flux distribution, there shall be no point in the core where certain limiting parameters are exceeded during operation under normal conditions. Two important limiting oper­ating parameters (or constraints) are the critical heat flux (§9.98) and the fuel temperature.1 A critical heat flux, even locally, can result in a boiling crisis accompanied by an increase in surface temperature, which could lead to cladding failure.

9.140. A principle that has been useful in reactor design is to relate the nominal (or average) performance to the maximum value that can be expected anywhere in the core. To this end, each of several relevant design specifications is assigned an adjustment or correction factor which repre­sents the ratio of the maximum to average values of such parameters as heat flux, coolant-flow rate, and enthalpy rise that would result from the most probable variations in the given specifications. The various factors are then combined in a suitable manner to yield the overall ratio of the maximum to average values of a particular operating parameter. The con­straints mentioned must then apply to the maxima determined in this manner.