[en] To study critical flow phenomenon in a thick orifice, and to generate technical data to evaluate the performance of break simulator design for small break accidents in a nuclear power plant, critical flow tests have been performed at the Blowdown and Condensation Loop. Steady state and blowdown critical flow tests have been performed using eight different-shaped thick orifices. The steady state flow data show that the critical mass flux can be expressed as a function of the discharge coefficient and initial conditions. Based upon the test results, a semi-empirical model has been developed. Comparison between the model prediction and test data from various sources showed that the critical mass flux through a thick orifice (0.4 ≤ L/d ≤ 2.0) with a small diameter (2.0 ≤ d ≤ 12.7 mm) can be accurately predicted. The characteristics of two typical break simulators, which simulate break geometries during a small break loss-of-coolant-accident, were analyzed and provisions for the design of a break simulator for a small scale test facility have been suggested. (author)