The Mathematical Basis of the PANAMA-I Code for Modelling Pressure Vessel Failure of TRISO Coated Particles under Accident Conditions

In the following, the computer code PANAMA (Ref. 1) and its underlying modelling assumptions are described. The models are based on independent measurements of the properties of TRISO particles with an SiC layer. Essential features are the calculation of internal gas pressure, and of the coating strength and its decrease during irradiation and its degradation under accident conditions. At very high temperatures, particle life is determined by SiC thermal decomposition. Good comparison is obtained in the temperature range from 1600 to 2500 °C when applying PANAMA to a wide variation of existing accident simulation experiments with spherical fuel elements. At lower temperatures, PANAMA tends to be overconservative. Predictions of particle failure during the depressurised accident sequence with the worst temperatures of the 200 MWth side-by-side Modular Reactor System remain below the level of normal operations. The same is true of the HTR-500 MWe accident sequence with the system under pressure. In the depressurised case, however, failure of all particles is predicted after approximately 100 hours in the least favourable core position.