9.102. The critical heat flux, at which burnout is expected to occur, is an important design consideration in water-cooled (and moderated) re­actors. A knowledge of burnout conditions is also necessary for possible accidental situations, such as might arise from loss of coolant flow or excessive fuel temperatures due to a power excursion. Consequently, meth­ods for estimating the critical heat flux in a reactor are required. Much experimental work has been done to develop the many correlations reported in the literature. In addition, much effort has been devoted to the development of theoretical models which would support empirical correlations.

9.103. For PWRs, the so-called W-3 correlation for the DNB condition has been widely used.[7] This is a many-termed complex expression for the critical heat flux with pressure, mass velocity, steam quality, enthalpy, and system dimensions as parameters [11]. In practice, the equation is readily solved as part of a digital computer calculation, and some general conclu­sions are of interest. The DNB flux decreases as the quality increases and hence reaches a minimum at the coolant channel exit. An increase in coolant-mass velocity tends to increase the DNB flux. The pressure effect is complex but an increase in critical heat flux can occur under PWR conditions as the system pressure is increased. As part of the core design effort, the DNB condition is calculated at various axial locations to deter­mine the closest approach to the design flux (see Fig. 9.22). Since the DNB flux and heat flux both decrease toward the exit, the closest approach is normally in the upper half of the channel.

9.104. The conditions in a BWR differ from those in a pressurized — water system largely because of the higher quality of the fluid in the former case. The approach generally used for determining the critical heat flux is based on numerous experiments with electrically heated rod bundles with fluid flow resembling that in an actual reactor. The results are incorporated into a thermal-hydraulic computational model, together with correlations that take into account the steam quality and coolant flow rate [12].