Neutron Multiplication in a Mass of Uranium Metal: Problem Assignment 307-x10P Small Experiment (Final Report)
- Clinton Laboratories, Oak Ridge, TN (United States)
Experiments are described in which measurements were made in a 35 ton mass of uranium metal. The measurements were made at site X, with the thermal neutron flux from the pile providing a source of fast neutrons in the metal. The report discusses: 1) Theory--in which a three group approximation--thermal, slow and fast--is used to derive the distribution of neutrons in a cube of metal with a source of thermal neutrons flowing into the bottom face. Expressions are derived for the relaxation length at large distances from the source, for the ratio of uranium fission to radiative capture, and for the ratio of U235 fission to U236 fission in terms of the cross sections for the various processes involved and other fundamental constants; 2) Description of experiment--a detailed description of the set-up of the pile and of the method of measurement is given; 3) Results--the results of various measurements are discussed. The measurements made consist of: a) Resonance and fission in the metal pile by means of the 23 min. U239 activity for the resonance and a catcher technique for fission. The ratio of capture to fission in the pile was found--by comparison with thermal neutrons--to be 11 ± 1; b) Iodine and indium detectors were used to explore the pile both along the axis and transversely. From these measurements the equilibrium relaxation length in an infinite mass of solid metal was found to be 9.6 cms. The indium measurements showed that the activity and indium foil far from the source was cut down by about 17% when the foil was covered by 1/2 mm of indium, while covering with Cd had no effect; c) The fast neutron threshold detectors show that, in the metal pile, the average energy of neutrons which have suffered no collision is greater than the average energy of fission neutrons--indicating a decrease of inelastic scattering for energies above about 1 mev; d) Fission chamber measurements were made using normal, enriched and depleted U, Pu, Pa, and Th detectors. the equilibrium relaxation lengths are found to be independent of detector. The counting rates per mg. of the various detectors are compared at one position; Pu and U235 are found to have about equal fission cross sections for the equilibrium neutron distribution in the metal pile. The ratio of U235 fission to U238 fission per atom is found to be about 336 for the equilibrium neutron distribution; e) The average energy of the metal pile neutrons as measured by their slowing down range in graphite and in paraffin, is 250 ± 150 kev; 4) Discussion--The results of the various measurements are compared in detail with the theory, and a set of consistent constants for U are determined. Critical enrichment--i.e., enrichment required to make U metal chain reactions determined to correspond to a U238 to U235 ratio of 12.1.
- Research Organization:
- Clinton Laboratories, Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP); US Atomic Energy Commission (AEC)
- NSA Number:
- NSA-10-004470
- OSTI ID:
- 4362953
- Report Number(s):
- CF-1627; A-2489
- Resource Relation:
- Other Information: Decl. Dec. 12, 1955. Orig. Receipt Date: 31-DEC-56
- Country of Publication:
- United States
- Language:
- English
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