Uranium pile

Jacob A. Marinsky ( 1918) as well as L. E. Glendenin and Charles D. Coryll ( 1912) detected the element at Oak Ridge. The first conclusive proof was in uranium piles. Uranium fission gives rise to fragments with nuclei of atomic number 61. 

The compounds of beryllium have little special value, except that beryllium oxide, BeO, is used in the uranium piles in which plutonium is made from uranium (Chap. S3),... 

In recent years hea w-ater has been used in the field of nuclear chemistry. It is mentioned in the Smyth Report (see Chap. 33) that heavy water can be used instead of graphite as the moderator in a uranium pile. The function of the moderator is to reduce the speed of the fast neutrons emitted when nuclei undergo fission. The Canadian pile at Chalk River is a heavy-water pile. 

Neutrons are also prepared by reactions in the cyclotron, and in uranium piles. 

The properties of isotopes. Packing fraction. Structure of atomic nuclei. Nuclear fission. Nuclear chain reaction. Manufacture of plutonium. Fission of U23 and Pu23 . Uranium reactors the uranium pile. Nuclear energy as a source of power. 

The reaction rate of a moderated uranium pile depends on temperature. For example, in a reactor in which the neutron spectrum is approximately Maxwellian, the average velocity will increase with temperature, thus decreasing the absorption cross sections for the low-energy neutrons which vary as 1/v. Resonance levels will be broadened by the Doppler effect, and if lumping of the uranium has been made use of to decrease the total resonance absorption as proposed by Szilard, and later found experimentally to be effective, increased temperature will decrease the advantage thus gained. 

When getting the first graphite-uranium pile close to critical conditions in Chicago in December 1942, the scientists were prepared to take necessary countermeasures and return the pile to a safe condition. They even had redundant and independently diverse possibilities to do this (control rods and liquid neutron poison). Enrico Fermi, the scientist responsible also took the human factor seriously. He asked his crew to go for a lunch break just before the experiment was entering the most interesting phase. 

Mill tailings are another form of nuclear waste. The residue from uranium ore extraction contains radium, the precursor of short-Hved radon and its daughters. Piles of tailings must be properly covered. 

Experiments were conducted during the Metallurgical Project, centered at the University of Chicago, and led by Enrico Fermi. Subcritical assembhes of uranium and graphite were built to learn about neutron multiphcation. In these exponential piles the neutron number density decreased exponentially from a neutron source along the length of a column of materials. There was excellent agreement between theory and experiment. 

Most uranium ore has a low, ca 1 part in 500, uranium content. Milling involves physical and chemical processing of the ore to extract the uranium. The mill tailings, which release gaseous radon-222 [13967-62-9] Ra, half-life 3.82 d, are placed in large piles and covered to prevent a local health problem. 

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

Isadore Perlman and I arrived in Chicago aboard the City of San Francisco. Although our trip from Berkeley took almost two full days, we feel that the time has not been wasted. Many lively discussions ensued in the privacy of our bedroom and, with appropriate care, in the club car regarding ways to separate element 94 chemically from uranium (that will be neutron-irradiated in chain-reacting piles) and from the fission by-products that will be produced concurrently in the neutron-irradiation process. 

Environmental Protection Agency, Environmental Surveys of the Uranium Mill Tailings Pile and Surrounding Areas, Salt Lake City, Report No. EPA-520/6-74-006, U.S. Environmental Protection Agency, Las Vegas, NV (1974). 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

Plutonium-239 also is produced from natural uranium by the so-called pile reactions in which irradiation of uranium-235 isotope with neutrons produces fission, generating more neutrons and high energy ( 200 MeV). These neutrons are captured by the uranium-238 to yield plutonium-239. 

The final answer came from the atomic pile. J. A. Marinsky, L. E. Glendenin, and C. D. Coryell at the Clinton Laboratories at Oak Ridge (20) obtained a mixture of fission products of uranium which contained isotopes of yttrium and the entire group of rare earths from lanthanum through europium. Using a method of ion-exchange on Amberlite resin worked out by E. R. Tompkins, J. X. Khym, and W. E. Cohn (21) they were able to obtain a mixture of praseodymium, neodymium, and element 61, and to separate the latter by fractional elution from the Amberlite column with 5 per cent ammonium citrate at pH 2.75. Neutron irradiation of neodymium also produced 61. 

The neptunium isotope first prepared by McMillan was Np239, but the atomic pile yielded larger amounts of Np287 which has a half-life of 2.25 X 10 years and a relatively low specific alpha-particle activity, only about one thousand times that of uranium. This isotope can be handled... 

Boron steel is used in controlling the operating rate of the uranium-graphite piles used to produce plutonium... 

Fermi s atomic pile was just a prototype. For manufacturing bomb plutonium, a plant was built at the tiny village of Hanford in Washington State. And so, drip by drip, the US war machine squeezed out its uranium-235 and plutonium, while the problem of how to build an atomic bomb was tackled by the physicists, chemists, and engineers at the Los Alamos complex in New Mexico. 

---