Parallel Simulation Techniques For Thermal-hydraulics Of Virtual Reactors

The virtual nuclear reactor is a complex software system built on supercomputers,and it conducts high-fidelity simulation for physical phenomena in reactors.The virtual reactor plays a significant role in many research domains including reactor design,power uprate and life extension.CVR(China Virtual Reactor)1.0 is a software system developed in the Development of CVR Protype and Demonstration project,which is supported by the National Key Research and Development Program of China.CVR 1.0 aims at leveraging the Chinese supercomputers to conduct high-fidelity simulation for physical phenomena in reactors including thermal-hydraulics,neutronics,and material radiation effects.The paper reports the key techniques of developing the thermal-hydraulic(T-H)simulation software in CVR 1.0.Based on the two widely used T-H simulation methods,subchannel method and computational fluid dynamics(CFD),the research is carried out from three aspects:reactor core object modeling and data automatic loading,adaptive parallel task partition,and parallel optimization on heterogeneous architectures.High-fidelity T-H simulation is realized on "Tianhe ?"and“Sunway TaihuLight" supercomputers,which proves the effects of the techniques raised in the paper.The main work and achievements of the paper are as follows:(1)The subchannel/rod amount is large and their relations are complex in full--core and high-resolution subchannel simulation.The widely-used manual methods to provide information are laborious and error-prone.A reactor core object modeling and data automatic loading technique for subchannel simulation is proposed in the paper.The technique is applicable for both quadrilateral and hexagonal assembly reactors.Based on the reactor core object model,input data can be loaded automatically,and the user input information is reduced from more than 200 thousand lines to about 400 lines.Based on the mathematical and physical models of the subchannel method,the paper analyses the requirements and challenges of preparing input data.The paper evaluates the simulation entities and identifies their main features,based on the simulation objects including assembly,rod and subchannel,a model is built to accurately describe the reactor core.The paper raises a technique to automatically load information based on the reactor-core-row-column-coordinate-system.The technique automatically loads attributes and automatically builds the relations of simulation objects.The technique reduces user input information and improves the flexibility of subchannel software.(2)The existing task partition methods of subchannel software have limitations on flexibility and scalability.The paper proposes a self-adaptive task partition technique.The technique chooses appropriate partition methods according to the core numbers of the computers/clusters used,which enables the subchannel simulation software to be deployed to various computing systems,including personal computers,small-scale servers and supercomputers.According to the structure features of reactors with quadrilateral or hexagonal assemblies,an"assembly equally partition" method based on domain decomposition is proposed.This method partitions the reactor core or assembly according to the computing core numbers of the computers/clusters,and it maps the computation tasks and processes flexibly.The paper investigates the appropriate core number for simulation by quantitively analysis of simulation with different geometry settings(different assembly,fuel rod and axial layer settings).(3)According to the architectures of Chinses home-grown supercomputers,the paper reports an optimization technique for CFD software using spectral element method(SEM).Based on the theory of SEM,the paper analyses the kernel of SEM CFD software and the challenges to optimize it.The paper investigates the features of some representative hybrid architectures of supercomputers and proposes the porting and optimizing methods accordingly.The paper describes the porting and optimizing work on the "Sunway TaihuLight" supercomputer in detail,including optimizing on manage processing elements and computation processing elements,and the static memory management.(4)Utilizing the techniques proposed in the paper,high-fidelity T-H simulation based on the subchannel and CFD methods are realized,which demonstrates the effects and parallel performance of the raised techniques.A high-fidelity subchannel simulation which calculates 157 assemblies with each subchannel divided into 125 axial layers is carried out on the "Tianhe ?" supercomputer using 4?6,280 cores.The running time is less than 8 minutes on 6,280 cores.A SEM-based CFD simulation is carried out on the "Sunway TaihuLight" supercomputer which calculating 621,610 spectral elements and about 8.5 billion grids.Scaling to 20,480 core groups(1,331,200 cores),the performance is improved about 40%compared with a typical SEM-based CFD code Nek5000.