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Determination of Two-Phase Flow Parameters for Nuclear Fuel Channels using a Real-Time Neutron Radiography Method

Glenn David Harvel

Abstract

Multi-dimensional modelling of two-phase flow requires accurate constitutive relationships for interfacial parameters such as interfacial heat transfer, void fraction distribution, interfacial area, etc. However, existing diagnostic systems for measurement of two-phase flow parameters have difficulty measuring two or three-dimensional void distributions required for determination of interfacial parameters.

In this work, a Real-Time Neutron Radiography (RTNR) system is developed for non-intrusive measurement of two-phase flow parameters in nuclear fuel channels at low thermal neutron fluxes (on the order of 10⁶n/cm²-s). This advanced radiation technique has the advantage of measuring two-phase flow in 3½ dimensions(x,∫dy,z,t) where the ½ dimension refers to an integrated or averaged space dimension. Pipe flow channels, annulus flow channels, MAPLE-type nuclear fuel flow channels, and CANDU-type nuclear fuel flow channels are investigated. Measurements of flow regime, void fraction, void fraction distribution, bubble diameter, bubble velocity, and interfacial area are conducted.

The RTNR system is compared to ultrasonic and optical video measurement systems in pipe flow channels. Good agreement is obtained for flow regime, void fraction, bubble diameter, and interfacial area measurements.

The RTNR system is compared to High-Speed X-ray Computed Tomography (X-CT) and optical video measurement systems in the annulus flow channel. Good agreement is obtained for the determination of flow regime, void fraction, void fraction distribution, and bubble velocity measurements.

Application of the RTNR system to the vertical MAPLE-type 37 rod hexagonal finned fuel bundle and the horizontal CANDU-type 37 rod cylindrical fuel bundle is conducted. Measurements of flow regime, void fraction, and void fraction distribution are obtained and interfacial wave motion is observed. Flow regime observations in the MAPLE-type nuclear fuel channel show good agreement with previous work whereas flow regime observations in the CANDU-type nuclear fuel channel show significant discrepancies with previous work. A new large amplitude stratified wavy (LASW) flow regime is observed by the RTNR images in the CANDU-type nuclear fuel channel. The MAPLE-type nuclear fuel channel is shown to inhibit void migration and bubble coalescence at the entrance to the bundle and significantly affect the flow. The CANDU-type nuclear fuel channel is also observed to influence the cross flow in the core of the rod bundle.

Based on this work, an RTNR diagnostic technique is shown to be successful in measuring two-phase flow parameters in nuclear fuel channels.