Subchannel analysis of fuel assemblies of a lead–alloy cooled fast reactor

Lead–alloy cooled fast reactor is one of the six Gen-IV reactors. It has many attractive features such as excellent natural circulation performance, better shielding against gamma rays or energetic neutrons and potentially reduced capital costs. A natural circulation lead–alloy cooled fast reactor with 10 MWth is under design in China (hereafter called LFR-10MW). Fuel assemblies thermal hydraulic analysis is of vital importance for a successful design. A subchannel analysis code with flow distribution model was used to carry out the thermal hydraulic analysis. This work briefly gave the thermal-hydraulic design for the LFR-10MW and analyzed the thermal-hydraulic characteristics under steady-state condition using the subchannel analysis code. Whole core analysis was performed to locate the hottest fuel assembly using the code. The hottest fuel assembly was analyzed to obtain the cladding temperature, fuel temperature and coolant velocity. The maximum cladding temperature, the maximum fuel center temperature and the maximum coolant velocity are all below the design constraints. These results imply that the thermal-hydraulic design of LFR-10MW is feasible.     

Thermal-hydraulic behavior of a marine reactor during oscillations

The effect of ship motion, such as heaving and rolling, on the thermal-hydraulic behavior of marine reactors was investigated. The COBRA-IV-I CODE was modified to analyse the thermal-hydraulic performance on the critical heat flux under oscillating acceleration conditions. The critical heat flux in the code was verified experimentally using freon as a comparison. The Critical Heat Flux Ratio (CHFR) at the hottest channel of the PWR subchannel was analysed using the same code. A system code RETRAN-02/MOD2-GRAV was developed by improving RETRAN-02/MOD2 to simulate the thermal hydraulic transient under ship motion. It was verified by comparison using the experimental results of both two-phase natural circulation flow under heaving motion and single-phase natural circulation flow at an inclined attitude. The code was used to analyse reactor plant behavior in the nuclear ship Mutsu. Natural circulation flow during rolling motion was investigated experimentally. The characteristics of loop flow and core flow rates were clarified. The core flow rate correlated well with the Reynolds number of rolling motion.     

钠冷快堆分析程序ATHAS-LMR的子通道模型

以子通道模型和绕丝分布式阻力模型为基础,研发了液态金属快中子增殖堆热工水力子通道分析程序ATHAS-LMR,以对液态金属快中子增殖堆燃料组件中的热工水力现象进行分析。与国外知名实验和类似子通道分析程序比较,结果表明：ATHAS-LMR与实验结果及其他子通道分析程序的结果相近,能够完成包括堵流工况的各种工况下液态金属快中子增殖堆组件的热工水力性能分析。     

Experimental validation of the thermal-hydraulic code SACATRI

A sub-channel analysis steady state thermal-hydraulic code (SACATRI) was developed for the Moroccan TRIGA MARK II research reactor. The main objective of the thermal-hydraulic study of the whole reactor core is to evaluate the main safety parameters of the reactor core, and to ensure that they are within the safety limits for any operating conditions. The thermal-hydraulic model used in SACATRI is based on four partial differential equations that describe the conservation of mass, energy and momentum. In order to assess the thermal-hydraulic mathematical model of SACATRI, the present paper focuses on the quantification of the physical model accuracy to judge if the code is capable to represent the thermal-hydraulic behaviour of the reactor core with sufficient accuracy. The methodology adopted is based on the comparison between responses from SACATRI computational model and experimentally measured responses performed on the IPR-R1 TRIGA research reactor. The results showed good agreement between SACATRI predictions and the experimental measurements where the discrepancies observed (simulation-experiment) are less than 6%.     

一个高效核反应堆热工水力分析方法

提供了一个高效率的核反应堆堆芯热工水力分析方法。以子通道概念为基础，给出了描述堆芯流体流动与传热特性的数学模型和控制方程。文中采用了两相流的滑移流模型，并考虑了过冷沸腾的影响。引入若干补充关系式，用以确定空泡份额、湍流掺混、阻力系数及热力学参数等的大小，与广泛应用的ＣＯＢＲＡ系列程序不同，本文求解的是压力梯度方程而不是关于速度的方程，大大提高了数值求解的稳定性和计算收敛速度。初步的数值结果与实验结果的比较表明。本文提供的方法和程序是令人满意的。     

Introducing and validating a new method for coupling neutronic and thermal-hydraulic calculations

In this study, a new and innovative method is introduced for analyzing neutronic and thermal-hydraulic calculation. For this aim, VVR-S research reactor was selected, and the calculation procedure was performed for it. WIMS, CITATION and COBRA-EN codes were used for investigation. Calculation model consists of two sub-models: neutronic and thermo-hydraulic. The neutronic model uses WIMS and CITATION codes for neutronic simulation of the reactor core and calculating flux and power distribution over it. WIMS code simulates the fuel assemblies and CITATION models the core. The thermal-hydraulic model uses COBRA-EN code for performing the relative calculation. In this study, FORTRAN 90 program is used for linking two sub-models and performing the calculation. The proposed procedure is performed for VVR-S analysis and finally, the obtained results are compared with the experimental results that show good agreement with it.     

