تحلیل نیروهای ارتعاشی القایی غیردائم وارد بر یک مجتمع سوخت هسته‌ای در جریان محوری مغشوش

در قلب (مرکز راکتور) یک نیروگاه‌های هسته‌ای، مجموعه‌ای از میله‌های سوخت که توسط شبکه‌های نگهدارنده در کنار هم نگه داشته شده‌اند، یک مجتمع سوخت را تشکیل می‌دهند. عبور جریان خنک‌کننده از اطراف میله‌ها و ایجاد آشفتگی جریان، به‌خصوص در اطراف شبکه‌های نگهدارنده، نیروهای عرضی، ارتعاشی و غیردائم به میله‌ها اعمال می‌کند. تنش دوره‌ای ناشی از این ارتعاشات، علاوه بر خوردگی سایشی غلاف میله‌ها، باعث خستگی و کاهش استحکام مکانیکی آن‌ها خواهد شد. در این پژوهش، اندازه، دامنه و شدت نوسانات این نیروها به دست آمده و مورد تجزیه و تحلیل قرار گرفته است. برای بررسی اثر پره‌های مغشوش‌کنندۀ موجود روی شبکه‌های نگهدارنده، شبیه‌سازی‌ها برای دو نوع شبکۀ نگهدارنده با پره‌های مغشوش‌کننده و بدون پره‌های مغشوش‌کننده انجام شده است. مقایسۀ نتایج به‌دست‌آمده با نتایج تجربی، تطابق خوبی را نشان می‌دهد. نتایج نشان می‌دهد که وجود پره‌های مغشوش‌کننده باعث افزایش حدود 17% در افت فشار می‌شود. نیروهای واردشده بر شبکۀ نگهدارنده که در پایین‌دست جریان قرار دارد، 10% کمتر از نیروهای واردشده به شبکۀ نگهدارندۀ بالادستی است. آنالیز نوسانات نیروهای واردشده به مجتمع سوخت نشان می‌دهد که دامنۀ ارتعاشات در فرکانس‌های کمتر از 300 هرتز، نسبتاً بالاست.

Analysis of Unsteady Fluid Induced Vibration Forces on a Nuclear Fuel Rod Bundle in Turbulent Axial Flow

