A thermal–hydraulic model has been developed to simulate loss of the ultimate heat sink in a typical
material testing reactor (MTR). The model involves three interactively coupled sub-models for reactor
core, heat exchanger and cooling tower. The model is validated against PARET code for steady-state operation
and verified by the reactor operation records for transients. Then, the model is used to simulate the
thermal–hydraulic behavior of the reactor under a loss of the ultimate heat sink event. The simulation is
performed for two operation regimes: regime I representing 11MW power and three cooling tower cells
operated, and regime II representing 22MW power and six cooling tower cells operated. In regime I, the
simulation is performed for 1, 2 and 3 cooling tower cells failed while in regime II, it is performed for 1, 2,
3, 4, 5 and 6 cooling tower cells failed. The simulation is performed under protected conditions where the
safety action called power reduction is triggered by reactor protection system to decrease the reactor
power by 20% when the coolant inlet temperature to the core reaches 43 C and scram is triggered if
the core inlet temperature reaches 44 C. The model results are analyzed and discussed.