Safety boundary of flow channel partial blockage in plate-type fuel assembly
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摘要: 为掌握板状燃料组件内多个流道堵塞下的流动换热特性，获得流动堵塞致传热恶化的触发边界，以提高板状燃料反应堆的运行安全性，以典型板状燃料堆JRR-3M的标准燃料组件为对象，基于定性分析将流道堵塞事故分为非相邻流道堵塞与相邻流道堵塞两类，采用计算流体动力学软件ANSYS Fluent对两类流道堵塞事故下的流动换热特性进行模拟。模拟结果表明：非相邻流道完全堵塞或相邻流道最大堵塞率低于35%，流道内不会发生局部沸腾且燃料最高温度低于许用温度。基于上述结果，可确定JRR-3M反应堆在堵流事故下的安全运行边界。Abstract: It is necessary to obtain the triggering boundaries of heat transfer deterioration by mastering the flow and heat transfer characteristics in plate-type fuel assembly with multiple channels blocked, to improve the operation safety of plate-type fuel reactors. Based on qualitative analysis, the flow channel partial blockage accidents can be divided into non-adjacent channel blockage accident and adjacent channel blockage accident for the standard fuel assembly of the typical plate-type fuel reactor JRR-3M. Furthermore, the simulations of the flow and heat transfer characteristics under the two types of accidents were carried out using the computational fluid dynamics software ANSYS Fluent. The simulation results show that local boiling will not occur in flow channels and the maximum fuel temperature will be lower than the allowable temperature when non-adjacent channels are completely blocked or the maximum blocking rate of adjacent channels is less than 35%. Therefore, the safety operation boundary of JRR-3M reactor under flow channel blockage accident can be determined.
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There is an opinion that the most sensitive to the effects of pulse disturbances are the technical systems incorporating semiconductor devices, while high-voltage equipment is immune against them. This conclusion is based on the results of tests of high-voltage equipment when the operating voltage has not been simulated (power off). But even short duration voltage pulses, such as those created by HEMP or HPEM threats, are capable to initiate a spark short circuit. This short circuit can outgrow into an arc flashover under the effect of the operating voltage. As a result, the destruction of insulators and the failure of the high-voltage equipment can take place. Such effects can lead to catastrophic phenomena in power supply systems. Therefore, it is important to investigate flashovers and damages of power supply system elements due to high voltage pulses with power on and power off. A set of publications were devoted to the investigation. For example, the results of investigation of flashovers and damages of power line insulators due to high voltage pulses with power on and power off are described in Refs.[1-4]. These investigations have shown that high-voltage insulators could be destroyed as a result of joint action of a pulse disturbance and operation voltage of a power line.
However, insulators are not the most important elements of power systems. High-voltage transformers are much more important and expensive elements. Our thoughts concerning ways of transformer test to joint action of pulse disturbance and operating voltage of a high-voltage power line are presented in this article.
1. Transformer test similar to the one used for testing high-voltage insulators
This test can be performed using the equipment developed for testing insulators of high-voltage power supply lines. General view of the experimental setup is shown in Fig. 1. The setup consists of two simulators, namely, a high-voltage pulse simulator and a power line operating voltage simulator.
Figure 1. General view of the experimental setupA block-diagram of the high-voltage pulse simulator is presented in Fig. 2. Basic elements of the simulator are: charger, capacitor store with controlled discharger, high-voltage generator, generator of delayed pulses (delayed-pulse oscillator), and ignition device.
Figure 2. Block-diagram of the high-voltage pulse simulatorA high-voltage pulse being formed by this simulator acts onto a device under test (DUT).Capacitive voltage divider, current transformer, optical isolator, as well as digital oscilloscope are used for measuring parameters of the simulated pulses. The charger is intended to charge the capacitive storage up to a certain voltage. By changing this voltage from 3 kV up to 20 kV, it is possible to change amplitude of a high-voltage pulse from 60 kV up to 400 kV.
After start of the controlled discharger the capacitor storage is discharging on the high-voltage generator, which forms a pulse with necessary parameters.
The generator of delayed pulses controls the work of the ignition device and starts the digital oscilloscope. Besides, it controls the work of the ignition system of the power line voltage simulator. Thus, a high-voltage pulse may be timed to occur at any point of waveform of a power line operating voltage.
As a source of high-voltage pulses the generator based on exploding wires was used. This generator is shown in Fig. 3. It consists of the inductance and the block of exploding wires. Explosion of conductors occurs in the cylindrical chamber with a diameter of 60 mm and a length of 1550 mm. It was filled with nitrogen at pressure up to 1 MPa. Copper wires with a diameter of 40, 50 and 80 μm were used in experiments. Parameters of a generated pulse can be changed by means of changing the diameter and quantity of exploding wires.
Figure 3. Pulse generator based on exploding wiresThe generator based on exploding wires forms the pulse with the following parameters: peak voltage 60-400 kV; rise time 30-100 ns; pulse duration 50-500 ns. The waveform of generated pulse is shown in Fig. 4.
Figure 4. Waveform of the generated pulseThe block-diagram of the test equipment used to reproduce a power line operating voltage is shown in Fig. 5.
Figure 5. Simulator of power line operating voltageThe basic element of the simulator of a power line operating voltage is the air-core pulse transformer. Fig. 6 shows the air-core transformer. It forms a voltage with an effective frequency from 30 Hz up to 100 Hz.
Figure 6. Air-core transformerBy changing the voltage of the capacitance storage one can control the amplitude of the reproduced power line operating voltage. Waveforms of a current in the primary winding of the air-core transformer and an open-circuit voltage in its secondary winding are presented in Fig. 7.
