Figure 3 shows a test assembly, which has a configuration accepted by the KSNP (Korean Standard Nuclear Power Plants), and has been developed to obtain a thermal margin increase when compared to the current fuel. The fuel assembly consists of 236 fuel rods, 4 guide thimbles and an instrumentation tube, 11 girds, which include 2 Inconel top/bottom grids, and 9 Zircaloy mid girds. The intermediate spacer girds have the localized design of hybrid mixing vane and H spring. The bottom end piece is designed as a wavy type capable of debris filtering. The guide thimbles/instrumentation tube, grids and top/bottom nozzles form the skeleton of the fuel assembly. The girds are fabricated from Zricaloy strips interlocked in an egg crate shape and welded together. The grids maintain the rod pitch over their fuel rods by providing a positive lateral restraint. The fuel rods are restrained from an axial motion by the frictional forces developed by the grid springs. Each cell of the grid contains two springs and four dimples. Since the grid cell size have an effect on the assembly stiffness and damping, the cell size has to be checked before fabrication. The assembly contains 4 instrumented fuel rods to measure the rod vibration. Each instrumented fuel rod contains two uni axial accelerometers, which are positioned with a right angle rotation to detect an x and y directional movement. Annular tungsten-carbide bushings are used in place of uranium pellets to allow for a route for the accelerometers signal cables through the rod. The instrumented fuel rods are located at the corner of the fuel assembly. The linear weight of the tungsten-carbide bushing is the same as that of the uranium pellets. The rest of the fuel rods are filled with lead to obtain the same weight as that of the uranium pellets.
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FIG. 3. Schematic diagram of test assembly.
4. INSTRUMENTATIONS
Test loop instrumentations include those that are necessary to monitor, record and control the loop operating parameters and also those required for an accurate determination of the pressure loss, lift-off flow rate, and the vibration for the fuel rods, assembly, housing and vessel.
4.1. Differential pressure
Differential pressure from the loop flow rate orifice, from the storage tank level, and from the test section measured differential pressure tap locations are all measured by a pressure transducer (Rosemount model 3051). Figure 4 shows the locations of the differential pressure taps in the test section. All the 1/8” diameter stainless steel pressure transmission lines attached to the pressure taps on the flow housing wall penetrate the pressure vessel through an instrumentation ring located above the test chamber. The pressure tap is located downstream of the spacer which minimizes turbulence effects generated from the mixing vane. The resulting 4-20 mmA current output signal from the transmitter can be transformed to a 0-5V voltage signal by a transformer (Myung Model M8DY1) and then through end plug (HPVXI E1419A) the signal is monitored and recorded by the HP VEE [3] as shown in Fig. 5.
11th Grid
FIG. 4. Location of the pressure taps, DVRT, and Housing accelerometer.
Test
FIG. 5. Data acquisition system.
4.2. Static pressure
Static pressure is measured by a pressure transmitter (Rosemount model 306)) with a range from 1 to 30 bar. The pressure transmitter is located at the pressureizer vessel, inlet, and outlet test section. The signal output from the pressurizer vessel is transformed to a voltage out and provided to both the pressureizer heater controller, which controls the SCR automatically and the VXI end plug to record it on the HPVEE. The pressurizer signal output from the inlet and outlet of the test section is displayed on the monitor.
4.3. Coolant temperature
The temperature is detected by the T type thermocouple (Okajaki Model T91) located at the test section inlet. One of the dual type temperature signals is compared with the pre-set point, the resultant controlled output is transferred to the control circuit of the SCR controlled loop heater. The manually controlled loop heater may be turned on or off as needed so that the SCR heater will remain in a controllable range. The other temperature signal is displayed and rescored by the HPVEE.
4.4. Fuel assembly vibration
DVRT (Differential Variable Reluctant Transducer) is used to measure the fuel assembly vibration and displacement. DVRT can measure a vibration from 0 to 7 KHz, with a range of 0–5 mm, which is more than twice that of the gap between the outer strip and the housing. The DVRT mounted on the flow housing walls measures the grid motion relative to the flow housing. The DVRT operated by the eddy currents induced in the target. The test assembly vibration is measure by the sensing transverse displacements at a mid gird in both the x and y directions. Transducers are mounted on the flow housing wall at the 6 spacer grid level which is expected to have high amplitude at mode 1 as shown in Fig. 4. Figure 6 shows the non contact DVRT, the threaded stainless nets can be mounted into the screw tap, available in advance, in the housing wall.
FIG. 6. Non-contact DVRT.
4.5. Rod vibration
Rod vibration will be monitored via the measurement of the acceleration signals obtained from the two uni-axial accelerometers (PCB model 352B01) axially positioned with a right angle rotation in a fuel rod [4, 5]. The accelerometer can measure a 0 to 10 KHz. The voltage signal from 7 to 11 V goes through the amplifier (B&K model nexus) and then it is monitored and recorded by the analyzer (MTS T-DAS) [6].
4.6. Housing vibration
Housing vibration is measured by two uni-axial accelerometers (RION Model PV-10B) mounted by screws on the flow housing. The locations of the accelerometers mounted on the pressure vessel and the flow housing are shown in Figure 4. The two sensors are positioned with a right angle rotation to measure the x and y directional movement. The sensors are located at the middle level of the housing, which is expected to have the maximum amplitude at the first mode. The accelerometer can measure a 3 to 8 KHz. The signals from the accelerometers pass through the B&K Model NEXUS amplifiers to the VXI end plug. The signals are annualized by the MTS T-DAS program.
4.7. Vessel vibration
angle rotation to measure the x and y directional movement. The sensors are located at the middle level of the pressure vessel which is expected to have the maximum amplitude at the first mode. The accelerometer can measure a 3 to 3 KHz. The signals from the accelerometers pass through the B&K Model NEXUS amplifiers to the VXI end plug. The signals are annualized by the MTS T-DAS program.
0o 270o 90o 180o
2818 mm2818 mm
Accelerometer 90o, 180o
FIG. 7. Location vessel accelerometer.
5. TEST METHODOLOGY