Extended AbstractIntroduction: In a nuclear power plant, a set of fuel rods held together by spacer grids form a fuel assembly. The motion of coolant flow around the rods for heat removal results in the creation of turbulent flow, especially around the spacer grids, which leads to the exertion of transverse, vibratory and unsteady forces to the rods. Periodic stress due to these vibrations, in addition to the fretting Vera, creates fatigue and reduces their mechanical strength. The vibration behavior of the coolant is also of great importance in other industrial equipment that uses turbulent flow for cooling. In most previous studies, the induced vibrations due to crossflow turbulence have been investigated and the models used to correctly analyze the applied forces and the factors affecting their behavior have been considered. In the present study, the cooling fluid flow parallel to the tube bundle and around the spacer grids will be investigated to analyze the unsteady forces applied. In this study, numerical simulation and unsteady Reynolds Averaged NavierStokes equations (URANS) will be employed. Transient forces of unsteady flow applied to rods calculated from the numerical method can be used for vibration analysis using vibrational codes such as vibration transient analysis.Materials and Methods: The geometry of the rod bundle is hexagonal. Since the transient simulation of the whole geometry requires a very high computational cost, a lot of time as well as a lot of computer memory, a part of the geometry is usually selected. Therefore, to investigate thermohydraulic parameters that cannot be obtained by experimental measurement and to compare numerical methods with experimental data, numerical simulations are performed for three rods with two spacer grids with mixing vanes.For this purpose, mass, momentum, and energy equations are solved. The equations are threedimensional and unsteady. After solving the above equations and obtaining the flow and temperature fields, other parameters such as pressure distribution, friction coefficient, and the amount of heat transfer and of heat transfer coefficient will be obtained. Finally, the analysis of vibration forces will be performed over time. Simulations have been performed for working pressure of 150 bar. The volumetric force applied to the fluid is gravity. The fluid properties of water depend on the temperature. The inlet velocity is equal to 5.6 m/s and the inlet temperature is 300 °C. Cosine heat flux is applied to the surfaces of the fuel rods, which are heat transfer boundaries. The outer boundaries of the system are selected as periodic. The governing equations are solved by a finitevolume method based on the incompressible pressurebased method for internal flows. SIMPLEC algorithm is used for pressure and velocity coupling using ANSYS FLUENT software by writing UserDefined Functions (UDF). Spatial discretization of all parameters has been performed by the secondorder upwind scheme. For the initial conditions, the velocity is zero, and the temperature is 291 °C.Results: To investigate the effect of mixing vanes on the spacer grid, simulations have been performed for two types of spacer grids with and without mixing vanes. First, to ensure the accuracy of the numerical results, the obtained results were compared with the experimental data showing a good agreement.In the study of pressure drop results, the trend of pressure changes has a decreasing behavior indicating a decrease in local pressure in passing through spacer grids and the recovery of part of the pressure.In the study of temperature, velocity, and viscosity for the rod assembly and their spacer grid with and without mixing vanes, it was observed that the presence of mixing vanes reduces the temperature of the rods. It was also observed that the presence of a spacer grid disturbs the flow uniformity. On the other hand, when the spacer grid does not have mixing vanes, the flow returns to its previous state in the shortest path after passing through the spacer grid. In the case of mixing vanes, the spacer grid assembly and the mixing vanes produce a pressure drop of nearly 4,000 Pa, while in the absence of mixing vanes the pressure drop due to the spacer grid is equal to 1700 Pa.In the study of the flow around the rods, it was observed that the Nusselt number increases locally as the flow passes through the fins. The range of Nusselt number changes from 400 to 450 in the area of ​​the flow passing through the spacer grid with mixing vanes. However, it is about 350 for the spacer grid without mixing vanes indicating an increase in heat transfer due to the presence of fins.Evaluation of the pressure forces applied to the spacer grid shows that the net pressure force applied to the spacer grid with mixing vanes is about four times that of the spacer grid without mixing vanes. Also, the pressure applied to the second spacer grid with and without mixing vanes is reduced by nearly 10% compared to the first spacer grid. The average amount of shear stress applied to the spacer grids is much less than the average net pressure stress. Therefore, it can be said that the maximum force applied to the spacer grid is the force perpendicular to the surface, i.e., the pressure stresses.Frequency analysis of the forces was performed using the distribution of forces over time to investigate the vibrations caused by the applied forces. By obtaining the power spectral density of the applied forces, the method of applying the forces and the vibration frequency and amplitude of the vibrations applied to the rods were obtained. It was observed that the amplitude of vibrations is very high at frequencies below 300 Hz, but the amplitude of vibrations is small at other frequencies. The jump occurs at frequencies between 2000 and 3000 Hz. It should be pointed out that the amplitude of this fluctuation is small.Discussion and Conclusion: In this study, thermohydraulic analysis and force analysis of parallel flow around the rods of a nuclear fuel bundle with two different types of spacer grids have been investigated. In the first case, the spacer grid had mixing vanes and in the second case, there were no mixing vanes. The results were compared with experimental data and other semiempirical results, and a difference of less than 10% was observed which was logical according to the assumptions. The results can be summarized as follows:1) Although the mixing vanes cause a large local pressure drop, they reduce the pressure drop in the distance between the two spacer grids by changing the upstream flow conditions. However, in any case, the overall pressure drop in the spacer grid with mixing vanes is greater than that without mixing vanes.2) The presence of fins increases heat transfer and, thus, reduces the wall temperature of the rods. Reducing the temperature of the rods can consequently reduce the risk of reaching a critical heat flux and can increase reactor safety. However, the presence of mixing vanes increases the induced vibrations.3) The amplitude of vibrations at very low frequencies, below 300 Hz, is very high. At other frequencies, the amplitude of the vibrations is very small. However, jumps have been observed at frequencies between 2,000 and 3,000 Hz. The amplitude of these fluctuations is very small.