Experiment of Bubbling Void Importance in the Core of JRR-2

One of the interesting problems in the core physics of a heavy water moderated reactor is the local importance for mass void and bubbling void. We published already a paper (J. Nucl. Sci. Tech., 1, No. 1, pp. 1117) about the influence of reactivity change for mass void in the central experimental hole of JRR-2. In order to understand the bubbling void importance, a series of experiments was undertaken in the core of the same reactor. Bubbling voids were introduced as nitrogen or helium gas bubbles at various locations in the central experimental hole, fuel element, dummy fuel element and upper reflector, and the relation between bubble flow rate, location of bubble generation and reactivity change was investigated. It is very interesting to see that bubbling void importance is almost linear to the square of thermal neutron flux comparing the mass void which has the flat importance distribution for the thermal neutron flux. In addition to these tests, checking the power fluctuation and reactor period owing to the bubble generation, we recognized less distinct change as reactivity change. All experiments were performed at room temperature.