Safety analysis of MTR type research reactor during postulated beam tube break inducing positive reactivity

MTR type research reactors are commonly equipped with beam tubes which functions as experimental facilities out off the core. When used for irradiation, the tubes are filled with air, in contrary they are flooded with water when idle. Should the break occur during irradiation, the water comes in and the air goes out. Beam tubes break should be viewed as potential accident to occur during reactor life time, caused, for instance by an earth quake. The break was assumed to occur inside the reactor pool and no water leaked out of the pool as the tube isolation valves worked properly. When being used for irradiation, the change from voided to flooded condition due to the break, inserts positive reactivity as more neutrons are moderated and reflected back to the core by the water. Functioning as moderator, water also reflects neutrons back to the core. More neutrons in the beam tube slowed down to a lower energy. For the above reason, safety analysis needs to be performed for beam tube break and its influence on reactor dynamics. The radial beam tubes gave higher positive reactivity feedback than that of tangential ones. The calculations on reactivity induced by beam tubes break were performed with MCNP5 code. It was treated as reactivity initiated accident analyzed using PARET/ANL code. Beam tube S5 gave the highest reactivity feedback of 0.062%dk/k and was treated as the initiating event. The maximum fuel and clad temperature reached 175.28, and 136.75 °C, respectively. All safety parameters were confirmed under acceptable limits.