Influence of thermal equivalent diameter on transient void behavior under conditions of cold shutdown reactivity initiated accidents

To accurately evaluate maximum fuel enthalpy during postulated reactivity initiated accidents (RIAs), well-verified methodology which correctly predicts transient void behavior of coolant in a reactor core is indispensable as the core power is influenced by void-reactivity feedback. We have thus performed series of out-of-pile experiments to provide a database on fast transient void behavior during RIAs under various thermo-hydraulic conditions. The present study focuses on clarifying the influence of thermal equivalent diameter (Dh) on the transient void behavior since different types of fuel bundle have their own Dh and the associated database is limited. The experiments were conducted under the atmospheric pressure using two types of test section: single-rod test section and bundle test section composed of four rods arranged in 2 x 2 arrays. The void fraction in the test section was measured using fast-response impedance-type void fraction meters. To clarify the effect of Dh on the transient void fraction variation, the time variations of the void fraction were evaluated using the corrected time based on thermal equilibrium quality in the flow channel. The results obtained through the single-rod tests indicate that the onset time of net vapor generation (ONVG) corresponding to the initiation of abrupt increase in the void fraction did not strongly depend on Dh. A similar tendency was found in the bundle tests as to the cross-sectional averaged void fraction, while the local void fraction had clear cross-sectional distribution which differed in both bundle test sections with different dimension. The evaluation of the discrepancy between the results and the prediction with the Saha-Zuber model as to ONVG suggests that the model for ONVG prediction needs to take a local characteristic length such as the thickness of thermal boundary layer into account. (author)