海洋条件下反应堆热工水力参数的子通道计算

针对海洋条件下反应堆的子通道热工水力分析,建立了海洋运动附加力模型和瞬态入口边界,将起伏、摇摆及复合运动的附加力关系式用于子通道模型的轴向和横向动量方程,并应用到COBRAⅢC程序将其改造为适应海洋条件的反应堆子通道分析程序。作为验证,计算了加热实验通道和"奥陆"堆在起伏运动情况下热通道的临界热流密度比(CHFR)、出口空泡份额和冷却剂流量,并与文献结果对比。还详细计算了"奥陆"堆在起伏、不同摇摆中心和复合运动情况下,热通道的CHFR和不同位置子通道出口的热工水力参数。研究表明:海洋条件下反应堆的子通道热工水力参数随运动呈周期性变化;起伏运动对子通道的压降影响较大,摇摆运动对子通道冷却剂的流量和温度影响较大。     

Code validation on a passive safety system test with the SMART-ITL facility

A thermal-hydraulic integral effect test facility, SMART-ITL, was constructed to examine the system performance of SMART, a 330 MWt integral type reactor, and to provide data for validation of related thermal-hydraulic models in the system analysis codes. SMART is equipped with various passive systems such as a passive residual heat removal system (PRHRS), a passive safety injection system (PSIS), and an automatic depressurization system (ADS). The PSIS of SMART is made up of four core makeup tanks (CMTs), four safety injection tanks (SITs), and related piping. Over 10 tests have been performed to investigate the behavior of a single train of a PSIS (a CMT and a SIT) in connection with PRHRSs and an ADS. Using a system analysis code, MARS-KS, we validated the experimental results for a representative test. All geometrical and thermal-hydraulic conditions of SMART-ITL were reflected in the code input construction. Through the validation process, several models, including a break flow model, heat transfer models, and pressure drop models, were examined. Overall, the major system parameters were well reproduced.     

RTDP方法在大型先进压水堆热工设计中的应用初步研究

偏离泡核沸腾（DNB）设计基准是反应堆热工水力设计中的重要基准之一,为评价该设计基准是否满足热工水力设计要求,首先需确定堆芯偏离泡核沸腾比（DNBR）设计限值。本工作详细论述了使用统计学方法确定运行参数及核设计参数等不确定性的RTDP原理,并应用该方法和堆芯子通道分析程序对大型先进压水堆DNBR设计限值及含汽率限值进行计算并给出结论,为DNBR设计基准的验证提供了关键判据。     

Application of Quick Subchannel Analysis Method for Three-Dimensional Pin-by-Pin BWR Core Calculations

Three-dimensional pin-by-pin core analysis is considered to be a candidate for the next-generation BWR core calculation method. In our previous study, the applicability of the transport and burnup calculations for a three-dimensional pin-by-pin BWR core analysis was investigated. However, the thermal-hydraulics calculation has not yet been studied in this framework. In the conventional core analysis code, the bundlewise thermal-hydraulics calculation is adopted. In the actual core analysis, the power distribution inside a fuel assembly is tilted at the region adjacent to a control blade or the core peripheral region. In these regions, the consideration of the subchannel-wise void distribution has an impact on the fission rate distribution. Therefore, an evaluation of the detailed void distribution inside an assembly, i.e., the incorporation of the subchannel wise void distribution, is desirable for the pin-by-pin BWR core analysis. Although several subchannel analysis codes have been developed, these subchannel analysis codes generally require a large computational effort to estimate the subchannel-wise void distribution in a whole BWR core. Therefore, to analyze a whole BWR core within a reasonable computation time, it was necessary to apply a fast subchannel analysis code. In this paper, a quick subchannel analysis code dedicated to pin-by-pin BWR core analysis is newly developed, and the void distribution of the present subchannel analysis code is compared with the prevailing subchannel analysis code NASCA using three-dimensional single-assembly geometries. Since the present subchannel analysis code is used for a coupled neutronics/thermal-hydraulics analysis, the results of the coupling calculation are also compared with those of NASCA. The calculation result indicates that the void distribution difference between NASCA and the present subchannel analysis code is slightly less than 10%. This result indicates that the prediction accuracy of the present subchannel analysis code will be reasonably appropriate for a pin-by-pin BWR core analysis. Furthermore, the results show that the calculation time of the present subchannel analysis code is only 10 min for a hypothetical three-dimensional ABWR quarter-core geometry using a single CPU. This calculation time is sufficient for a pin-by-pin BWR core analysis.     

