## Nuclear Enthalpy Rise Hot Channel Factor – F^{N}_{ΔH}

The Nuclear Enthalpy Rise Hot Channel Factor – F^{N}_{ΔH} is defined as:

- The ratio of the integral of linear power along the fuel rod on which minimum departure from nucleate boiling ratio occurs (during AOOs) , to the average fuel rod power in the core.
- The ratio of the integral of linear power along the fuel rod with the highest integrated power [kW/rod] to the average rod power [kW/rod].

Operation within the Nuclear Enthalpy Rise Hot Channel Factor – F^{N}_{ΔH} limits prevents departure from nucleate boiling (DNB) during accidents, that are limiting from DNB point of view. For example, a loss of forced reactor coolant flow accident, a loss of normal feedwater flow or an inadvertent opening of a pressurizer relief valve. The Nuclear Enthalpy Rise Hot Channel Factor F^{N}_{ΔH} is an assumption in these and other analyses as well as it is an assumption for Safety Limits (SLs) calculations. Its merit is that F^{N}_{ΔH} provides with an information about power distribution as well as about the coolant temperature (enthalpy). Both are crucial for DNB occurrence. Operation beyond the Nuclear Enthalpy Rise Hot Channel Factor – F^{N}_{ΔH} could invalidate core power distribution assumptions used in these analyses (Safety Analyses and Safety Limits derivation).

See also: What is Enthalpy

DNB occurs when a fuel rod cladding surface is overheated, which causes the formation of a** local vapor layer**, causing a dramatic reduction in heat transfer capability. Proximity to the DNB condition is expressed by the departure from nucleate boiling ratio (DNBR), defined as the ratio of the cladding surface heat flux (known as the critical heat flux) required to cause DNB to the actual cladding surface heat flux. The minimum DNBR value during both normal operation and anticipated transients is limited to the DNBR correlation limit for the particular fuel design in use, and is accepted as an appropriate margin to DNB. The DNB acceptance criterion for an anticipated operational occurrence (AOO) is met when there is a 95 percent probability at a 95 percent confidence level (the 95/95 DNB criterion) that DNB will not occur, and the fuel centerline temperature stays below the melting temperature.

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