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Aircraft Reactor Experiment

The Aircraft Reactor Experiment (ARE) was an experimental molten salt nuclear reactor developed by Oak Ridge National Laboratory (ORNL) under the U.S. Air Force's Aircraft Nuclear Propulsion program in the early 1950s to demonstrate the feasibility of high-temperature, high-power-density reactors for powering military aircraft.^[1]^[2] Designed as a liquid-fueled system using circulating molten fluoride salts (initially NaF-ZrF₄-UF₄ with 93.4% enriched uranium), the ARE featured a heterogeneous core with 66 fuel passages, beryllium oxide (BeO) moderator blocks cooled by sodium, and construction primarily from Inconel alloy to withstand extreme conditions.^[1]^[3] Initiated in the late 1940s amid Cold War efforts to extend aircraft range without refueling, the ARE aimed to operate at temperatures of 1000–1500°F (810–1100 K) and power levels of 1–3 MWt, heating air for turbine propulsion while testing inherent safety features like a negative temperature coefficient through fuel expansion.^[1]^[3] The reactor achieved criticality on November 3, 1954, at ORNL in Tennessee, marking it as the world's first functional molten salt reactor, and ran for a total of 221 hours of nuclear operation, including 74 hours at megawatt power.^[4]^[2] During high-power runs starting November 9, it reached a maximum steady-state fuel temperature of 1580°F (1130 K) and 2.5 MWt, producing 96 MW-hours of energy while demonstrating stable control with boron carbide shim rods and no significant xenon poisoning.^[4]^[5] The experiment successfully validated key molten salt reactor concepts, including low-pressure operation and rapid shutdown capabilities via electromagnetic fuel dumps, but faced challenges such as minor gaseous leaks and discrepancies in heat balance measurements.^[4] Operations concluded on November 12, 1954, with fuel and coolant drained the following day, and the facility was decommissioned shortly thereafter.^[4] Although the broader aircraft propulsion program was canceled in 1961 due to the rise of intercontinental ballistic missiles and unresolved issues like material corrosion, the ARE's results influenced subsequent molten salt research, including the Molten Salt Reactor Experiment in the 1960s.^[2]^[3]

Historical Context

Origins of Nuclear Aircraft Propulsion

In the immediate aftermath of World War II, the U.S. military sought advanced propulsion technologies to maintain strategic superiority amid rising tensions with the Soviet Union, particularly the need for long-range bombers capable of evading defenses and striking distant targets without logistical constraints. The U.S. Army Air Forces initiated investigations into nuclear energy applications for aircraft in early 1946, leading to the formal launch of the Nuclear Energy for the Propulsion of Aircraft (NEPA) project on May 28, 1946, under a contract with Fairchild Engine and Airplane Corporation at Oak Ridge.^[6] This effort was driven by the recognition that nuclear power could enable aircraft with virtually unlimited endurance, eliminating the need for frequent refueling and allowing sustained operations over intercontinental distances, including supersonic flight profiles essential for penetrating Soviet airspace.^[6] The NEPA project evolved into the broader Aircraft Nuclear Propulsion (ANP) program, established in 1951 under the oversight of the Atomic Energy Commission (AEC), which coordinated research with the U.S. Air Force, National Advisory Committee for Aeronautics (NACA), and other agencies.^[7] Initial studies emphasized solid-fuel, sodium-cooled reactors, such as those explored by General Electric for direct air-cycle systems, aiming to heat air directly for jet propulsion while leveraging sodium's efficient heat transfer properties.^[7] However, challenges with solid-fuel designs, including neutron economy limitations and buildup of fission product poisons like xenon-135—which could suppress reactivity and hinder reactor control—prompted a pivot toward molten salt reactors by the early 1950s.^[7] These fluid-fuel systems allowed for continuous removal of volatile poisons through gas sparging, improving operational flexibility for high-power-density applications. By 1950, the AEC had deepened involvement of national laboratories, designating Oak Ridge National Laboratory (ORNL) in September 1949 to lead advanced reactor research under the nuclear aircraft propulsion program, building on its Manhattan Project expertise.^[7] The program, which continued until its cancellation in 1961, represented a $1 billion investment in pursuit of nuclear-powered strategic aviation, ultimately influencing subsequent prototypes like the Aircraft Reactor Experiment as a molten-salt testbed within the ANP structure.^[6]

