Further Investigations of NIST Water Sphere Discrepancies

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Measurements have been performed on a family of water spheres at the National Institute of Standards and Technology (NIST) facilities. These measurements are important for criticality safety studies in that, frequently, difficulties have arisen in predicting the reactivity of individually subcritical components assembled in a critical array. It has been postulated that errors in the neutron leakage from individual elements in the array could be responsible for these problems. In these NIST measurements, an accurate determination of the leakage from a fission spectrum, modified by water scattering, is available. Previously, results for 3-, 4-, and 5-in. diam. water-filled spheres, both ... continued below

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48 pages

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Broadhead, B.L. January 11, 2001.

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Description

Measurements have been performed on a family of water spheres at the National Institute of Standards and Technology (NIST) facilities. These measurements are important for criticality safety studies in that, frequently, difficulties have arisen in predicting the reactivity of individually subcritical components assembled in a critical array. It has been postulated that errors in the neutron leakage from individual elements in the array could be responsible for these problems. In these NIST measurements, an accurate determination of the leakage from a fission spectrum, modified by water scattering, is available. Previously, results for 3-, 4-, and 5-in. diam. water-filled spheres, both with and without cadmium covers over the fission chambers, were presented for four fissionable materials: {sup 235}U, {sup 238}U, {sup 237}Np, and {sup 239}Pu. Results were also given for ''dry'' systems, in which the water spheres were drained of water, with the results corresponding to essentially measurements of unmoderated {sup 252}Cf spontaneous-fission neutrons. The calculated-to-experimental (C/E) values ranged from 0.94 to 1.01 for the dry systems and 0.93 to 1.05 for the wet systems, with experimental uncertainties ranging from 1.5 to 1.9%. These results indicated discrepancies that were clearly outside of the experimental uncertainties, and further investigation was suggested. This work updates the previous calculations with a comparison of the predicted C/E values with ENDF/B-V and ENDF/B-VI transport cross sections. Variations in the predicted C/E values that arise from the use of ENDF/B-V, ENDF/B-VI, ENDL92, and LLLDOS for the response fission cross sections are also tabulated. The use of both a 45-group NIST fission spectrum and a continuous-energy fission spectrum for {sup 252}Cf are evaluated. The use of the generalized-linear-least-squares (GLLSM) procedures to investigate the reported discrepancies in the water sphere results for {sup 235}U, {sup 238}U, {sup 239}Pu, and {sup 237}Np is reported herein. These studies should be a valuable exercise to demonstrate the utility of the GLLS methodology and to attempt to understand the discrepancies seen.

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48 pages

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  • Other Information: PBD: 11 Jan 2001

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  • Report No.: ORNL/TM-2000/173
  • Grant Number: AC05-96OR22464
  • DOI: 10.2172/777643 | External Link
  • Office of Scientific & Technical Information Report Number: 777643
  • Archival Resource Key: ark:/67531/metadc716764

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  • January 11, 2001

Added to The UNT Digital Library

  • Sept. 29, 2015, 5:31 a.m.

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  • March 21, 2016, 6:09 p.m.

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Broadhead, B.L. Further Investigations of NIST Water Sphere Discrepancies, report, January 11, 2001; Tennessee. (digital.library.unt.edu/ark:/67531/metadc716764/: accessed September 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.