the thermal shield is supported by a concrete pad topped by cast-iron blocks. The graphite composition was selected to moderate the nuclear reaction fueled by of uranium slugs approximately diameter long (the approximate size of a roll of quarters), sealed in aluminum cans, and loaded into the aluminum tubes. The reactor was water-cooled, with cooling water pumped from the Hanford Reach of the Columbia River through the aluminum tubes around the uranium slugs at the rate of per minute. The water was discharged into settling basins, then returned to the river after allowing time for the decay of radioactive materials, the settling out of particulate matter, and for the water to cool so it could be returned to the Columbia River. Water returning to the river was prohibited from raising the temperature of the river by more than 11 °F. Reactor operation The B Reactor had its first nuclear chain reaction in September 1944, the D Reactor in December 1944 and the F Reactor in February 1945. The initial operation was halted by a problem identified as neutron absorption by the fission product Xe-135, first identified in a research paper of Chien-Shiung Wu that was shared with Fermi. It was overcome by increasing the amount of uranium charged. The reactor produced plutonium-239 by irradiating uranium-238 with neutrons generated by the nuclear reaction. It was one of three reactors – along with the D and F reactors – built about six miles (10 km) apart on the south bank of the Columbia River. Each reactor had its own auxiliary facilities that included a river pump house, large storage and settling basins, a filtration plant, large motor-driven pumps for delivering water to the face of the pile, and facilities for emergency cooling in case of a power failure. Emergency shutdown of the reactor, referred to as a SCRAM, was attained either by rapidly fully inserting the vertical safety rods or, as a backup method, by the injection of borated water into the reactor. In January 1952, the borated water system was replaced by a "Ball-3X" system that injected nickel-plated high-boron steel balls into the channels occupied by the vertical safety rods. The plutonium for the nuclear bomb used in the Trinity test in New Mexico and the Fat Man bomb dropped on Nagasaki, Japan was created in the B reactor. The B Reactor ran for two decades, and was joined by additional