page_title: Project Y
section_title: Water boiler
hierarchical_section_title: Project Y / Water boiler
caption_attribution_description: English: Los Alamos laboratory Water Boiler
context_page_description: The Los Alamos Laboratory, also known as Project Y, was a secret laboratory established by the Manhattan Project and operated by the University of California during World War II. Its mission was to design and build the first atomic bombs. Robert Oppenheimer was its first director, serving from 1943 to December 1945, when he was succeeded by Norris Bradbury. In order to enable scientists to freely discuss their work while preserving security, the laboratory was located in a remote part of New Mexico. The wartime laboratory occupied buildings that had once been part of the Los Alamos Ranch School.
  The development effort initially concentrated on a gun-type fission weapon using plutonium called Thin Man. In April 1944, the Los Alamos Laboratory determined that the rate of spontaneous fission in plutonium bred in a nuclear reactor was too great due to the presence of plutonium-240 and would cause a predetonation, a nuclear chain reaction before the core was fully assembled.
context_section_description: The Water Boiler was an aqueous homogeneous reactor, a type of nuclear reactor in which the nuclear fuel in the form of soluble uranium sulfate is dissolved in water. Uranium sulfate was chosen instead of uranium nitrate because sulfur's neutron capture cross section is less than that of nitrogen. The project was proposed by Bacher in April 1943 as part of an ongoing program of measuring critical masses in chain-reacting systems. He saw it also as a means of testing various materials in critical mass systems. T Division were opposed to the project, which was seen as a distraction from studies related to the form of chain reactions found in an atomic bomb, but Bacher prevailed on this point. Calculations related to the Water Boiler did take up an inordinate amount of T Division's time in 1943. The reactor theory developed by Fermi did not apply to the Water Boiler.
  Little was known about building reactors in 1943. A group was created in Bacher's P Division, the P-7 (Water Boiler) Group, under the leadership of Donald Kerst, that included Charles P. Baker, Gerhart Friedlander, Lindsay Helmholz, Marshall Holloway and Raemer Schreiber. Robert F. Christy from the T-1 Group provided support with the theoretical calculations, in particular, a calculation of the critical mass. He calculated that 600 grams of uranium-235 would form a critical mass in a tamper of infinite size. Initially it was planned to operate the Water Boiler at 10 kW, but Fermi and Samuel K. Allison visited in September 1943, and went over the proposed design. They pointed out the danger of decomposition of the uranium salt, and recommended heavier shielding. It was also noted that radioactive fission products would be created that would have to be chemically removed. As a consequence, it was decided that the Water Boiler would only run at 1 kW until more operating experience had been accumulated, and features needed for high power operation were shelved for the time being.
  Christy also calculated the area that would become contaminated if an accidental explosion occurred. A site in Los Alamos Canyon was selected that was a safe distance from the township and downstream from the water supply. Known as Omega, it was approved by the Governing Board on 19 August 1943. The Water Boiler was not simple to construct. The two halves of the 12.0625-inch (306.39 mm) stainless steel sphere that was the boiler had to be arc welded because solder would be corroded by the uranium salt. The CM-7 (Miscellaneous Metallurgy) Group produced beryllia bricks for the Water Boiler's tamper in December 1943 and January 1944. They were hot pressed in graphite at 1,000 °C (1,830 °F) at 100 pounds per square inch (690 kPa) for 5 to 20 minutes. Some 53 bricks were made, shaped to fit around the boiler. The building at Omega Site was ready, if incomplete, by 1 February 1944, and the Water Boiler was fully assembled by 1 April. Sufficient enriched uranium had arrived by May to start it up, and it went critical on 9 May 1944. It was only the third reactor in the world to do so, the first two being the Chicago Pile-1 reactor at the Metallurgical Laboratory and the X-10 Graphite Reactor at the Clinton Engineer Works. Improved cross-section measurements allowed Christy to refine his criticality estimate to 575 grams. In fact, only 565 grams were required. The accuracy of his prediction surprised Christy more than anyone.
  In September 1944, the P-7 (Water Boiler) Group became the F-2 (Water Boiler) Group, part of Fermi's F Division. On completion of the planned series of experiments in June 1944, it was decided to rebuild it as a more powerful reactor. The original goal of 10 kW power was discarded in favor of 5 kW, which would keep the cooling requirements simple. It was estimated that it would have a neutron flux of 5 x 10¹⁰ neutrons per square centimeter per second. Water cooling was installed, along with additional control rods. This time uranium nitrate was used instead of uranium sulfate because the former could more easily be decontaminated. The tamper of beryllia bricks was surrounded 
caption: Water Boiler
exif: {"Image ImageWidth": "1352", "Image ImageLength": "1775", "Image BitsPerSample": "[8, 8, 8, 8]", "Image PhotometricInterpretation": "1", "Image Orientation": "Horizontal (normal)", "Image SamplesPerPixel": "4", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS5 Macintosh", "Image DateTime": "2014:06:06 10:47:02", "Image ExifOffset": "232", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "382", "Thumbnail JPEGInterchangeFormatLength": "6871", "EXIF ExifVersion": "0221", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "1352", "EXIF ExifImageLength": "1775"}