Initiation of the ARE Project

The Aircraft Reactor Experiment (ARE) was formally initiated in 1950 as part of the U.S. Aircraft Nuclear Propulsion (ANP) program, which sought to develop nuclear reactors for powering military aircraft. In the summer of 1950, the Atomic Energy Commission (AEC) selected Oak Ridge National Laboratory (ORNL) to lead the ARE development, recognizing the laboratory's expertise in nuclear reactor design and operation. This choice leveraged ORNL's foundational work on molten-salt fuels, initiated by physicist Alvin Weinberg in the late 1940s, who advocated for liquid-fuel systems to enable higher operating temperatures and improved safety compared to traditional designs.^[8] The project's core objectives centered on testing the feasibility of a circulating fluid-fuel reactor using molten salts, designed to achieve high temperatures up to 860°C (1580°F) and high power density in a compact form suitable for aviation. As a scaled proof-of-concept, the ARE aimed to validate key technologies for an eventual 350 MW prototype system, focusing on criticality achievement, fuel circulation, and sustained operation without the mechanical complexities of solid-fuel elements. These goals addressed critical needs for lightweight, efficient nuclear propulsion in aircraft, where extreme thermal and radiation environments posed significant challenges.^[8]^[9] Funding for the ARE was provided through the AEC's ANP budget, enabling rapid planning and design phases starting in 1950. To support the effort, ORNL established collaborations with Argonne National Laboratory and Knolls Atomic Power Laboratory, drawing on their complementary expertise in reactor physics, materials testing, and engineering. The ORNL team, under Weinberg's influence as research director, proposed the experiment specifically to mitigate limitations of prior solid-fuel reactor concepts, such as susceptibility to swelling under neutron bombardment and inefficient heat removal at elevated temperatures.^[8]

Technical Design

Reactor Core and Fuel System

The reactor core of the Aircraft Reactor Experiment (ARE) featured a heterogeneous architecture consisting of hexagonal beryllium oxide (BeO) moderator blocks interlaced with Inconel tubes that formed the fuel channels for circulating molten salt. These BeO blocks, each containing 1.25-inch diameter holes lined with Inconel tubing, were arranged in a cylindrical configuration to optimize neutron moderation and fuel circulation. The active core measured approximately 33 inches in diameter and 36 inches in height, housed within an Inconel shell with an inner diameter of about 48 inches and a total height of 44 inches, providing structural integrity under high-temperature conditions. This design facilitated a compact volume of roughly 1.3 cubic feet for the core region, enabling high power density in a fluid-fueled system.^[1]^[9] The fuel system employed a molten fluoride salt mixture as both fissile material carrier and coolant, with the composition comprising 53.09 mole% NaF, 40.73 mole% ZrF₄, and 6.18 mole% UF₄, enriched in uranium-235. This salt had a melting point around 1,000°F (538°C) and was circulated through 66 parallel fuel passages in the core at a rate of 46 gallons per minute, ensuring turbulent flow (Reynolds number exceeding 14,000) for effective heat removal and mixing. The total fuel salt inventory in the primary system was approximately 50 gallons (about 6 cubic feet when hot), containing a critical mass of 32.8 pounds of U-235 at full loading. This integrated fuel-coolant approach minimized component complexity while supporting the high-temperature, compact requirements of aircraft propulsion concepts.^[10]^[9] Operational parameters targeted 1-3 MW thermal power, with achieved levels up to 2.5 MW and maximum fuel temperatures of 1,580°F (860°C) during steady-state runs, approaching the design limit of 1500°F (816°C). Power density was enhanced by the core's geometry and salt properties, aiming for 10-100 times that of typical solid-fuel reactors through efficient fission and moderation; fundamentally, power P is given by

P = (\text{fission rate}) \times (\text{energy per fission}),

where the compact design maximized fission events per unit volume. Fluid dynamics in the molten salt emphasized low viscosity at operating temperatures (around 1,300°F or 704°C mean) for rapid circulation and inherent stability against perturbations.^[1]^[10] Key innovations centered on the BeO moderator's low neutron absorption cross-section combined with high slowing-down power, which complemented the fluoride salt's inherent moderation without introducing excessive parasitic losses, thus supporting elevated temperatures and power densities unattainable in water-moderated systems. The circulating fuel design also mitigated xenon poisoning effects by continuously flushing fission products from the core, demonstrating feasibility for dynamic reactor control in high-flux environments.^[11]

