The Heat Flux Hot Channel Factor – FQ(z) is defined as:
- The ratio of the maximum local linear power density, where there is a minimal margin to limiting fuel temperature (during AOOs), to the average local linear power density in the core.
- The maximum local linear power density [kW/ft] in the core is divided by the core average fuel rod linear power density [kW/ft].
Operation within the Heat Flux Hot Channel Factor – FQ(z) limits prevents power peaks that exceed the loss of coolant accident (LOCA) limits derived from the analysis of the Emergency Core Cooling Systems (ECCS). In this analysis, fuel cladding failure during a postulated LOCA is limited by restricting the maximum linear heat rate (LHR) so that the peak cladding temperature does not exceed the acceptance criterion (e.g., 2200°F or 1204°C). Cladding temperatures below this criterion exclude severe cladding failure by oxidation due to a cladding-steam reaction. The Heat Flux Hot Channel Factor – FQ(z) is an assumption in these and other analyses, as well as it is an assumption for Safety Limits (SLs) calculations. Operation beyond the Heat Flux Hot Channel Factor – FQ(z) could invalidate core power distribution assumptions used in these analyses (Safety Analyses and Safety Limits derivation).
Now, consider a case where power distribution is limited only by the FQ(z). But FQ(z) can be limiting at each elevation of the core (axially flat distribution). The maximum heat flux at several core elevations can occur in the same channel. In this case, the coolant enthalpy rise would be very high, most probably over-limit. In this case, DNB is more likely to occur because the critical heat flux is significantly lower when coolant enthalpy is higher. Therefore, the FQ(z) cannot prevent DNB occurrence, and the enthalpy rise hot channel factor must be introduced.