Simulation tools incorporated to DIONISIO in the frame of the FUMAC project

Several models were integrated to the DIONISIO code within the framework of the IAEA Research Project "Fuel Modeling in Accident Conditions (FUMAC)", to take account of accidental conditions, in particular the loss of coolant accidents (LOCA). A specially designed thermal-hydraulic subroutine provides a simplified description of the rod environment in normal or accidental conditions. The heat transfer coefficients that account for the different coolant regimes, in single or double phases, are activated as the corresponding conditions occur. The simulation of a considerable number of experiments has shown that, despite its simplicity this subroutine gives adequate predictions of the conditions in a vertical cooling channel, quite similar to those given by the thermal-hydraulic codes. The description of the fuel rod atmosphere is improved with the incorporation of this subroutine since it provides fairly realistic boundary conditions for the simulation of the fuel rod behavior, without requiring the intervention of external specific codes. Models of high temperature oxide growth (ZrO2) and hydrogen capture and release by the cladding in steam were also included. Moreover, the model of cladding creep predicts the conditions for ballooning and eventually, those for catastrophic failure (burst) and its localization. The calculation scheme makes a partition of the rod length into a number of segments defined by the user. In each segment the local conditions are considered to calculate, with the synchronous work of all the subroutines, the physical and chemical parameters in one representative pellet. Then, a description of the whole rod is obtained by coupling all the segments. This strategy has yielded accurate simulations of a wide variety of cases, either in normal or LOCA type conditions. Exhaustive comparisons were carried out with several thermal-hydraulic codes (COBRA-IV, RELAP5-Mod3.1, SOCRAT, ATHLET-Mod 1.1) and with a number of experiments like those of the IFA–650 series (-1,-2,-9,-10,-11), PUZRY, QUENCH-L0/L1 (for which a new working scheme was specially developed in DIONISIO), CORA-15, IAEA-SPE-4, among others. (author)