US recommended slow overpower transient ramp rate for PFR-TREAT test program

Analyses of US and UK fuel rods with several fuel rod mechanics codes resulted in identification of three major loading mechanisms that cause cladding failure: differential thermal expansion between solid fuel and cladding prior to initial fuel melting; pressurization of the central molten fuel region by constriction of the void volume available to contain fission gas; and processes related to extreme thermal conditions. These processes would include fuel vapor pressure loading, cladding melt-through due to coolant boiling disruption of heat transfer, and low (plenum) fission gas pressure loading of very hot cladding. Ramp rate regimes are not defined in terms of these loading mechanisms because they are not uniquely associated with any single ramp rate regime.