RELAP5 Calculates The Applicability Analysis Of Low Pressure Natural Circulation Flow Instability

The RELAP5 program has the significant advantages of accurate and reliable ,model diverse ,improved over years ,so is approved by IAEA and many countries' Nuclear Safety Review departments and widely used in a large number of reactors system security analysis work. However, much of the safety analysis work are based on the following basic fact :most research is targeted at high-pressure systems. In recent years, an increasing number of scholars have continued to point out that, in the description of some of the complex flow characteristics in low pressure conditions,such as the natural circulation and two-phase flow instability , results of RELAP5 calculation are unreliable. In view of the above questions ,The purpose of present study was to do the "Applicability of RELAP5 calculation program running under low pressure working conditions of natural circulation system", followed by the application of some typical low-pressure natural circulation system features.Firstly, two typical low pressure natural circulation system operating conditions (geyser boiling mode and flashing mode) were studied experimentally , the results show that, under the condition of low pressure natural circulation , The essential reason is caused by the pressure head periodic changes driven by the geyser boiling or flashing. Then, two typical sytems of low pressure natural circulation loop were simulated using the RELAP5/MOD4.0 codes,the calculated results and the experimental results were carried out a detailed comparative analysis. In particular, in the calculation of the flashing mode, with the the use of mixture gase (vapour + air + helium) as the heat source in the experiment, called two kinds of non condensable gas model in the RELAP5 codes .parameters involved in the comparison are the temperature,pressure and mass flow rate,the maximum error is 14.0%,9.38% and 20.0% ,respectively. The analytical results show that, RELAP5/MOD4.0 program can be applied to the calculation of two typical condition of low pressure natural circulation system,the program reliability of mixture model is relatively large.Finally, based on the capability of the program, apply the RELAP5 codes to do the follow-up study of two typical low pressure natural circulation systems. First do the sensitivity study of geyser boiling mode, studied the effects of loop length diameter ratio,horizontal section length and the heating power of three factors on the flow characteristics of the system. Subsequently ,for the flashing mode ,the application of RELAP5 on the heating section outer wall temperature, the ascending pipe end diameter and length of horizontal pipe section were analyzed . The calculation results show that: for the geyser boiling mode of natural circulation loop, increasing the loop pipe diameter and reducing the natural circulation loop length diameter ratio can significantly reduce the flow resistance in the circuit, then the two-phase natural circulation flow rate increase significantly. In the long-running stage, the length of the horizontal pipe section have no effect on the natural circulation loop circulation flow, but in the startup phase, the longer the horizontal pipe section is not conducive to the establishment of a two-phase natural circulation flow. Increasing the heating power is conducive to the establishment of natural circulation, so that the loop circulation flow is significantly increased;For the flashing mode of natural circulation system,the difference of wall temperatures can result in a different flashing point in different positions, resulting in different segments within the rising density, thereby affect the mass flow. Changing the ascending pipe end diameter does not affect the flashing point, but it can greatly reduce the average density of the gas-liquid two-phase within the whole ascent, making pressure head greatly increased. In the long-running stage of flashing mode system, the length of the horizontal pipe section also has no effect on circulation flow, just change the transition time from the single-phase to the two-phase. This conclusion is consistent with the geyser boiling natural circulation systems.The last part of the conclusion aims to provide some references for the natural circulation systems under low pressure actual engineering conditions.