Subchannel analysis in BWR fuel bundles

Steady-state two-phase-flow calculations have been performed with the multidimensional drift-flux code canal. Flow-regime-dependent drift-flux parameters have been used to evaluate the flow quantities in the subchannels. Consistent modeling of the mixing components, e.g. divergence cross-flow, turbulent mixing and void drift effect, has resulted in the good prediction capability of canal. Measurements of subchannel exit mass flux and quality from simulated BWR rod bundles have been used to assess the code capability. A wide range of operating conditions has been taken into consideration in addition to variations in uniform and nonuniform radial heat-flux profile. Comparison has been made with the familiar subchannel code cobra iiic. Prediction of corner subchannel quality and mass flux by canal are nearly always found to be better than cobra iiic. The overall performance of the drift-flux code canal is comparable to that obtained from advanced two-fluid codes. A review of the conservation equations and constitutive relations shows that the countercurrent transverse flow velocities are essential for accurate prediction of subchannel flow conditions.     

快堆系统分析程序FASYS堆芯分析模块验证

中国原子能科学研究院自主开发了快堆系统分析程序FASYS,已用于中国实验快堆的调试试验分析,目前正用于中国示范快堆的事故分析。FASYS程序包含堆芯分析模块、一二回路模块、事故余热排出系统模块等,其中堆芯分析模块包括点堆、衰变热、反应性反馈、堆芯通道热工水力模型等。本文采用解析解、DINROS程序、SAS4A/SASSYS-1程序验证FASYS程序的点堆模型;采用SAS4A/SASSYS-1程序验证FASYS程序的衰变热、反应性反馈和堆芯通道热工水力模型,各模型的验证结果均符合良好。对FASYS程序堆芯分析模块各模型的计算偏差和整体计算偏差进行评估,为中国示范快堆的事故分析提供参考。     

COSINE子通道均相流分析程序的研发

COSINE一体化软件包的子通道安全分析程序cosSubc基于子通道控制体三维网格模型,采用轴向及横向的热工水力控制方程,包括两流体和均相流两种求解算法。本文介绍了子通道均相流程序的物理模型和数值算法,并用cosSubc均相流程序和参考程序COBRA-TF分别对典型1 000MW核电厂稳态算例进行计算分析,结果表明:cosSubc均相流程序与COBRA-TF吻合较好,具备堆芯子通道的热工水力计算能力。     

Deformation analysis of fuel pins within the wire-wrap assembly of an LMFBR

Deformation of fuel pins within the wire-wrap fuel assembly of a fast breeder reactor is analyzed by two computational codes, the subchannel deformation analysis code SHADOW and the thermal-hydraulic analysis code DIANA. Coupling these codes makes it possible to analyze percisely the mechanical interactions between all fuel pins in an assembly, and the deviation of coolant temperature distribution in deformed flow channels from the nominal distribution.In this paper, particular attention is paid to the effect on fuel pin deformation of the following factors: dimensional changes in the fuel assembly components, displacement of wrapper tube walls and changes in the radial power gradients.     

Assessment of TAPINS code for application to thermal-hydraulic analysis of an integral reactor,REX-10

The thermal-hydraulic analysis program for integral reactor system (TAPINS) is a thermal-hydraulic system code developed by Seoul National University for transient analysis of an integral reactor, REX-10. Specialized for a fully passive integral pressurized water reactor, TAPINS adopts a one-dimensional four-equation drift-flux model for two-phase flows. It also consists of component models for the core, the helical-coil steam generator, and the steam-gas pressurizer. This paper presents the developmental assessment of TAPINS to validate its applicability to the thermal-hydraulic analysis of REX-10. Assessment problems are determined by taking into account thermal-hydraulic phenomena expected during design basis accidents of REX-10, including the loss-of-feedwater accident and the small-break loss-of-coolant accident. To confirm the predictive capability of TAPINS for these phenomena, the TAPINS model is validated against four sets of separate effects problems, including the pressurizer insurge test, the subcooled boiling experiment, the critical flow test, and the Edwards pipe problem. In addition, the calculation results of TAPINS are compared with the experimental data obtained from a series of integral effects tests using a scaled apparatus of REX-10. From the validation results, it is demonstrated that TAPINS can provide the reasonable prediction on the thermal-hydraulic responses of REX-10 during the transient and accident conditions.     

