This paper describes preliminary results of a dynamic system model for a closed-loop Brayton-cycle that is coupled to a nuclear reactor. The current model assumes direct coupling between the reactor and the Brayton-cycle, however only minor additions are required to couple the Brayton-cycle through a heat exchanger to either a heat pipe reactor or a liquid metal cooled reactor. Few reactors have ever been coupled to closed Brayton-cycle systems. As such their behavior under dynamically varying loads, startup and shut down conditions, and requirements for safe and autonomous operation are largely unknown. Sandia National Laboratories has developed steady-state and dynamic models for closed-loop turbo-compressor systems (for space and terrestrial applications). These models are expected to provide a basic understanding of the dynamic behavior and stability of the coupled reactor and power generation loop. The model described in this paper is a lumped parameter model of the reactor, turbine, compressor, recuperator, radiator/waste-heat-rejection system and generator. More detailed models that remove the lumped parameter simplifications are also being developed but are not presented here. The initial results of the model indicate stable operation of the reactor-driven Brayton-cycle system and its ability to load-follow. However, the model also indicates some counter-intuitive behavior for the complete coupled system. This behavior will require the use of a reactor control system to select an appropriate reactor operating temperature that will optimize the performance of the complete spacecraft system. We expect this model and subsequent versions of it to provide crucial information in developing procedures for safe start up, shut down, safe-standby, and other autonomous operating modes. Ultimately, Sandia hopes to validate these models and to perform nuclear ground tests of reactor-driven closed Brayton-cycle systems in our nuclear research facilities.
Heat pipes are often proposed as cooling system components for small fission reactors. SAFE-300 and STAR-C are two reactor concepts that use heat pipes as an integral part of the cooling system. Heat pipes have been used in reactors to cool components within radiation tests (Deverall, 1973); however, no reactor has been built or tested that uses heat pipes solely as the primary cooling system. Heat pipe cooled reactors will likely require the development of a test reactor to determine the main differences in operational behavior from forced cooled reactors. The purpose of this paper is to describe the results of a systems code capable of modeling the coupling between the reactor kinetics and heat pipe controlled heat transport. Heat transport in heat pipe reactors is complex and highly system dependent. Nevertheless, in general terms it relies on heat flowing from the fuel pins through the heat pipe, to the heat exchanger, and then ultimately into the power conversion system and heat sink. A system model is described that is capable of modeling coupled reactor kinetics phenomena, heat transfer dynamics within the fuel pins, and the transient behavior of heat pipes (including the melting of the working fluid). The paper focuses primarily on the coupling effects caused by reactor feedback and compares the observations with forced cooled reactors. A number of reactor startup transients have been modeled, and issues such as power peaking, and power-to-flow mismatches, and loading transients were examined, including the possibility of heat flow from the heat exchanger back into the reactor. This system model is envisioned as a tool to be used for screening various heat pipe cooled reactor concepts, for designing and developing test facility requirements, for use in safety evaluations, and for developing test criteria for in-pile and out-of-pile test facilities.