Outcomes of an activity on development and verification of models for Fission Gas Release in Light-Water Reactor fuels during Loss-of-Coolant Accidents are described. The GRSW-A model of the FALCON code is extended to simulate the processes of fuel fragmentation and concomitant transient Fission Gas Release. The FRELAX code is redesigned with a view to simulation of bulk flow and diffusion of the gases in the rod free volume. Finally, close coupling of the updated FALCON and FRELAX codes is set up, referred to as the F2F coupled code system, due to which the two codes are being exchanging data during their simultaneous analysis. Three Halden LOCA experiments, IFA-650.12/13/14, using fuel irradiated in the Swiss Leibstadt NPP to a high level of fuel burnup are analyzed with the F2F coupled code system. Overall, the results of calculation for the selected three tests of the IFA-650 series are in good agreement, or conservative in comparison to the data.