Cooling and Moderation Features

The cooling system of the Aircraft Reactor Experiment (ARE) employed separate circuits: the molten fuel salt was cooled via finned-tube heat exchangers using helium in a closed loop with a water-cooled radiator, while liquid sodium was circulated at a maximum flow rate of 150 gallons per minute through the reflector-moderator assembly and heat exchangers, with an air-cooled radiator system dissipating heat to maintain outlet temperatures around 1235°F. This setup utilized four electromagnetic pumps for redundancy, ensuring continuous cooling even during transients, and transferred heat via intermediate exchangers to isolate the primary fuel circuit. The design prioritized efficient heat removal at elevated temperatures, achieving a mean sodium temperature of about 1,300°F during full-power runs without significant pressure drops exceeding 50 psi. Moderation in the ARE was provided by hexagonal beryllium oxide (BeO) blocks arranged in a lattice to thermalize neutrons for efficient U-235 fission in the circulating fuel. The core blocks contained 1.25-inch diameter axial holes lined with Inconel for the fuel channels, positioned symmetrically to optimize the neutron spectrum while minimizing parasitic absorption. These hot-pressed BeO blocks, cooled by the sodium circuit, occupied a significant portion of the core volume to achieve the required moderation ratio, supporting a heterogeneous assembly that enhanced neutron economy in the high-flux environment. The arrangement allowed for a compact core diameter of about 33 inches, critical for aircraft integration. Safety features emphasized inherent stability and passive shutdown mechanisms, leveraging the liquid fuel's properties for aviation safety. The reactor exhibited a strong negative temperature coefficient of reactivity, measured at -6 × 10^{-5} Δk/k per °F, which automatically reduced reactivity during temperature rises due to fuel expansion, ensuring self-stabilization without active intervention. Power regulation was achieved using one regulating control rod and by adjusting fuel flow rates, with three boron carbide shim rods for overall reactivity control. In emergencies, the entire fuel inventory could be dumped by gravity into Inconel-lined drain tanks, quenching the reaction and facilitating post-shutdown cooling via integrated flues. Engineering challenges centered on material compatibility in the aggressive environment, particularly corrosion resistance of Inconel-X to the molten fluoride salts at temperatures up to 1,500°F. Pre-operational tests demonstrated corrosion depths of 7-10 mils after 1,000 hours, primarily from selective chromium leaching, mitigated by maintaining high salt purity and a favorable UF₃/UF₄ ratio to limit mass transfer. The Inconel components, including core shells and piping, were Heliarc-welded and subjected to dynamic corrosion loops, confirming acceptable degradation rates below 2 mils per 50°F increment, which supported the experiment's 100 MW-hr operation without structural failure.

Development and Construction

Facility Preparation and Engineering Studies

The facility for the Aircraft Reactor Experiment (ARE) was constructed at Oak Ridge National Laboratory (ORNL) as part of the broader Aircraft Nuclear Propulsion (ANP) program, which aimed to develop a compact, high-temperature reactor suitable for aircraft applications. Construction of the dedicated test facility building progressed rapidly following the program's expansion in 1951, with the structure completed in time for reactor assembly and initial operations by November 1954. This timeline aligned with the need to demonstrate molten-salt reactor feasibility under extreme conditions, including high power density and elevated temperatures.^[8] Parallel to the physical construction, extensive engineering studies were conducted from 1950 to 1954 to address key challenges in materials compatibility and system performance. A primary focus was corrosion testing of Inconel, the primary structural material, in molten fluoride salts at temperatures up to 1500°F (815°C). These tests, including thermal-convection loops operated for up to 5000 hours, revealed that Inconel experienced intergranular attack depths of 2-4 mils under simulated fuel conditions, with mass transfer increasing at higher temperatures but remaining acceptable for short-term operation when chromium was present in the salt mixture. Loop experiments simulating fuel flow and heat transfer further validated the design, demonstrating stable circulation at velocities yielding Reynolds numbers around 4500 and temperature differentials up to 300°F, essential for ensuring efficient heat removal without excessive material degradation.^[12] Safety considerations were integral to these preparatory efforts, culminating in the issuance of the Hazards Summary Report (ORNL-1407) in November 1952. This document detailed potential nuclear and chemical hazards associated with the fluid-fuel reactor, emphasizing radiation shielding requirements—such as thick concrete barriers and beryllium oxide reflectors to contain neutron flux—and emergency protocols for containment breaches, including rapid salt drainage and ventilation isolation to mitigate off-site exposure.^[13] The engineering studies benefited from broader ANP program collaborations, incorporating expertise from Argonne National Laboratory on fluid dynamics for circulating-fuel systems and from Knolls Atomic Power Laboratory on high-temperature materials selection, building on prior lower-temperature reactor experience to inform the ARE's innovative design.

