<p dir="ltr">Nuclear mobile microreactors will serve as a unique, transportable power source, especially for remote communities. Because mobile microreactors are meant to remain operable after initial startup, keeping the microreactor cool during transport is a safety concern that must be taken into consideration. Due to the compact nature of shipping containers, there is no significant room for the installation of active cooling mechanisms. The thermal limitations imposed by current regulatory guidelines lead to a reactor shipment external maximum temperature of 85◦C. Transporting heat from the microreactor's exterior walls to its surrounding air within the shipping container under natural convection will serve as the greatest source of resistance to the decay heat removal. In the event of mechanical failure or local regulations restricting forced cooling systems within the shipping container, natural convection will be the primary method for transferring heat. Before mobile microreactors can reach commercial status, research must be conducted on ensuring continued passive safety. </p><p><br></p><p dir="ltr">During the unavailability of helium circulation, the internal reactor core is designed to cool by block-to-block conduction and radiation, and the reactor vessel surface is cooled by the ambient air. This scenario is anticipated during the transport of the micro-high temperature gas-cooled reactor (HTGR) in a shipping container. The conduction and radiation between the prismatic micro-HTGR blocks in the core can be influenced by variances in the thermal contacts. This work investigated the conduction within a simulated horizontal micro-HTGR core. An experimental setup was used to validate a numerical model for conduction radiation cooldown with postulated thermal contact conductances (TCC). The experimental setup consisted of a hexagonal assembly with scaled prismatic blocks placed within a high-temperature vacuum environment. The gaps between the blocks were well controlled and monitored. The experimental setup was designed in such a way that the temperature variation in the axial direction was minimal, such that the experiment could be observed as a 2D (r,θ) heat transfer problem. The experimental scenario was computationally modelled with a finite element analysis (FEA) program. Once validated, the computational model was used to investigate the impact of gap conductance on overall decay heat removal. Using a conservative estimate for gap conductance value (100 W/m2 − K) between the prismatic blocks, there is a negligible increase in temperature observed during decay heat generation with constant natural convection coefficients. </p><p><br></p><p dir="ltr">However, the internal temperature profile may change drastically depending on the exterior conditions of the microreactor. A second model for the worst case scenario of exterior cooling being limited to natural convection flows was validated against existing benchmark experimental data on natural convection in closed cavities. The investigations have been performed for several configurations, including different reactor sizes, power levels, and scenarios with or without shielding around the reactor pressure vessel (RPV). This conservative safety analysis restricts the power level of the reactor to be equal to 1 MWe. A more realistic analysis with intermittent shutdown of shipping container air circulation demonstrates that a 4 MWe reactor will reach 85◦C Code of Federal Regulations (CFR) limitations after one hour while a 5 MWe reactor reaches the limit after 34 minutes. </p><p><br></p><p dir="ltr">Finally, both models were combined into a conjugate heat transfer model to examine whether thermal contact conductance (TCC) values would affect the external temperature profile as well as the maximum temperature reached by the core to ensure material limitations would be maintained. Studies have been conducted on a micro-HTGR design that originates from the fuel block design of the MHTGR-350 with changes to the overall power level, TCC values, and outer shipping container wall temperatures. Changes to TCC values do not significantly change microreactor exterior temperatures. In addition, the internal temperatures under all examined conditions remained under 875◦C. </p>