The Aircraft Reactor Experiment was designed for operation at temperatures in the region of 1500ºF (1100 K) at a power of 1-3 MWt with a fluoride-salt fuel circulating in a heterogeneous core. The moderator was hot-pressed BeO blocks cooled by circulating sodium. The heat produced was dissipated in water through hot liquid-to-helium-to-water heat exchange systems. All sodium and fuel circuit components were made of Inconel fabricated by inert-gas (Heliarc) welding. The system was heated to design temperature by means of electrical heating units applied over all parts of the system. Instrumentation and control of the experiment were fairly conventional. For the most part, standard instruments were modified slightly for the hightemperature application. The reactor system was constructed and operated in a building specifically provided for the purpose.The Aircraft Reactor Experiment (ARE) to be described in this paper was originally conceived in a very different form from that in which it was ultimately constructed. In fact, the original high-temperature reactor design did not employ a fluoride-salt fuel, and, in order to understand some of the features incorporated in the ARE, a certain amount of historical background is necessary.By 1950, at various places in the country, work had progressed on the handling of hightemperature sodium metal to the point that it was being seriously considered as a coolant for nuclear reactors. Accordingly, a group of engineers and physicists at ORNL started design work on a solid-fuel-pin sodium-cooled reactor, with the fuel consisting of 235 U (as UO 2 ) canned in stainless steel. It was decided to make this a thermal reactor and to use BeO blocks as the moderator. The circulating sodium was to extract heat from the fuel pins and at the same time to remove heat from the moderator blocks. The design of this solid-fuel-pin, BeO-moderated, sodium-cooled reactor proceeded to the point of purchase of the BeO moderator blocks. These blocks, as fabricated for the original reactor design, are shown in Figure 1.The solid-fuel-pin thermal reactor design was found to possess a serious difficulty when the design concept was projected to cover a relatively high-power reactor. The problem was the positive temperature coefficient of reactivity associated with the cross section of xenon at elevated temperatures. This xenon instability was considered to be serious enough to warrant abandoning the solid-fuel design concept, because of the exacting requirement placed on any automatic control system by this instability.An obvious way to avoid the control problem would be to incorporate a liquid fuel that would have a large density change for a given change in temperature. If, upon heating and expanding, a portion of the fuel could, in effect, be made to leave the critical lattice, a selfstabilizing reactor would result.

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As power plant combustion turbines (CTs) are pushed towards higher thermal efficiencies, increased attention is being given to operating requirements for their fuel gas supply such as the maximum allowable rate-of-change in pressure. It is important to perform detailed analyses for multi-unit plants to ascertain whether pressure transient events, such as those caused by initial trip of one or two combustion turbines, will cause additional combustion turbines to trip off. In this paper, single and dual CT trips were postulated in a near-realistic combined cycle power plant. Predictions of the gas flow behavior, along with propagation and superposition of pressure waves, was carried out using the method of characteristics (MOC) for compressible flows. Specifically, the rate of change in fuel gas supply pressure to each CT was monitored and compared against a typical manufacturer limit of 0.8 bar/s. Instances where simulations showed this threshold exceeded were noted, since such events correspond to automatic valve closure that would shut down one more CT and thereby further reduce plant electrical output. The overall goal of fuel gas transient analyses is to improve pipeline designs, iteratively when necessary, such that those additional trips are avoided. To that end, this paper presents several simulation cases to illustrate pressure transient phenomena and to show the impact of various pipeline design alterations, some of which caused 40% reductions in the worst pressure rate-of-change during simulations.