Structural Integrity Analysis On Fuel Assembly With Ballast For Lead-Bismuth Cooled Reactor

As the good service performance of neutronics, thermal-hydraulic and safety characteristics, lead-based reactor is one of the most promising nuclear energy system for Generation-IV reactors. Fuel assembly is one of the key components in the reactor, so core structure and service environment are recognized as the important factors in the structure design of fuel assembly. There are many differences between lead-based cooled reactor and commercial reactor, so structure design and safety analysis of fuel assembly within and without the core are essential. This paper will mainly introduce the design criteria and structure design of fuel assembly serviced in the 10MW forced circulation lead-bismuth cooled reactor. Structural mechanics and modal analysis of fuel element and fuel assembly are studied by the finite element analysis method.On the basis of extensive investigation in service characteristics and fuel assembly of lead-based reactors in abroad, the design principles and limits of fuel assembly were proposed in the first place. Then, the study of structure design on fuel assembly with ballast in lead-based reactor was systematically carried out. The UO2 with 19.75% enrichment was chosen as the prime candidate. The structure material of cladding tube was 15-15Ti. The ballast material was depleted uranium. The fuel elements are arranged in a triangle and the wrapper was hexagonal. Wire was used to fix the elements and block was used to fix the fuel assemblies. With a view to the core characteristics, the design scheme of fuel element with large diameter and high enrichment was provided to decrease the core volume and increase the fuel ratio. The calculation results of temperature field showed that the central temperature of fuel pellet was 880.712℃ and the max temperature of cladding tube was 488.313℃. The simulation results of heat stress showed that the max strain of active zone was 0.1% in the normal condition and 0.4% in the serious accident condition. All the analysis results met the limit value of fuel assembly design. Therefore, the design scheme of fuel element with large diameter and high enrichment could meet the requirements of the safety assessment. It is well known that a conventional FA would float in the liquid lead-alloy due to the higher density. The design scheme of an integrated system of ballast and fuel element was provided to guarantee the correct position of fuel assembly during the running and refueling operation. The calculation results of temperature field showed that the max temperature of ballast and cladding was 406℃ and 402.053℃. The simulation results of heat stress showed that the max strain of ballast was 0.00245%. The mechanical analysis proved that components’strain was belower than limit values. All the above calculation results illustrated that the design scheme of an integrated system of ballast and fuel element could meet the requirements of the safety assessment.According to the numerical modal analysis results, the inherent frequencies and vibration types of fuel assembly under transport and service environment were obtained to guide the transportation and installation procedure, as well as structure design and seismic safety evaluation of other core components. The modal analysis of one box of fuel assembly showed that operation head had the max deformation. The weak parts of fuel assembly could be distinguished and optimized by modal analysis. Furthermore, seismic frequency was exerted to the simulation model to verify the structural integrity and support the final safety analysis report.Through the overview of state-of-the-art structure design and analysis of fuel assembly in lead-based cooled reactor abroad, this paper systematically carried out the study of structure design and simulation analysis of fuel assembly with ballast which services in lead-bismuth cooled reactor. The simulation results will support the final safety evaluation in the near future.