@article{shaw_mahajan_hassan_2023, title={Critical Evaluation of a Novel Analysis Technique for Assessment of Printed Circuit Heat Exchangers in High-Temperature Nuclear Service}, volume={145}, ISSN={["1528-8978"]}, DOI={10.1115/1.4057061}, abstractNote={
 Application of Printed Circuit Heat Exchangers (PCHEs) to very high-temperature reactors (VHTRs) requires mechanical performance assessment methodologies. The PCHE morphology consists of thousands of millimeter-scale channels, for enhanced thermal efficiency, enclosed in a meter-scale PCHE core. PCHE geometry under thermomechanical creep-fatigue transients results in multiaxial interactions between its different segments, such as channeled core, walls, and headers. These global-level interactions influence the local channel-level responses. Hence, developing a PCHE performance assessment methodology, following the ASME Code, Section III, Division 5 provisions, is a critical gap to be filled. There is no analysis or design methodology available in ASME Code to assess a PCHE for its global and local level performances under high temperature and pressure loadings. This paper critically evaluates a recently proposed two-step analysis technique to estimate global interactions and local channel level responses of PCHEs. In this novel analysis technique, the channeled PCHE core is replaced with orthotropic solid blocks of representative stiffness properties for the global thermomechanical analysis. Subsequent channel scale submodel analysis with detailed channel geometry, loading, and elastic-perfectly plastic material model estimates the local responses for PCHE performance assessment. This paper critically evaluates this novel technique for its effectiveness in PCHE performance assessment. Finite element (FE) models imitating various analysis issues are developed, and FE analysis results are scrutinized. An important outcome of this study is the validation of the novel two-step PCHE analysis technique for application to the performance assessment of PCHEs in VHTRs.}, number={3}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Shaw, Avinash and Mahajan, Heramb P. and Hassan, Tasnim}, year={2023}, month={Jun} }
 @article{shaw_mahajan_hassan_2022, title={A Practical Analysis Framework for Assessment of Printed Circuit Heat Exchangers in High-Temperature Nuclear Service}, volume={144}, ISSN={["1528-8978"]}, DOI={10.1115/1.4052697}, abstractNote={
 Printed Circuit Heat Exchangers (PCHEs) have high thermal efficiency because of the numerous minuscule channels. These minuscule channels result in a high thermal exchange area per unit volume, making PCHE a top contender for an intermediate heat exchanger in high-temperature reactors. Thousands of minuscule channels make finite element analysis of the PCHE computationally infeasible. A two-dimensional analysis is usually performed for the PCHE core, which cannot simulate the local channel level responses reasonably because of the absence of global constraint influence. At present, there is no analysis technique available in the ASME Code or literature that is computationally efficient and suitable for engineers to estimate PCHE local responses. A novel but practical two-step analysis framework is proposed for performing PCHE analysis. In the first step, the channeled core is replaced by orthotropic solids with similar stiffness to simulate the global thermomechanical elastic responses of the PCHE. In the second step, local submodel analysis with detailed channel geometry and loading is performed using the elastic-perfectly plastic material model. The proposed two-step analysis technique provides a unique capability to estimate the channel corner responses to be used for PCHE performance assessment. This study first developed a methodology for calculating the elastic orthotropic properties of the PCHE core. Next, the two-step analysis is performed for a realistic size PCHE core, and different issues observed in the results are scrutinized and resolved. Finally, a practical finite element analysis framework for PCHEs in high-temperature nuclear service is recommended.}, number={4}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Shaw, Avinash and Mahajan, Heramb P. and Hassan, Tasnim}, year={2022}, month={Aug} }
 @article{mahajan_lima_hassan_2022, title={Mechanical and Microstructural Performance Evaluation of Diffusion Bonded Alloy 800H for Very High Temperature Nuclear Service}, volume={144}, ISSN={["1528-8889"]}, DOI={10.1115/1.4052825}, abstractNote={
 Very high temperature reactors (VHTRs) are planned to be operated between 550 to 950°C, and demand a thermally efficient intermediate heat exchanger (IHX) in the heat transport system (HTS). The current technological development of compact heat exchangers (CHXs) for VHTRs is at the ‘proof of concept’ level. A significant development in the CHX technologies is essential for the VHTRs to be efficient, cost-effective, and safe. CHXs have very high thermal efficiency and compactness, making them a prime candidate for IHXs in VHTRs. Photochemically etched plates with the desired channel pattern are stacked and diffusion bonded to fabricate CHXs. All plates are compressed at an elevated temperature over a specified period in the diffusion bonding process, promoting atomic diffusion and grain growth across bond surfaces resulting in a monolithic block. The diffusion bonding process changes the base metal properties, which are unknown for Alloy 800H, a candidate alloy for CHX construction. Hence, developing mechanical response data and understanding failure mechanisms of diffusion bonded Alloy 800H at elevated temperatures is a key step for advancing the technology of IHXs in VHTRs. The ultimate goal of this study is to develop ASME BPVC Section III, Division 5 design rules for CHXs in nuclear service. Towards this goal, mechanical performance and microstructures of diffusion bonded Alloy 800H is investigated through a series of tensile, fatigue, creep, and creep-fatigue tests at temperatures 550 to 760°C. The test results, failure mechanisms, and microstructures of diffusion bonded Alloy 800H is scrutinized and presented.}, number={2}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Mahajan, Heramb P. and Lima, Lucas M. A. and Hassan, Tasnim}, year={2022}, month={Apr} }
 @article{mahajan_mckillop_keating_hassan_2022, title={Proposed Material Properties, Allowable Stresses, and Design Curves of Diffusion Bonded Alloy 800H for the ASME Code Section III Division 5}, volume={144}, ISSN={["1528-8978"]}, DOI={10.1115/1.4054073}, abstractNote={
 Increased interest in compact heat exchangers (CHXs) to serve as an intermediate heat exchangers of very high temperature reactors resulted in significant research and development on their design, analysis, and construction. Printed circuit heat exchangers are a type of CHXs with high thermal efficiency and compactness achieved through diffusion bonding a stack of etched plates with millimeter scaled channels. The diffusion bonding process changes the microstructural and mechanical properties of the wrought metal plates. The current non-nuclear design code ASME Section VIII, Division 1 captures the material property change through a 'joint efficiency factor.' However, the current nuclear design code ASME Section III, Division 5 does not address or support the diffusion bonded material properties. Hence, there is a need to develop allowable stresses, isochronous curves, and fatigue life curves for various diffusion bonded alloys. In this study, Alloy 800H material was selected to establish the diffusion bonded material properties under tension, creep, fatigue, and creep-fatigue loads at elevated temperatures in the range 550 to 760C. A set of tests on diffusion bonded Alloy 800H (DB 800H) are performed and the acquired data are used in developing allowable stresses Sy, Su, Sr, Sm, St, Smt, So, isochronous curves and fatigue life curves according to the ASME Section III, Division 5 requirements. This paper presents detailed procedures used in developing the ASME code section III Division 5 design provisions for diffusion bonded Alloy 800H.}, number={6}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Mahajan, Heramb P. and McKillop, Suzanne and Keating, Robert and Hassan, Tasnim}, year={2022}, month={Dec} }