Editing Boiling water reactor (section)

==== Maximum fraction limiting critical power ratio (MFLCPR) ====
Specifically, MFLCPR represents how close the leading fuel bundle is to "dry-out" (or "departure from nucleate boiling" for a PWR). [[Transition boiling]] is the unstable transient region where nucleate boiling tends toward [[film boiling]]. A water drop dancing on a hot frying pan is an example of film boiling. During film boiling a volume of insulating vapor separates the heated surface from the cooling fluid; this causes the temperature of the heated surface to increase drastically to once again reach equilibrium heat transfer with the cooling fluid. In other words, steam semi-insulates the heated surface and surface temperature rises to allow heat to get to the cooling fluid (through convection and radiative heat transfer). [[Nuclear fuel]] could be damaged by film boiling; this would cause the fuel cladding to overheat and fail.

MFLCPR is monitored with an empirical correlation that is formulated by vendors of BWR fuel (GE, Westinghouse, AREVA-NP). The vendors have test rigs where they simulate nuclear heat with resistive heating and determine experimentally what conditions of coolant flow, fuel assembly power, and reactor pressure will be in/out of the transition boiling region for a particular fuel design. In essence, the vendors make a model of the fuel assembly but power it with resistive heaters. These mock fuel assemblies are put into a test stand where data points are taken at specific powers, flows, pressures. 
Experimental data is conservatively applied to BWR fuel to ensure that the transition to film boiling does not occur during normal or transient operation. Typical SLMCPR/MCPRSL (Safety Limit MCPR) licensing limit for a BWR core is substantiated by a calculation that proves that 99.9% of fuel rods in a BWR core will not enter the transition to film boiling during normal operation or anticipated operational occurrences.<ref name="NUREG0800">http://pbadupws.nrc.gov/docs/ML0523/ML052340664.pdf NUREG-0800, (67:234) Chpt 4, Section 4.4, Rev. 1, Thermal and Hydraulic Design, of the Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants. LWR Edition. (10 page(s), 7/31/1981)</ref> Since the BWR is boiling water, and steam does not transfer heat as well as liquid water, MFLCPR typically occurs at the top of a fuel assembly, where steam volume is the highest.