The fission gas behavior computer model LAKU has been used to analyze annealing experiments performed on thermal reactor fuel and the transient in-pile experiment CABRI All. The results of the calculations revealed some minor weaknesses of the code. Consequently, changes were made in the modeling of the grain boundary gas bubbles, the release of intergranular gas, and the gas behavior in molten fuel. These modeling changes are presented together with the results of the recent calculations.