Two experiments with different types of PAR unit (from the vendors Areva and NIS, respectively) from the international experimental project THAI (organized by OECD/NEA) were simulated with the lumped parameter code MELCOR 1.8.6. A simple model of PAR units, based on identification of sinks and sources of mass and energy in each time step, was implemented in the code, taking advantage of FORTRAN functions and vendors-furnished correlations for the recombination rate. The two experiments, HR-12 and HR-14, were characterised by elevated pressure and temperature in steam-saturated atmospheres with lack of oxygen (i.e. oxygen starvation conditions). The MELCOR code with the simple PAR model succeeded to accurately predict the recombination rate, including the efficiency drop due to oxygen starvation. The precision achieved in calculation of other quantities such as pressure and gas concentrations was less accurate, reflecting the possibilities of a lumped parameter code.

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