PSI - Issue 60

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

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4.1.1. Separate geometric discretization for thermal and structural calculations Another key aspect of this analysis method is the provision of separate geometric discretization and specialized analysis tools for the thermal hydraulic and structural analysis. As mentioned in the Section 3.2 and Section 3.3 respectively, the considerations and the requirements for thermal hydraulic and structural analyses are different. Therefore, these are carried out separately using different geometric model, mesh as well as specialized, dedicated tools/ software. The thermal hydraulics analysis has been carried out using the Accident Source Term Evaluation Code (ASTEC) V2.1 (2016), developed by the French “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN), with PHWR specific addition/ modifications to some of its modules. Thermal analysis results are input as loadings to the finite element analysis for the structural assessment. These inputs include temperature time history transients of the different parts and regions of Calandria-end shield assembly, and the remaining water inventories- moderator, end shield water, Calandria vault water. 4.1.2. Geometric domain for structural analysis The Calandria has a stepped shell geometry. It is connected to end shield to form an integrated Calandria- end shield assembly. The assembly has parts such as annular plate and diaphragms to accommodate thermal expansions and limit the thermal stresses. It also has very specific fixity conditions (constraints), as well as displacement coupling between the Calandria side tube sheet (CSTS) and fueling machine side tube sheet (FSTS) through the lattice tubes. It is required to model the realistic boundary conditions and member flexibility of different parts/ regions to evaluate the structural response- displacement, stress, strain distributions in the Calandria- end shield assembly correctly. The structural analysis aspects, as mentioned in Section 3.3, necessitated that the finite element model comprised of the integral Calandria-end shield assembly. 4.2. Important failure modes and assessment criteria Failure assessment of Calandria assembly is carried out by the post processing of FEA results for the different failure modes, including those specified in IAEA TECDOC-1594. These are briefly summarized as following: 4.2.1. Plastic instability/ collapse With continuous rise in Calandria temperatures, the material strength diminishes. As a result, large plastic deformations set in, finally resulting into the complete plastic instability. This correlates with the rise in temperatures up to the level at which the Calandria does not have strength to even sustain the self-weight and debris dead weight. At this juncture, the force equilibrium requirements are not met and finite element analysis does not converge further. 4.2.2. Large inelastic strains Under the continuous plastic and creep deformations, the material ductility may get exhausted which is not explicitly modeled in the FEA. To assess the failure, a strain based failure criteria is defined under which the failure is considered when the through-thickness equivalent inelastic strain (sum of equivalent plastic and equivalent creep strain) exceeds a threshold value in a local region. Based on tensile and creep test data of SS304L, the threshold is specified as 10% for present case. 4.2.3. Failure due to creep-stress rupture Failure due to creep stress rupture is assessed by evaluating the accumulated life fractions, based on the linear summation of the partial life fractions. For this purpose, the partial life fractions are evaluated based on the FEA results and Larson-Miller Parameter (LMP) based correlations between stress, temperature and creep stress rupture life. For present study, the LMP correlations for Calandria material were obtained from the creep tests for temperatures up to 1100 °C.