Top>Research and Development>Wire mesh void fraction distribution measurement technology

Spatiotemporal data on the spatial distribution of void fraction and interfacial velocity are important to evaluate the two-phase flow behavior within a nuclear reactor fuel assembly. However, due to the presence of structures in the rod-bundle system, there are issues in that it is difficult to simultaneously measure multiple points or obtain spatial distribution with high temporal resolution using conventional measurement methods.
In this research, in order to solve these issues, we developed a measurement method using a three-layer (three-wire) wire mesh sensor (WMS) that has a structure in which wires are passed inside the rod, and constructed an experimental system that can independently obtain the spatial distribution of void fraction in the rod bundle flow path.
Specifically, a three-layer WMS was incorporated into a 4×4 rod bundle specimen, and the cross-sectional distribution and temporal fluctuations of void fraction were measured with high temporal resolution in water-steam two-phase flow under high temperature and high pressure conditions (up to 2.6 MPa). Furthermore, by using the cross-correlation of upstream and downstream signals, it is also possible to evaluate the interfacial velocity.
As a result, we were able to obtain the spatial distribution of void fraction within the subchannel and in the inter-rod region, and successfully captured flow characteristics such as an increase in void fraction with increasing steam flow rate and an increase in void fraction at the center of the subchannel. We also observed changes in void fraction and interfacial velocity with increasing pressure.
Through this research, we established a method to obtain the void fraction spatial distribution of two-phase flow in rod bundles using three-layer WMS, and provided experimental data that will contribute to CFD code verification under high temperature and high pressure conditions.

4×4 rod bundle test system under high temperature and high pressure conditions

Structure of three-layer (3-wire) wire mesh sensor (WMS)

Example of time-average void rate obtained by WMS

reference:Taku Nagatake & Hiroyuki Yoshida (2023) Measurement of the water-vapor void fraction in a 4x4 unheated rod bundle, Journal of Nuclear Science and Technology, 60:11, 1417-1430