Pre-Operational Testing and Safety Assessments

Prior to achieving criticality, the Aircraft Reactor Experiment (ARE) underwent extensive non-nuclear mock-up and loop tests to validate the hydraulic behavior of the molten salt fuel circulation system. Hydraulic simulations using full-scale water and glass models confirmed turbulent flow conditions in the core, with a Reynolds modulus exceeding 5,000, ensuring adequate mixing and heat transfer without stagnation.^[1] Forced-circulation loop tests operated at maximum fuel temperatures of 1,600°F and wall temperatures of 1,700°F for up to 1,000 hours, evaluating corrosion rates under varying flow velocities (Reynolds numbers 5,200–14,250) and demonstrating material integrity for the Inconel components in contact with the NaF-ZrF4-UF4 fuel salt.^[14] Thermal-convection loop tests at 1,500°F hot-leg temperatures and 1,300–1,350°F cold-leg temperatures for 500 hours further assessed passive flow stability, revealing deep corrosive attack on Inconel but informing design adjustments for leak prevention.^[14] Tests on the moderator and cooling systems addressed potential thermal stresses and fluid integrity. Beryllium oxide (BeO) blocks were subjected to thermal tests using electrical heaters up to 1,500°F (45 kW maximum) to simulate operational heating, verifying compatibility with sodium coolant without excessive degradation.^[1]^[14] The design utilized hot-pressed beryllium oxide (BeO) moderator blocks cooled by circulating sodium, with pre-operational verification through convection and forced-circulation loops confirming minimal corrosion (0.5–1.5 mils) in sodium environments up to 1,500°F.^[14] For the NaK cooling circuit in the fuel-to-air heat exchanger, loop tests with temperature gradients of 150–400°F demonstrated low mass transfer rates and deposit formation limited to 12–20 mils, establishing the system's reliability for heat dissipation at design flows.^[14] These simulations also addressed risks of salt freezing by confirming preheat protocols to maintain the fuel carrier above 1,150°F using helium blowers at rates of 10–25°F/min.^[9] Safety assessments focused on reactivity control and operational boundaries to mitigate transients in the novel fluid-fuel design. Analyses of potential reactivity transients, including those from pump failure or flow disruptions, established scram thresholds at a 1-second period and neutron flux limits, with safety rods providing 15% Δk/k shutdown margin via boron carbide insertion.^[9] Pre-operational modeling of xenon buildup predicted poisoning effects requiring less than 1% Δk/k compensation, addressed through helium scrubbing in the expansion tank to remove fission products.^[1] Operating limits were set at a maximum thermal power of 2.5 MW, fuel outlet temperature of 1,200°F, and sodium moderator temperatures below 1,300°F, with non-drainable fuel necessitating backup emergency drainage within 3 minutes to a cooled dump tank.^[1]^[9] Personnel training protocols involved three-shift operating crews practicing system handling from August 1, 1954, including rod drop simulations (over 25 tests at 1,200–1,300°F) and helium flow adjustments, ensuring familiarity with scram procedures and remote monitoring.^[9] In late 1954, final inspections integrated these validations into the facility. By September 26, 1954, the sodium cooling system completed circulation tests at 600°F and 1,300°F, with leaks repaired by October 16; fuel carrier loops achieved full flow at 1,300°F by October 25, followed by auxiliary system leak checks using mass spectrometry before October 30.^[9] Instrumentation for real-time monitoring was calibrated and verified, including over 500 devices such as 1,000 thermocouples for temperature profiling every 3 feet along pipes, 27 strip chart recorders for flow and pressure, BF3 counters and fission chambers for neutron flux, and ion chambers for servo control signals.^[9]^[4] These efforts confirmed design feasibility, resolving issues like potential leaks through welded joint inspections (266 fluoride and 225 sodium lines, all leak-tight) and corrosion monitoring via carrier samples showing acceptable chromium levels (90 ppm).^[9]^[14] The pre-operational phase culminated in subcritical measurements on October 30, 1954, demonstrating system readiness without nuclear activation.^[9]

