Sodium boiling situations in a fast-reactor fuel element can be calculated with the BLOW-2 code from their initiation until fuel-pin failure occurs. The model, which is used, is substantiated by means of a theoretical analysis of the cause of the typical piston-type bubble generation, and it is checked against water and sodium tests. Typical examples of sodium boiling incidents are given. The probability of occurrence of severe nuclear excursions generated by sodium boiling is calculated.