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Title: Validation of Dosimetry Data Using Historic and Recent Measurements on the Flattop Critical Assembly

  1. Los Alamos National Laboratory
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
Report Number(s):
DOE Contract Number:
Resource Type:
Resource Relation:
Conference: ND2016 International Conference on Nuclear Data for Science and Technology ; 2016-09-11 - 2016-09-16 ; Brugge, Belgium
Country of Publication:
United States

Citation Formats

White, Morgan Curtis. Validation of Dosimetry Data Using Historic and Recent Measurements on the Flattop Critical Assembly. United States: N. p., 2016. Web.
White, Morgan Curtis. Validation of Dosimetry Data Using Historic and Recent Measurements on the Flattop Critical Assembly. United States.
White, Morgan Curtis. 2016. "Validation of Dosimetry Data Using Historic and Recent Measurements on the Flattop Critical Assembly". United States. doi:.
title = {Validation of Dosimetry Data Using Historic and Recent Measurements on the Flattop Critical Assembly},
author = {White, Morgan Curtis},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 9

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  • According to Russian federal norms and the safety guide of the nuclear regulatory body of Russia, the maximum fast neutron fluence above 0.5 MeV at critical positions of the reactor pressure vessel (RPV) of VVER-type reactors is used for prediction of the RPV lifetime. For the computation of neutron fluences in the RPV near the reactor core midplane level, the three-dimensional (3-D) synthesis method based on two- and one-dimensional S{sub N} calculations may be acceptable but needs validation. The present validation analysis was carried out on the basis of neutron transport calculations for a VVER-1000 model by means of themore » well-known codes DORT (R, {Theta}- and R, Z geometry) and ANISN (R geometry) using the multigroup library BUGLE-96. The 3-D spatial neutron source distribution, including pin-to-pin power variations and the complex baffle construction, were modeled in detail.« less
  • Burnable absorbers such as gadolinium are used in different, advanced reactor types. The use of burnable absorbers in VVERs (Russian-designed pressurized water reactor, characterized by hexagonal lattice with low H/U ratios) had not been studied earlier, so the International Atomic Energy Agency (IAEA) started a coordinated research program in this field. An important task in this research program was to validate different computer codes that calculate VVER problems for lattices with gadolinium. This validation could be best performed on the basis of experimental results. Therefore, experiments have been carried out with absorbers of different gadolinium content in a critical assembly.more » The activities described here were carried out under a research contract with the IAEA.« less
  • We describe new dosimetry (radiochemical) ENDF evaluations for yttrium, iridium, and thulium. These LANL2006 evaluations were based upon measured data and on nuclear model cross section calculations. In the case of iridium and yttrium, new measurements using the GEANIE gamma-ray detector at LANSCE were used to infer (n,xn) cross sections, the measurements being augmented by nuclear model calculations using the GNASH code. The thulium isotope evaluations were based on GNASH calculations and older measurements. The evaluated cross section data are tested through comparisons of simulations with measurements of reaction rates in critical assemblies and in Bethe sphere (sometimes called Wymanmore » sphere) integral experiments. Two types of Bethe sphere experiments were studied - a LiD experiment that had a significant component of 14 MeV neutrons, and a LiD-U experiment that additionally had varying amounts of fission neutrons depending upon the location. These simulations were performed with the MCNP code using continuous energy Monte Carlo, and because the neutron fluences can be modeled fairly accurately by MCNP at different locations in these assemblies, the comparisons provide a valuable validation test of the accuracy of the evaluated cross sections and their energy dependencies. The MCNP integral reaction rate validation testing for the three detectors yttrium, iridium, and thulium, in the LANL2006 database is summarized as follows: (1) (n,2n)near 14 MeV: In 14 MeV-dominated locations (the LiD Bethe spheres and the outer regions of the LiD-U Bethe spheres), the (n,2n) products are modeled very well for all three detectors, suggesting that the evaluated {sup 89}Y(n,2n), {sup 191}Ir(n,2n), and {sup 169}Tm(n,2n) cross sections are accurate to better than about 5% near 14 MeV; (2) (n,2n)near threshold: In locations that have a significant number of fission spectrum neutrons or downscattered neutrons from 14 MeV inelastic scattering (the central regions of the LiD-U spheres and the fast critical assemblies), the (n,2n) products are overpredicted by 5-30 % for the three detectors, suggesting either the threshold region (n,2n) cross sections are too high, or that the MCNP-simulated neutron flux is too large for neutron energies above about 8 MeV; (3) Capture: The capture products for yttrium are modeled accurately for the LiD Bethe spheres, but are underpredicted by about 20% for the LiD-U Bethe spheres and the critical assemblies; for iridium-191 they are predicted accurately in the critical assemblies; and for thulium they are generally overpredicted by 10-30 %; (4) Inelastic scattering in iridium: The evaluated {sup 193}Ir(n,n{sup '}){sup 193m}Ir cross section performs well over a very wide range of neutron spectra (where the 193m/190 spectrum hardness index varies by over three orders of magnitude), the differences between simulation and experiment typically being better than 10-15%; (5) Iridium 193m/190 spectrum hardness index: Our simulations reproduce the measured 193m/190 data typically to better than 10-20% over three orders of magnitude in the 193m/190 ratio. The aforementioned indications from data testing involving assemblies containing actinides - that the (n,2n) products are overpredicted by 5-30% - could be used to motivate a decrease in the evaluated (n,2n) cross sections in the approximately 8-12 MeV range. However, at this stage we have not modified these cross sections since: (a) They are consistent with the cross section laboratory measurements; and (b) It is possible that the cross sections are correct and instead the simulated integral assembly neutron spectrum is too high for neutron energies above 8 MeV. The latter possibility is particularly intriguing given all three detector materials showed a bias in the same direction, and that the evaluated actinide prompt fission spectra and inelastic scattering data are probably uncertain to at least 20% above 8 MeV. We also discuss refinements needed in the transport methods to faithfully represent the evaluated data.« less