Subchannel analysis of a fast reactor cooled by supercritical light water

The concept of a high temperature fast reactor cooled by supercritical water (SCFR-H) was developed for achieving high thermal efficiency and a compact reactor system. The core characteristics were obtained from single channel thermal-hydraulic analysis. Thus, it is necessary to carry out subchannel analysis to estimate the effect of local power peaking and cross flows. For this purpose, a subchannel analysis code is developed. It is verified by comparing the results with experimental data of High Conversion Pressurized Water Reactor (HCPWR). Sensitivities of the outlet coolant and cladding temperature to the subchannel flow area and local power peaking are high. One of the reasons is that the ratio of the coolant flow rate of SCFR-H to the power is smaller than that of LWR. Another reason is that, temperature of supercritical water is more sensitive to the enthalpy change above 450°C. The outlet coolant temperature distribution can be flattened by reducing the area of the peripheral subchannels and by enhancing the mixing between the subchannels.     

Three-dimensional transport coefficient model and prediction-correction numerical method for thermal margin analysis of PWR cores

Combustion Engineering Inc. designs its modern PWR reactor cores using open-core thermal-hydraulic methods where the mass, momentum and energy equations are solved in three dimensions (one axial and two lateral directions). The resultant fluid properties are used to compute the minimum Departure from Nucleate Boiling Ratio (DNBR) which utlimately sets the power capability of the core. The on-line digital monitoring and protection systems require a small fast-running algorithm of the design code. This paper presents two techniques used in the development of the on-line DNB algorithmFirst, a three-dimensional transport coefficient model is introduced to radially group the flow subchannel into channels for the thermal-hydraulic fluid properties calculation. Conservation equations of mass, momentum and energy for these channels are derived using transport coefficients to modify the calculation of the radial transport of enthalpy and momentum.Second, a simplified, non-iterative numerical method, called the prediction-correction method, is applied together with the transport coefficient model to reduce the computer execution time in the determination of fluid properties.Comparison of the algorithm and the design thermal-hydraulic code shows agreement to within 0.65% equivalent power at a 95/95 confidence/probability level for all normal operating conditions of the PWR core. This algorithm accuracy is achieved with 1/800th of the computer processing time of its parent design code.     

A two-dimensional analysis of in-vessel buoyancy driven flows during a degraded core scenario

A transient two-dimensional thermal-hydraulic analysis of a full scale PWR uncovered degraded core heat up scenario based on the full Navier-Stokes equations was performed. The results show that the buoyancy forces produce significant two-dimensional circulations that induce strong mixing within the reactor pressure vessel. The results were also used to quantitatively assess the inadequacy of neglecting multidimensional dynamic effects in subchannel modeling of the in-vessel details of the reactor pressure vessel degraded core thermal-hydraulics.     

压水堆燃料辐照性能多物理耦合并行分析程序开发

核燃料元件是反应堆的核心部件,其性能影响反应堆的安全性与经济性,利用燃料元件性能分析程序开展燃料堆内稳态辐照性能分析对于燃料设计及安全评价具有重要意义。通过开发燃料温度分布、变形计算、裂变气体释放及内压等模型,结合燃料元件热工-力学多物理耦合计算分析耦合方案,基于先进并行计算方法构建了高性能并行化燃料性能分析程序Athena。利用典型商用压水堆核电站数据及同类程序计算结果进行了程序初步验证,结果表明Athena程序计算结果合理可靠。通过定义堆芯功率及热工水力边界条件,程序能够并行开展压水堆全堆芯燃料辐照性能分析,提高燃料辐照性能分析效率,是数值反应堆原型系统（CVR1.0）的重要组成。     

The reactor dynamics code DYN3D – models,validation and applications

The article provides an overview of the reactor dynamics code DYN3D. The code comprises various 3D neutron kinetics solvers, a thermal-hydraulics reactor core model and a thermo-mechanical fuel rod model. The implemented models and methods and the capabilities and features of the code are described. Latest developments of models and methods are delineated. An overview on the status of verification and validation is given. Code applications for selected safety analyses are described. Furthermore, multi-physics code couplings to thermal-hydraulic system codes, CFD and sub-channel codes as well as to the fuel performance code TRANSURANUS are outlined. Developments for innovative reactor concepts, in particular Molten Salt Reactor, High Temperature Gas-cooled Reactor and Sodium Fast Reactor are delineated. The management of code maintenance is briefly described. An outlook on further code development is given.