PSI - Issue 60

Keshav Mohta et al. / Procedia Structural Integrity 60 (2024) 36–43 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

39

4

3. Important considerations: analysis of Calandria assembly under severe accident Severe accidents are inherently complex, multi-physics phenomena. However, all the phenomena during the accident progression need not to be modelled for structural assessment of Calandria, and only the relevant ones may be considered with reasonable engineering approximations. Key considerations are summarized in the following text. 3.1. Initial conditions for the severe accident analysis The events during the LCDA phase and during the transition from LCDA to SCDA phase, i.e. before the core collapse, do not pose major concern for the structural integrity of Calandria. Therefore, the LCDA phase analysis can be carried out separately using the relevant modeling and simulation tools, to determine the reactor core degradation till core collapse. The state of the reactor core at the instant of core-collapse is then considered as the initial condition for SCDA phase analysis for structural assessment of Calandria. 3.2. Considerations for thermal hydraulic analysis Key considerations for thermal hydraulic analysis are as following: a) Heat inputs due to decay heat and metal water reaction, heat exchanges up to the vault environments. b) Modeling of conduction, convection and radiation heat transfer modes. c) Modeling of PHWRs specific reactor components and features: horizontal cylinder, debris bed shape etc. d) Modeling of heat sinks (various water inventories- moderator, end shield water, Calandria vault water) and their gradual depletion during the accident progression. e) Modeling of core debris melting, corium- Calandria wall interaction, ablation, wall-thinning etc. f) Temperature dependent properties and correlations for material, as well as debris properties. g) Adequate geometric discretization for evaluating temperature- time histories relevant to structural analysis. 3.3. Considerations for structural analysis Key considerations for structural analysis are as following: a) Realistic modeling of constraints owing to boundary conditions and flexibility of different parts/ regions b) Temperature dependent deformation properties of Calandria material- base and weld c) Consideration of material and geometric non-linearity d) Various mechanical and transient temperature loads e) Modeling of material discontinuity (weldment) and geometric discontinuity (nozzle openings etc.) 4. Analysis methodology The salient features of the analysis methodology used for structural assessment of Calandria assembly are as following: 4.1. Sequentially coupled thermo-mechanical analysis During the accident progression, the Calandria vessel would be subjected to high temperatures and steep temperature gradients. It would lead to large thermal stresses in the vessel due to internal and external material constraints. While it is crucial to account for the temperature gradients as well as their evolution during the accident for estimating the stress/ strain distribution, the resulting Calandria deformations are not deemed to influence the heat transfer significantly. In view of this, sequentially coupled thermo-mechanical analysis has been considered suitable for present assessment. With inputs received from the thermal analysis, 3D, detailed elastic-viscoplastic finite element analysis with temperature dependent plastic and creep material properties and non-linear geometric considerations has been carried out. The tensile and creep stress rupture properties of the Calandria material were obtained from in-house tests for temperature up to 1100°C (Mohta et al., 2020a)