Fig. 1-5 Typical geometries of test sections simulating fuel assemblies (cross sections) Real-time measurement of void fraction in various complicated channels has been achieved.

Fig. 1-6 Results of calibration test The measured results agree well with theory (Maxwell equation).

Fig. 1-7 Real-time measurements under BWR conditions Real-time measurements were made during a test under BWR conditions. Void fraction oscillations with flow rate oscillations were measured.

To gain a high conversion ratio of 238U to 239Pu in nuclear fuel by suppressing the slowing down of neutrons with reducing the water in a fuel assembly of a reduced moderation water reactor (RMWR), a tight lattice structure having a complicated geometry is used, as shown in Fig. 1-5. In addition, void fraction in coolant should be increased in the lattice with securing core cooling. Therefore, highly accurate void fraction measurements in a complicated geometry are needed for core design. Moreover, real-time void fraction measurements are necessary to predict core power, because core power is strongly dependent on void fraction. Previously, however, there was no practical method of such void fraction measurements because of the difficulties in real-time measurement of void fraction in a complicated flow channel, such as the RMWR core. We developed a practical void fraction meter. This made real-time measurement of area-averaged void fraction possible even in a complicated flow channel. The developed meter measures the electrical conductance or capacitance, which changes with void fraction in gas-liquid two-phase flow, between electrodes. The features of the meter are as follows. (1) A ceramic cover insulates the electrodes; the thermal expansion of the ceramic is almost equal to that of metal. Therefore, the meter can be applied in high-temperature environments. (2) A channel wall is used as one electrode. Thus, the meter can be easily applied to high-pressure applications. (3) Various shapes of electrodes can be applied. The plate-shaped electrode shown in Fig.1-5 was adapted to the test channel for the thermal-hydraulic study of RMWR. (4) The frequency of the carrier current between electrodes can be optimized depending on water properties. Fig. 1-6 shows the results of a calibration test performed with water-air two-phase flow. The measured results agree with theory, indicating good meter performance. Fig. 1-7 shows an example of a real-time measurement of a test under BWR conditions. Void fraction oscillation with flow rate oscillation was measured. The developed void fraction meter is now used under various conditions in RMWR tests. It is also used in stability tests and transient void tests for reactivity initiated accidents of BWR cores where fast response of void fraction measurements is needed.

Reference H. Watanabe et al., Development of Quick-response Area-averaged Void Fraction Meter, JAERI-Research 2000-043 (2000).

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