Numerous nuclear microreactor designs are currently under development, with an ambitious timeline that includes demonstrations within the next five years. The key attribute of these reactors is their relatively compact size, which makes them able to be factory-assembled and shipped to site, rather than requiring large-scale construction onsite. Many of these reactors employ heat pipe technology for cooling and heat transfer. Heat pipes have been used for decades in a wide variety of applications because of their extreme thermal efficiency, compactness, manufacturing simplicity, and reliability. While reliable, heat pipes have limitations on their heat throughput, due to a variety of thermal hydraulic phenomena. These limits motivate the need for modeling and simulation, as traditional methods of analysis are largely limited to steady-state analysis and usually employ additional, restrictive assumptions. To support this effort, the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program began development of Sockeye, an engineeringscale heat pipe tool to be used for the design and safety analysis of nuclear microreactors. Sockeye is built on the Multiphysics Object-Oriented Simulation Environment (MOOSE) framework, which allows for seamless multiphysics coupling with other NEAMS tools for full microreactor modeling.Sockeye development is largely driven by the needs of key stakeholders, such as Westinghouse Electric Company. Stakeholders have provided ample feedback and numerous discussions on their functional requirements, which led to the identification of a number of tasks, which this report discusses. These tasks include new capabilities, model improvements, user documentation and examples, and verification and validation.New capabilities were implemented to increase the flexibility of the existing model and provide new tools to facilitate heat pipe design exploration, allowing users to quickly estimate performance from design parameters, before engaging in a more comprehensive design evaluation via simulation. Flexibility improvements made to the existing heat pipe model include the ability to provide a general working fluid distribution, the ability to provide non-constant permeability for the wick, and the ability to provide a closure relationship for the interfacial curvature. An alternative heat pipe model provides users with a simple, robust surrogate model to complement Sockeye's higher fidelity model. Design utilities were created to allow the user to evaluate heat pipe designs based on analytic approximations, which are very useful for heat pipe design exploration.Numerous modeling improvements were implemented in this work to support customer functional requirements and address issues raised by users. This included some major improvements to the capillary pressure model that were designed in collaboration with Argonne National Laboratory. Other modeling improvements improved the accuracy of pressure loss calculations as well as wall heat transfer and wall boiling, which is a prerequisite for mo...
