Accident A is a severe accident with an intact containment (IC) and the release at a normal containment leakage rate.
Accident B is a severe accident with a containment bypass scenario (BP) resulting from steam generator tube failure (either as the initiating event, or resulting from failure of one or more tubes due to high temperature during accident progression).
Source term of accident A results from a severe accident, with the containment staying intact. The release to the environment happens at normal containment leakage rates.
Accident B is assumed to occur during shutdown, with an open Reactor Pressure Vessel (RPV), when cooling of the core is lost. In this scenario fission products have a direct path to the environment via the open RPV and containment. The Passive Flooder System is assumed to be operating, so the core-concrete interaction can be prevented.
Accident A is a severe accident with an intact containment, and a release at a normal containment leakage rate. It considers deposition in the annulus and fuel/safeguards buildings without building ventilation.