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CA9700777 A NEUTRON SCATTERING DEVICE FOR VOID FRACTION MEASUREMENT IN CHANNELS OF THE RD-14M THERMALHYDRAULICS TEST FACILITY P. HAN and E.M.A. HUSSEIN Department of Mechanical Engineering University of New Brunswick P.O. Box 4400, Fredericton, N.B., E3B 5A3
P.J. INGHAM Thermalhydraulics Branch AECL, Whiteshell Laboratories Pinawa, Manitoba, ROE 1LO
Abstract This paper presents a neutron scattering device designed for measuring the void fraction of two-phase flow in the channels or heated sections of the RD-14M Thermalhydraulics Test Facility, located at the AECL Whiteshell Laboratories. The results of an on-line test of the device are presented. The performance of the scatterometer is assessed and is shown to be in agreement with the results inferred from other independent process-parameter measurements.
INTRODUCTION The RD-14M Thermalhydraulics Test Facility at AECL Research, Whiteshell Laboratories is a full-elevation model of the heat transport loop in a CANDU reactor. It consists of ten parallel horizontal high-pressure flow channels, each containing electrically heated fuel element simulators (FES) in the form of seven-element rod-bundles. A cross section of an RD-14M channel is shown in Figure 1. The area of metal in this section is about three times larger than the flow area. This flow area is equivalent to that of a 28.4 mm ID pipe. Some postulated loss-of-coolant accidents can result in boiling of the water coolant in the channel. Due to the large metal content of the FES rod bundle, conventional techniques for void-fraction measurement are not suited for nonintrusive monitoring offlowboiling conditions in RD-14M heated sections. As a consequence, void fraction has to be inferred from other measurements. The presence of a strong electromagnetic field, generated by the heating current, renders electromagnetic techniques useless. While ultrasonic waves can easily penetrate the metal, the large mismatch between the acoustic impedance of the metal and that of the flow material leads to a strong signal reflection that makes it difficult to obtain useful bulk information. Radiation techniques based on photons, such as x-rays and gamma rays, have the disadvantage of being very sensitive to metals, because of their high electron density, and much less susceptible to the usually low electron-density flow material. Neutrons are less affected by the presence of the metal and are strongly influenced by 1
the hydrogen present in most liquid flows. This makes the neutron a natural probe for diagnosing hydrogenous flows, particularly in the presence of a significant amount of metal. Fast neutrons are more suited for this purpose than thermal neutrons, due to the large loss in neutron energy caused by collisions with the low mass-number nuclei of the flow (in contrast with the very small change due to collisions with the metal's large-mass number nuclei). Fast neutrons are utilized in this work to design a portable neutron-scattering device (scatterometer) for on-line transient measurement of the void fraction at some stations along the length of an RD-14M heated channel. This paper describes the scatterometer and presents the results of testing it on the RD-14M Test Facility.
SCATTEROMETER The scatterometer is schematically shown in Figure 2. It consists of a fast neutron source (californium252) positioned at the side of the channel and two pairs of thermal neutron (helium-3) detectors. The detectors are arranged such that one pair is located at the top of the channel and the other at its bottom, to allow the identification of flow stratification. However, by averaging the readings of the top and bottom detectors the effect of flow stratification on the neutron count rate can be reduced, which further minimizes the dependence on flow regime configuration. Cadmium sheets were used to surround the detectors and the shielding walls in order to minimize the number of background thermal neutrons within the detection cavity. The scatterometer is designed such that it has a linear response (for ease of calibration) and is not very sensitive toflow-regimevariations. A prototype scatterometer was constructed and successfully tested on the RD-14M loop. The design process of this scatterometer is discussed elsewhere [1]. The design of the scatterometer was first tested using bench-top experiments [2]. In these tests, lucite rods or varying levels of stratified water were used to simulate the liquid phase while air represented the vapour phase. These tests were performed at room temperature and pressure. Various design configurations were considered to improve the performance of the scatterometer by maximizing its contrast and resolution. The device was then ready for testing under field conditions.
TESTING SETUP The scatterometer was installed during a natural circulation test on the RD-14M loop. The scatterometer was mounted on a channel at the bottom of the loop (about 0.457 m above the floor) at a distance of 4.995 m from the inlet hydraulic boundary, as shown in Figure 3. The test lasted about nine hours; during which the scatterometer's counts were sampled every twelve seconds for a period of ten seconds. Some other measurements were used during the test to help verify the scatterometer's signal. A gamma-ray densitometer was used to measure the void fraction at the outlet of the channel. A single-phase flow meter provided the volumetric flow rate at the inlet of the channel. A pressure gauge was also used at the outlet of the channel. The temperature at the outlet of the channel was measured. A thermocouple was located on a top FES, 5.317 m from the inlet hydraulic boundary of the channel, while another thermocouple was located on a bottom FES at 5.336 m from the inlet hydraulic boundary. The measurements for the two thermocouples were recorded for the test period from 28,000 to 31,000 s. The location of the above measurements are shown in Figure 3.
These measurements are normally used to infer the value of the void fraction in the heated section. All the measurements, except for the scatterometer, were sampled every two seconds. The scatterometer's count rate for empty and full-of-water measurements were measured on the RD-14M loop. The calibration measurements were recorded at a temperature of 22°C and a pressure of 0.3 MPa. These calibration data were used in estimating the void fraction, a, using the linear relationship:
where C refers to the measured count rate and C\ and CQ are the calibration count rate for an empty and full-of-liquid channels, respectively.