Operational Phase

Achieving Criticality and Initial Runs

The Aircraft Reactor Experiment (ARE) achieved initial criticality on November 3, 1954, at 3:45 p.m., following the loading of approximately 23.4 quarts of fuel concentrate containing about 30 pounds of U-235 into the core.^[9] This milestone marked the first successful operation of a circulating molten-salt-fueled reactor, with the core reaching a clean critical mass of 14.9 kg of U-235 at an uranium concentration of 384 g/liter and a core volume of 38.8 liters at 1300°F.^[15] Low-power experiments commenced the following day on November 4, confirming basic reactivity and system integrity prior to escalation.^[9] Power ramp-up progressed steadily, attaining the megawatt range by November 9 and reaching full operational levels of 2.5 MW thermal on November 12 during high-power runs starting November 9.^[4] Initial high-power operations, spanning November 9 to 12, 1954, proceeded under the oversight of Colonel Clyde D. Gasser, Chief of the Nuclear Powered Aircraft Branch at Wright Air Development Center.^[9] During these runs, fuel flow was adjusted in real time—typically to rates of 44–46 gallons per minute—to maintain core stability, with the total integrated nuclear energy output amounting to 96 MW-hours over 221 hours of operation, including 74 hours in the megawatt regime.^[9]^[4] Key technical milestones included the demonstration of continuous molten-salt circulation through the Inconel-tubed, beryllium oxide-moderated core, with the fuel system accumulating 462 hours of fluoride circulation overall.^[15] The reactor exhibited excellent inherent stability, evidenced by a negative temperature coefficient of reactivity—approximately -6 × 10^{-5} (Δk/k)/°F for the overall system and -9.8 × 10^{-5} (Δk/k)/°F for the fuel— which effectively damped power excursions without reliance on control rods.^[9]^[4] Minor challenges arose during the initial high-power phase, including flow imbalances that caused power fluctuations of up to 15% on monitoring instruments; these were promptly corrected through pump adjustments and helium flow management to restore equilibrium.^[9] Pre-operational safety assessments, including subcritical mockups and leak tests, had verified system readiness prior to fuel loading.^[4]

Experimental Objectives and Key Results

The experimental objectives of the Aircraft Reactor Experiment (ARE) centered on validating the performance of a circulating-fuel, molten-salt reactor for nuclear aircraft propulsion, emphasizing operation at extreme temperatures to enable efficient heat transfer to an intermediate coolant loop. Key goals included demonstrating the high-temperature stability of the NaF-ZrF4-UF4 fuel salt and Inconel structural materials up to 1500°F with a 350°F temperature rise, achieving reactivity control through dynamic fuel flow adjustments, mitigating xenon-135 poisoning via continuous off-gassing in the salt circulation system, and attaining elevated power densities suitable for compact, lightweight aircraft designs. The project also sought to evaluate the overall fluid behavior and nuclear characteristics of the system, though direct validation of thorium breeding was not pursued in this uranium-fueled configuration, as that aspect emerged in subsequent molten-salt programs.^[9]^[8]^[16] Following the achievement of initial criticality on November 3, 1954, the ARE successfully operated at a maximum sustained fuel outlet temperature of 1580°F, with transient peaks reaching 1620°F during power ramps, exhibiting no corrosion failures in the core components or fuel salt degradation over the test period. Reactivity control was effectively demonstrated using the standard equation \rho = \frac{k-1}{k}, where the effective multiplication factor k_\text{eff} was dynamically adjusted via fuel pump flow rates from 0 to 46 gallons per minute, yielding reactivity changes of up to 0.4% \Delta k/k compensated by control rod movements of about 12 inches. Xenon poisoning proved negligible, with buildup limited to an upper bound of 0.01% \Delta k/k during a dedicated 25-hour run at 2.12 MW, far below the theoretical 0.2% \Delta k/k without off-gassing, due to the salt's low retention (≤5%) and efficient gas stripping.^[9] Measured data highlighted the system's inherent safety features, including an overall reactor temperature coefficient of -6.0 to -6.1 \times 10^{-5} \Delta k/k per °F and a fuel-specific coefficient of -9.8 \times 10^{-5} \Delta k/k per °F, which contributed to self-regulating power responses during transients. The NaF-ZrF4-UF4 fuel salt (53.09-40.73-6.18 mole %) maintained fluidity with viscosities of 4.2-8.5 cP and density of 3.36 g/cm³ at 1300°F, showing no solidification or chemical instability. Core power densities of 94-153 W/cm³ were achieved at peak thermal outputs of 2.5 MW, underscoring the design's suitability for high-performance aviation reactors by enabling smaller volumes compared to contemporary solid-fuel alternatives. These results affirmed the molten-salt approach's advantages in temperature tolerance and power handling for propulsion applications.^[9] Operational limitations included a brief high-power duration of 73.8 hours above 1 MW across roughly 4 days of the total 220.7-hour critical runtime, curtailed by program funding constraints rather than technical issues, with the experiment generating approximately 96 MW-hours of nuclear energy overall.^[9]^[8]

