Fraction limiting linear heat generation rate (FLLHGR) FLLHGR (FDLRX, MFLPD) is a limit on fuel rod power in the reactor core. For new fuel, this limit is typically around 13 kW/ft (43 kW/m) of fuel rod. This limit ensures that the centerline temperature of the fuel pellets in the rods will not exceed the melting point of the fuel material (uranium/gadolinium oxides) in the event of the worst possible plant transient/scram anticipated to occur. To illustrate the response of LHGR in transient imagine the rapid closure of the valves that admit steam to the turbines at full power. This causes the immediate cessation of steam flow and an immediate rise in BWR pressure. This rise in pressure effectively subcools the reactor coolant instantaneously; the voids (vapor) collapse into solid water. When the voids collapse in the reactor, the fission reaction is encouraged (more thermal neutrons); power increases drastically (120%) until it is terminated by the automatic insertion of the control rods. So, when the reactor is isolated from the turbine rapidly, pressure in the vessel rises rapidly, which collapses the water vapor, which causes a power excursion which is terminated by the Reactor Protection System. If a fuel pin was operating at 13.0 kW/ft prior to the transient, the void collapse would cause its power to rise. The FLLHGR limit is in place to ensure that the highest powered fuel rod will not melt if its power was rapidly increased following a pressurization transient. Abiding by the LHGR limit precludes melting of fuel in a pressurization transient.
Fraction limiting linear heat generation rate (FLLHGR) FLLHGR (FDLRX MFLPD) is a limit on fuel rod power in the reactor core For new fuel this limit is typically around 13 kW/ft (43 kW/m) of fuel rod This limit ensures that the centerline temperature of the fuel pellets in the rods will not exceed the melting point of the fuel material (uranium/gadolinium oxides) in the event of the worst possible plant transient/scram anticipated to occur To illustrate the response of LHGR in transient imagine the rapid closure of the valves that admit steam to the turbines at full power This causes the immediate cessation of steam flow and an immediate rise in BWR pressure This rise in pressure effectively subcools the reactor coolant instantaneously; the voids (vapor) collapse into solid water When the voids collapse in the reactor the fission reaction is encouraged (more thermal neutrons); power increases drastically (120%) until it is terminated by the automatic insertion of the control rods So when the reactor is isolated from the turbine rapidly pressure in the vessel rises rapidly which collapses the water vapor which causes a power excursion which is terminated by the Reactor Protection System If a fuel pin was operating at 130 kW/ft prior to the transient the void collapse would cause its power to rise The FLLHGR limit is in place to ensure that the highest powered fuel rod will not melt if its power was rapidly increased following a pressurization transient Abiding by the LHGR limit precludes melting of fuel in a pressurization transient