a). Ratio of the 133Xe and 135Xe activities (the vertical axis on the left) during steady-state reactor operation in the course of cycle N-1. b). Threshold value corresponding to the fuel failure criterion for NGs in the zero degassing rate approximation (the vertical axis on the left) and for the iodine radionuclides (the vertical axis on the right). c). Ratio of the normalized release rates for 131I and 134I