Finned Fuel Element Two-Phase Flow Parameter Measurements Using Real-Time Neutron Radiography

The measurement of two-phase flow parameters for development of constitutive relationships for the HANARO/MAPLE type finned fuel using Real-Time Neutron Radiography (RTNR) is discussed in this paper. A single element finned Fuel Element Simulator (FES) was used with Freon 134a as the working fluid. To observe the effect of a spacer device on void distribution, single pin tests were performed with and without a spacer present. By analyzing the RTNR images using image processing, the effects of the spacer on the time-averaged and instantaneous void fraction distribution were studied. For the experimental results without a spacer, the time-averaged local void distribution is radially asymmetric and the degree of void fluctuation increases with a decreasing frequency along the heated channel, where the observed asymmetry may be caused by flow induced vibration. For the experimental results with a spacer, the spacer clearly limits any significant vibration and the local void distribution becomes more symmetrical. The spacer however does generate an axial void fraction maximum at the upstream of the spacer with a small depletion zone at the exit of the spacer. By analysis of the instantaneous local void distribution, void fluctuation at the heated wall due to boiling was clearly observed. Also, the agglomeration and breakup of the cold-wall void was evident. The dynamic effects of the local void transients will be discussed in detail.