neutron flux distributions, and reactivity coefficients of a wide variety of materials were measured. Construction of the test facility building began on July 6, 1951. Operation and experiments The ARE was operated successfully. It became critical with a mass of uranium-235. It was very stable as a result of its strong negative fuel temperature coefficient (measured at -9.8e-5 dk/k/°F). The assembly was first sufficiently assembled on August 1, 1954, at which point a three-shift operation commenced for tests. Hot sodium metal was flowed through the system beginning on September 26 to test the process equipment and instrumentation. Problems with the sodium vent and sodium purification systems required lengthy repairs. After several sodium dumps and recharges, carrier salt was introduced to the system on October 25th. Fuel was first added to the reactor on October 30, 1954. Initial criticality was reached at 3:45 P.M. on November 3, after a painstaking and careful process of adding the enriched fuel. Much of the four days was spent removing plugs and repairing leaks in the enrichment line. A series of fuel samples were taken periodically. Most notably, they showed an increase in chromium content at a rate of 50 ppm/day, indicating rapid corrosion of the fuel pipes. A series of experiments were performed in the ARE supporting its mission. Critical experiment Subcritical measurement of reactor temperature coefficient Power determination at 1 watt (nominal) Regulating rod calibration vs. fuel addition Fuel system characteristics Power determination at 10 watts Regulating rod calibration vs. reactor period Calibration of shim rod vs. regulating rod Effect of fuel flow on reactivity Low-power measurement of reactor temperature coefficient Adjustment of chamber position Approach to power: 10-kW run Test of off-gas system Approach to power: 100-kW to 1-MW runs High-power measurement of the fuel temperature coefficient High-power measurement of the reactor temperature coefficient Reactor startup on temperature coefficient Sodium temperature coefficient Effect of a dollar of reactivity High-power measurement of reactor temperature coefficient Moderator temperature coefficient Xenon run at full power Reactivity effects of sodium flow Xenon buildup at one-tenth full power Operation at maximum power At 4:19 P.M. on November 8, during the ascent to high power, the reactor was shut down due to high airborne radioactivity measurements in the basement. It appeared that the gas fittings to the main fuel pump were leaking fission-product gases and vapors into the pits, and the pits were leaking into the basement through