RESULTS A five-point moving average smoothing procedure was applied to the measured "raw data" to eliminate the fluctuations observed in the data. For measurements sampled over a two-second period, the raw data were accumulated over twelve seconds to provide an equivalent sampling period to that of the scatterometer. The smoothing process was then applied over the accumulated data. Using the calibration measurements, the void fraction was calculated using equation (1). The void fraction was first estimated using the original "raw data", then the five-point smoothing process was applied. Figure 4 shows the results obtained for the scatterometer, gamma-ray densitometer and a flow meter at the channel inlet, during the entire test period. The scatterometer results are the average of the response of individual detectors. The response of the gamma-ray densitometer is the output voltage of the detector of the central radiation beam; the voltage is approximately proportional to the void fraction in the test section. A comparison of the three graphs of Figure 4 indicates that both the scatterometer, gamma-ray densitometer and the flow meter reflected the same trend. During the first 4,000 s of the test, the scatterometer showed that the void fraction was fluctuating around some mean value, while the output of the densitometer showed almost no change. This may indicate that while the conditions at the outlet of the channel were quite stable, some local boiling was taking place in the heated channel near the scatterometer's site. From 4,000 to about 14,440 s, the gamma-ray densitometer showed a significant amount of boiling and an oscillating behaviour. Within the same time period, the scatterometer reflected a somewhat smoother increase in void fraction. There were also some oscillations in the flow rate at the inlet of the channel. These may be due to the presence of an intermittent buoyancy-induced flow in the channel. In the period from 14,400 to about 30,300 s, all measuring devices indicated that the situation in the system was beginning to stabilize, and a bubbly flow regime may have been established. The change in void fraction at the end of the test was captured by the two devices. This change was due to the apparent refilling of the channel, as indicated by the reversal of the flow. The scatterometer did not reflect this refilling process as dramatically as the other devices. This may be due to the fact that the FES in the channel did not have a chance to completely cool-down, thus resulting in the continuation of some boiling in the part of the heated channel monitored by the scatterometer. In summary, the three measuring devices behaved, at least qualitatively, in a consistent manner.
Further consistency verification of the scatterometer's performance against other independent measurements are given in reference [2]. Fast Fourier Transform (FFT) was used to obtain the frequency spectra of the signals shown in Figure 4, to determine whether they reflected the same physical driving force. FFT was applied to the period from 16,800 to 24,000 s, where the measurements seemed to behave in a periodic fashion. The frequency characteristics for the scatterometer, gamma-ray densitometer and flow meter are shown in Figure 5. It can be seen that the shape and peak of the spectrum for the scatterometer's signal is very close to those of the densitometer and flow meter. This further validates the performance of the scatterometer. All the measurements discussed above were taken at locations far away from the position of the scatterometer; either at the inlet (flow) or outlet (gamma-ray densitometer, pressure and temperature). The only available measurement in the proximity of the scatterometer site was that of the temperature of a top FES and a bottom one. The data for the last portion of the test and are shown in Figures 6 and 7. The saturation temperature at the end of the test should be somewhat larger than 192°C, the saturation temperature corresponding to an outlet pressure of 1.3 MPa. The bottom FES temperature, as Figure 7 shows, does not exceed significantly this value. The variation in temperatures of the bottom detector is within the change in saturation temperature due to change in pressure. It is therefore reasonable to assume that the bottom FES remained covered with liquid during this test period. The temperature of the top FES indicates clearly that the FES was uncovered and exposed to superheated steam. Since during the same period, the temperature of the bottom FES stayed close to the saturation value, one can conclude that a stratified flow pattern was established within this time period. The scatterometer confirms these changes, Figure 4, by showing that the void fraction increases at the same time the temperature of the top FES increases (Figure 6). Moreover, the top detectors seem to overestimate the void fraction, while the bottom detectors predicted a lower void fraction [2]. This is characteristic of the scatterometer's response, when a stratified flow is established, where the bottom detectors reflect an increase in count rate due to their proximity to the liquid phase.
CONCLUSIONS This paper presented the results of an on-line testing of the scatterometer on one of the test sections of the RD-14M loop during a natural circulation test. The scatterometer's results were shown to be consistent with those inferred using other independent measurements, i.e. gamma-ray densitometer, flow meter, pressure gauge and thermocouples. In particular the scatterometer was able to reflect the strong boiling activity indicated by the temperature excursions of a top FES recorded in the neighbourhood of the scatterometer. The liquid stratification, inferred by the temperature of the bottom FES was also indicated by the scatterometer. In general, the test results confirmed that the scatterometer can directly measure the void fraction and provide flow regime information in the RD-14M heated sections. The estimated void fraction exceeded unity during the late stages of the test. This was attributed to a possible increase in the detector temperature. One needs however to verify this conclusion experimentally. This conclusion also indicates that a proper calibration process, or correction procedure, needs to be established to account for such temperature changes.
The scatterometer counting period was ten seconds, using a two-microgram californium-252 source that was about half decayed. A fresh ten-microgram source should enable the counting period to be reduced to two seconds, to match those of the other measurements.
ACKNOWLEDGEMENT This work was funded by the CANDU Owners Group as a part of an instrumentation development package.
References [1] HAN, P., HUSSEIN, E.M.A., INGHAM, P.J. AND HENSCHELL, R.M., "Nonintrusive Measurement of Transient Flow Boiling in Rod-Bundle Channels using Fast-Neutron Scattering", Nuclear Instruments and Methods in Physics Research, Vol. A353, pp. 695-698, 1994. [2] HAN, P., "Neutron Scatterometer for Void-Fraction Measurement of Two-Phase Flow in Channels with Metallic Inclusions", PhD Thesis, University of New Brunswick, Fredericton, N.B., March 1994.
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