Shutdown and Aftermath

Decommissioning Process

The shutdown of the Aircraft Reactor Experiment (ARE) was ordered on November 12, 1954, by Colonel Clyde D. Gasser of the U.S. Air Force, who performed the scram at 8:04 p.m. during the final high-power run at approximately 2.5 MW, marking the conclusion of operations after achieving the targeted integrated power of 96 MW-hr.^[17] Following the scram, the molten salt fuel and sodium coolant continued to circulate overnight to prevent freezing in the heat exchangers, with helium flow halted to facilitate safe cooldown; on the morning of November 13, the materials were drained into dedicated storage tanks to isolate them from the reactor system.^[17] This process was executed without significant incidents, as the experiment had operated successfully overall, though early termination was influenced by the attainment of core objectives amid broader program shifts.^[17] Dismantling commenced in February 1955, after sufficient radioactive decay to allow access, involving systematic disassembly of the reactor core, pumps, valves, heat exchangers, and associated piping to assess corrosion, structural integrity, and radiation-induced damage.^[18] Components were decontaminated through rinsing and mechanical cleaning where feasible, with radiation surveys conducted to ensure worker safety and compliance; highly contaminated equipment was then packaged and transported to designated burial grounds for long-term disposal under Atomic Energy Commission (AEC) protocols.^[18] Post-operation analysis focused on extracted salt samples from the fuel system, which were examined for fission product accumulation, revealing buildup primarily of short-lived isotopes with effective off-gassing during runs and no evidence of major retention issues; corrosion products, such as increased chromium levels, were noted but within expected bounds for the high-temperature fluoride environment.^[17] No major leaks or operational incidents were reported, with radiation monitoring confirming levels below thresholds—such as off-gas activity under 0.8 μC/cm³—and subatmospheric pit pressures preventing fission gas escape.^[17] Environmental safety measures adhered to AEC guidelines, including off-gas discharge via pipeline to an isolated area and comprehensive facility surveys prior to release for unrestricted use.^[18]

Legacy and Subsequent Developments

Following the successful operation of the Aircraft Reactor Experiment (ARE), the U.S. Aircraft Nuclear Propulsion (ANP) program advanced plans for a larger follow-on facility, the 60 MW thermal Aircraft Reactor Test (ART), designed in the mid-1950s to further demonstrate high-temperature molten salt reactor technology for aircraft propulsion.^[19] The ART was intended to operate at core power densities significantly higher than the ARE, using similar fluoride salt fuels, but it was never constructed due to programmatic shifts and budget constraints within the ANP effort.^[20] The ARE's contributions extended to broader advancements in molten salt reactor technology, particularly through its influence on the Molten-Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory, which operated from 1965 to 1969.^[21] Key ARE findings on fluoride salt chemistry, fuel processing, and structural materials like Hastelloy-N enabled the MSRE to achieve stable operation at 7.4 MW thermal power, testing both uranium-235 and uranium-233 fuels in a homogeneous configuration.^[20] This validation of high-temperature operations (up to 650°C) supported the feasibility of future breeder reactors, including those exploring thorium-uranium fuel cycles, where molten salts facilitate efficient breeding and waste reduction.^[20] The ANP program, encompassing the ARE and planned ART, was fully terminated in March 1961 by President John F. Kennedy as part of his budget recommendations to Congress, citing escalating costs exceeding $1 billion, unresolved safety challenges related to radiation shielding and aircraft integration, and the rapid evolution of conventional jet propulsion technologies that diminished the military need for nuclear aircraft.^[6] Despite the cancellation, the ARE's legacy persists in contemporary nuclear research, where its data on thorium-compatible molten salts informs advocacy for thorium fuel cycles in advanced reactors and potential applications in small modular reactors (SMRs) for enhanced safety and fuel efficiency.^[20] Modern initiatives, such as China's Thorium Molten Salt Reactor program initiated in 2011, draw on these early experiments to pursue sustainable, high-temperature SMR designs.^[20]

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