Research On Structural Design For Cladding For Central Measuring Shroud Against Thermal Shock In A Fast Reactor

The central measuring shroud in a fast reactor is used to provide protection for in-pile measuring equipment and control rod driving mechanism.Its integrity is closely related to safety of the reactor.The central measuring shroud is located 500mm above the core outlet and is affected by coolant(liquid sodium)temperature for a long period of time.In the normal operating conditions,the central measuring shroud keeps in high temperature.However,in the scram conditions,the sudden decrease of power causes the temperature at the core outlet decreases rapidly,which makes the temperature of the surface of the central measuring shroud reduces sharply,resulting in thermal shock phenomenon.Severe thermal shocks may cause thermal fatigue or even failure of the central measuring shroud,which must be paid attention to in a fast reactor.In order to protect the central measuring shroud,cladding is usually added at the outside to reduce the temperature change of its surface.The reasonable design for cladding is important to reduce the influence of thermal shock on the structure.Theoretical analysis method and finite element method(FEM)can be used in structural design in engineering.Although at present,at home and abroad,there are a lot of researches on temperature fields,stress fields and structural designs under thermal shock,the theoretical analysis model of the structural design for cladding for central measuring shroud against thermal shock is not found.The finite element analysis and design model suitable for the structure and thermal shock temperature characteristics is not found either.Therefore,it is necessary to carry out researches on the theoretical design and finite element design models for cladding for central measuring shroud against thermal shock.The design for cladding can be divided into two aspects:one is the total thickness design to ensure the integrity of the central measuring shroud.The other is the single layer thickness design to ensure the integrity of cladding.As the first step of the research,a set of thermal shock transient analysis model was developed by adopting the theory of heat transfer differential equations,thermoelastic mechanics equations.Then,the formulary method for structural design path of cladding for central measuring shroud against thermal shock was established by combining ASME fatigue assessment method and the above model.The process of heat conduction,stress solving and fatigue assessment in any thickness interval of the model were studied in detail,and were verified by FEM.The central measuring shroud and cladding are independent of each other,so there can be two layout methods,including stacking(contact exists between layers)and separate(gaps exist between layers).There can also be two analysis methods,including elastic analysis and elastoplastic analysis.Based on this discussion,in the second step of the research,a detailed analysis method design procedure for cladding was proposed.The design steps for cladding under different layout and analysis methods were given.The steady and transient temperature simulation method,the analysis process when there are gaps and contact,the stress and strain solving based on elastic and elastoplastic analysis,and fatigue assessment method of central measuring shroud and cladding under thermal shock were studied in detail.The elastoplastic analysis method was verified by experiment.The third step of the research was applying formulary method and the analysis method design procedure to design for cladding for actual central measuring shroud against thermal shock,and the results were compared and analyzed.The design results of the formulary method show that the total thickness of cladding is 6mm,cladding is divided into two layers,and the single layer thickness is 3mm.When there are gaps between layers,the structural design results of the cladding based on FEM elastic analysis is that the total thickness is 6mm,cladding is divided into two layers and the single layer thickness is 3mm,the gap distance(between the central measuring shroud and each cladding layer)between layers is 2.54mm.When there are gaps between layers,the design results of the cladding based on FEM elastoplastic analysis is that the total thickness is 2mm,there is no need for layering,which means the single layer thickness is 2mm as well,and the gap distance between layers is 2.648mm.When there is contact between layers,the design results of cladding based on FEM elastic analysis is that the total thickness is 4mm,but the single layer thickness cannot be designed.When there is contact between layers,the design results of cladding based on FEM elastoplastic analysis is that the total thickness is 2mm,there is no need for layering,but the single layer needs to be increased to 9mm.The above results are the critical values to meet the design requirements.The comparison and analysis show that the elastic stress of cladding in gap models is obviously smaller than that in contact models.The total strain range of cladding in elastoplastic analysis is less than that in elastic analysis.Therefore,the single layer thickness of cladding in gap model is significantly reduced compared with contact model,the reduction range is 77.8%.The total thickness and single layer thickness of cladding in elastoplastic analysis are both obviously decreased compared with that in elastic analysis.The average reduction range of total thickness is 58.35%,and the reduction range of single layer thickness is 33.3%.It can be seen that the stacking layout(contact model)for central measuring shroud and cladding may be more beneficial for engineering assembly,but the problem of increase of cladding single layer thickness in design process needs to be considered.Separate layout(gap model)may be beneficial to reduce the size and volume of cladding single layer,but it may have difficulty in engineering assembly.Compared with elastic analysis,although the elastoplastic analysis occupies more computational resources,the design conservatisem can be reduced.In this paper,the in-depth theoretical and numerical simulation researches on the structural design for cladding for central measuring shroud against thermal shock in a fast reactor were carried out.The structural design model for cladding based on formulary method was developed.Analysis method design procedures based on FEM were proposed.The convenience of formulary method after programming can make it play a great role in preliminary design.The accuracy of analysis method make it have a good application prospect in detailed design.The results can provide method references for the structural design for cladding for central measurement shroud against thermal shock in a fast reactor.Meanwhile,several cladding design results were given,the results of temperature,stress,strain and fatigue were compared and analyzed.These data can be used as benchmark data for cladding design and analysis.