• Energy Research

This paper proposes a new concept of a passive safety system for small modular reactors as a heat pipe type double-containment vessel. The proposed system configures an additional vessel outside the existing one and functions as a heat pipe that utilizes phase change heat transfer to reduce the requirement of a large pool design. The double-containment vessel features a design consisting solely of the evaporation and condensation sections for heat pipe function with compactness. The ultimate heat sink only serves as a condenser part for the double-containment vessel, therefore significantly reducing the volume of stored fluid. The study numerically evaluates several postulated accidental cases to compare the performance of the double-containment vessel using the thermal hydraulic system code, MARS-KS. Even for serious conditions such as multiple failure accidents without safety systems operation, the double-containment can secure extra golden time to core damage by more than 2,090 sec compared to the original design. This study highlights the feasibility and potential of the heat pipe-based double-containment vessel as a passive safety system, enhancing the safety and reliability of small modular reactors. This kind of design can provide the advantages of safety and economics in the construction of small modular reactors.

The CRUD on the fuel cladding under the pressurized water reactor (PWR) operating condition causes several issues. The CRUD can act as thermal resistance and increases the local cladding temperature which accelerate the corrosion process. The hideout of boron inside the CRUD results in axial offset anomaly and reduces the plant's shutdown margin. Recently, there are efforts to revise the acceptance criteria of emergency core cooling systems (ECCS), and additionally require the modeling of the thermal resistance effect of the CRUD during the performance analysis. There is an urgent need for the evaluation of the effect of the CRUD deposition on the cladding heat transfer under PWR operating conditions, but the experimental database is very limited. The experimental facility called DISNY was designed and constructed to analyze the CRUD-related multi-physical phenomena, and the performance analysis of the constructed DISNY facility was conducted. The thermal-hydraulic and water chemistry conditions to simulate the CRUD growth under PWR operating conditions were established. The design characteristics and feasibility of the DISNY facility were validated by the MARS-KS code analysis and separate performance tests. In the current study, detailed design features, design validation results, and future utilization plans of the proposed DISNY facility are presented.

Abstract This paper proposes the new pattern of wire wrap spacer, called U-pattern to enhance coolant mixing and reduce pressure drop for liquid metal cooled reactor fuel assembly. It is designed by designating its winding direction and pin arrangement without any additional structures for fuel assembly. To evaluate its thermal-hydraulic performance, both experimental and numerical approaches are performed; flow visualization experiments using a combined particle image velocimetry (PIV) and matching index-of-refraction (MIR) technique and CFD simulation. The visualized flow field in both axial and lateral planes shows that U-pattern wire wrap spacer has a stronger swirl flow in center subchannel compared to the conventional case. To extend the application ranges to the normal LMFR operation condition, CFD analysis is conducted using ANSYS-CFX. Simulation results indicate 8.60 °C decrease of peak cladding temperature and 5.2% reduced pressure drop respectively. Rod bundle with U-pattern wire spacer has a similar range of bundle loss coefficient at Re 10,000. Measurement and calculation results for the U-pattern exhibit that flow is re-distributed to have flatter temperature profile and larger flow channel with the removal of the wire wrap spacer at the center pin brings a lower flow resistance. Therefore, the U-pattern wire wrap spacer for LMFR can enhance the thermal margin and improve economics by promoting thermal mixing and reduction in pressure loss.

This paper presents the construction results and design of the UNIST Reactor Innovation platform for small modular reactors as a versatile testbed for exploring innovative technologies. The platform uses simulant fluids to simulate the thermal-hydraulic behavior of a reference small modular reactor design, allowing for cost-effective design modifications. Scaling analysis results for single and two-phase natural circulation flows are outlined based on the three-level scaling methodology. The platform's capability to simulate natural circulation behavior was validated through performance calculations using the 1-D system thermal-hydraulic code-based calculation. The strategies for evaluating cutting-edge technologies, such as the integration of a solid oxide electrolysis cell for hydrogen production into a small modular reactor, are presented. To overcome experimental limitations, the hardware-in-the-loop technique is proposed as an alternative, enabling real-time simulation of physical phenomena that cannot be implemented within the experimental facility's hardware. Overall, the proposed versatile innovation platform is expected to provide valuable insights for advancing research in the field of small modular reactors and nuclear-based hydrogen production.