Cladding designs for the lower head of central measuring shroud against thermal shock in SFRs

Abstract

In the normal operating conditions of sodium-cooled fast reactors (SFRs), the coolant near the core outlet keeps high temperature. However, in the scram conditions, the temperature of coolant from core outlet decreases sharply due to the reduction of core power. It makes the surface temperature of the central measuring shroud above the core outlet, which is used to supply the guiding of control rods and in-vessel measuring equipment, suddenly decrease. The maximum temperature change of the lower head of central measuring shroud (LHCMS) can reach 175 K in 20 s. Such condition will bring a severe thermal shock to the LHCMS. Several thermal shocks may bring fatigue damage to the LHCMS and threaten the safety of the reactors.Cladding can be used to protect the LHCMS from the fatigue damage of thermal shocks. Currently, there is no standard procedure for designing cladding of the LHCMS. In this paper, a numerical design procedure for designing cladding of LHCMS was proposed. The thermo-structural coupling method and ASME fatigue assessment method were used. Firstly, the models with different cladding thicknesses were established in finite element method (FEM). Secondly, the fatigue damage factors were calculated based on the strain results and fatigue assessment method, then, the design cladding thickness was determined according to Miner’s linear fatigue rule. In the meanwhile, if there is contact between the LHCMS and cladding, there will exist friction and squeeze that increase the stress and strain on the structures. Therefore, a reasonable gap distance should also be designed between the LHCMS and cladding after determining the cladding thickness. It should be noted that when designing the cladding thickness, in order to avoid the influence of the possible contact on the deformation of the LHCMS, special model settings were used so that the LHCMS and cladding can deform without interaction. Thirdly, the deformation values of the model with the design cladding thickness at different times were obtained, then the gap distance was determined according to the relative deformation values of the LHCMS and cladding. Finally, a model with the design cladding thickness and gap distance was established to verify design results.From the simulation results, under the boundary conditions of this paper, the cladding thickness should be 4 mm, and the gap distance between the LHCMS and cladding should be 4 mm. In addition, the effects of different cladding thicknesses on the temperature, stress and strain of the LHCMS were also studied. The results showed that under the scram conditions (temperature decrease), the temperature of the outside surface of the LHCMS increases by 10 K or 1.5%, the stress and strain of the outside surface of the LHCMS (away from the stress and strain concentration area) decrease by about 9% with every 1 mm increase of the cladding thickness at 20 s. Besides, the stress and strain concentration will increase the stress and strain by 17% to 21%. The numerical design procedure and results of this paper can provide reference and data for the design of the LHCMS cladding in SFRs, and thereby having good potential application.