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Title: Kinetics of the Autoreduction of Hexavalent Americium in Aqueous Nitric Acid

The rate of reduction of hexavalent americium due to self-radiolysis was measured across a range of total americium and nitric acid concentrations. These so-called autoreduction rates exhibited zero order kinetics with respect to the concentration of hexavalent americium, and pseudo-first order kinetics with respect to the concentration of total americium. This indicates that reduction is due to reaction of hexavalent americium with the radiolysis product of total americium decay. The concentration changes of Am(III), Am(V) and Am(VI) were determined by UV-vis spectroscopy. The Am(III) molar extinction coefficients are known; however, the unknown values for the Am(V) and Am(VI) absorbances across the range of nitric acid concentrations studied were determined by sensitivity analysis in which mass balance with the known total americium concentration was obtained. The new extinction coefficients and reduction rate constants have been tabulated here. Multi-scale radiation chemical modeling using a reaction set with both known and estimated rate coefficients was employed to achieve excellent agreement with the experimental results, and indicates radiolytically-produced nitrous acid from nitric acid radiolysis, and hydrogen peroxide from water radiolysis are the important reducing agents. Since these species also react with each other, modeling indicated the highest concentrations of these species occurred at intermediatemore » nitric acid concentrations. In conclusion, this is in agreement with the empirical finding that the highest rate constant for autoreduction occurred at the intermediate acid concentration.« less
Authors:
ORCiD logo [1] ; ORCiD logo [2] ;  [3] ;  [4] ;  [5] ;  [1]
  1. Idaho National Lab. (INL), Idaho Falls, ID (United States)
  2. California State University at Long Beach, Long Beach, CA (United States); Univ. of Notre Dame, Notre Dame, IN (United States)
  3. Florida Intl Univ., Miami, FL (United States)
  4. The Univ. of Manchester, Cumbria (United Kingdom); The Univ. of Manchester, Manchester (United Kingdom)
  5. California State University at Long Beach, Long Beach, CA (United States)
Publication Date:
Report Number(s):
INL/JOU-17-41567-Rev000
Journal ID: ISSN 0020-1669
Grant/Contract Number:
AC07-05ID14517
Type:
Accepted Manuscript
Journal Name:
Inorganic Chemistry
Additional Journal Information:
Journal Volume: 56; Journal Issue: 14; Journal ID: ISSN 0020-1669
Publisher:
American Chemical Society (ACS)
Research Org:
Idaho National Lab. (INL), Idaho Falls, ID (United States)
Sponsoring Org:
USDOE Office of Nuclear Energy (NE)
Country of Publication:
United States
Language:
English
Subject:
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS; Americium Reduction; Reduction Kinetics; Hexavalent Americium Reduction
OSTI Identifier:
1471064