Figure 7. Current (I) in primary winding and the open-circuit voltage (U) in secondary winding of the air-core transformerThe open-circuit voltage of the transformer has an amplitude of 15 kV when the voltage of the charger is equal to 3 kV. The open-circuit voltage delays on 90° from the current in primary winding of the air-core transformer.
Required amplitude of the power line current may be reproduced in the secondary short-circuit winding. Waveforms of the current in the primary winding and the short-circuit current in the secondary winding are presented in Fig. 8. One can see that the current in the primary winding is about 10 kA. In this case the short-circuit current in the secondary winding is about 1 kA. This current will be a current of arc after overlapping of the DUT. Currents in the primary and secondary windings are in phase. Thus, the line operating voltage and the arc current will be 90° out of phase at power-on testing.
Figure 8. Currents in windings of the transformer at short circuitControl pulses of the delayed pulse generator start ignition systems of the high-voltage pulse simulator and the power line operating voltage simulator. Thus, time delay between the high-voltage pulse and maximum of the line operating voltage may be in range from several microseconds up to several milliseconds.
To measure parameters of the reproduced pulses and to register processes of DUT overlapping the following measuring tools are used:
— Digital oscilloscope;
— Digital camera (exposition time is 0.03 ms, shooting frequency is 300 Hz);
— Rogovski coil;
— Capacity divider (rise time 5 ns, factor of division 1∶350 000);
— Fiberoptic line;
— Resistive high-voltage divider.
Photos of the resistive high-voltage divider and the capacity divider are presented in Fig. 9 and Fig. 10.
Figure 9. Resistive high-voltage divider
Figure 10. Capacity dividerIt is necessary to remind that the experimental setup described above has been used for high-voltage insulators tests. Naturally, the scheme of transformers tests should be different, as is shown in Fig. 11.
Figure 11. Transformers test scheme based on the equipment developed for insulators testApparently, this scheme does not need comments as it is analogous to the scheme of insulators tests described above. However, it should be noted that it has a shortcoming: only one phase is being exposed to influence of test pulses. The schemes overcoming this shortcoming are presented in the following section.
2. Transformer test methods using diesel-generator
The first variant of the transformer test with the use of a serial 10 kV diesel-generator is shown in Fig. 12. Besides the diesel-generator, this scheme includes three high-voltage generators and a device for their synchronization.
Figure 12. Scheme of the transformer test by means of a diesel-generator and three high-voltage generatorsThis scheme is much simpler in realization in comparison with the previous one, as it does not demand building of the simulator of an operating voltage for the high-voltage power line. However it has two shortcomings, one is that the extremely powerful high-voltage diesel-generator must be used, the other consists in using three synchronously functioning generators of high voltage pulses. It is possible to eliminate this difficulty by using the optimized scheme shown in Fig. 13.
Figure 13. Scheme of the transformer test by means of a diesel-generator and influencing loop circuitPractical application of the optimized scheme shows that it is the simplest in realization in comparison with the other two of the three schemes. Yet it still has shortcomings. The main shortcoming is that the maximum voltage concerning the earth which can be induced by means of an influencing circuit does not exceed 100 kV.
3. Conclusions
The previous researches showed that joint action of a high-voltage pulse disturbance and an operating voltage of a power line leads to destruction of insulators of this line. This fact testifies about need of assessing immunity of other elements of power infrastructure to similar influences. A set of ways which can be used for test of high-voltage transformers is presented in the article. One of them is similar to the way used for test of insulators. It includes the high-voltage pulse simulating an electric disturbance and the pulse which simulates operating voltage of a power line being given to the transformer. The main shortcoming of this way is high cost of the simulator of the line operating voltage. For this reason, as a rule, only one phase of the transformer is being tested.
In the article two more ways of tests, free from this shortcoming, are offered. They allow applying the testing pulses to three phases simultaneously. A high-voltage pulse disturbance is simulated by means of three generators or by means of the loop circuit with a current which induces disturbances in all wires of the line simultaneously. An operating voltage of the line is simulated by means of the high power diesel-generator.
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图 3 非相邻流道完全堵塞事故分析结果
Figure 3. Analysis results of non-adjacent channel complete blockage accident
material density/(kg·m−3) specific heat/ (J·kg−1·K−1) thermal conductivity/(W·m−1·K−1) 6061Al 2700 896 170 U3Si2-Al 6000 406.7 32 
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表 2 不同网格量下的平均组件计算结果
Table 2. Average assembly calculation results under different grid sizes
number of grids pressure drop/kPa mean convective heat transfer coefficient / (W·m−2·K−1) maximum fuel temperature /K 104466 57.99 33196.41 339.65 200500 57.52 33197.97 339.44 453855 57.33 33120.20 339.92
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表 3 不同堵塞率及堵塞位置的分析结果
Table 3. Analysis results of different blockage rates and positions
No. blockage rate/% plug position Tfluid,max/K Tfuel,max/K Vch/(m·s−1) Tout/K 1 30 middle 337.82 365.61 5.29 329.12 2 35 middle 340.07 368.32 5.07 329.88 3 40 middle 343.56 371.82 4.89 330.68 4 45 middle 345.68 375.51 4.66 331.76 5 50 middle 358.15 390.59 4.39 333.14 6 55 middle 368.59 397.68 4.10 335.04 7 60 middle 383.58 413.68 3.75 337.67 8 30 side 345.35 402.03 5.44 328.44 9 35 side 369.32 415.48 5.29 329.13 10 40 side 387.61 436.60 5.08 330.02
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