Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) (2001) / Chapter Skim
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4. Radiation Transport Calculations
4. Radiation Transport Calculations
Pages 60-73
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From page 60... ...
Therefore, the conclusion of this chapter on neutron measurements must be that the neutron doses are in doubt until further work is done. The unsatisfactory performance of DS86 in calculating thermal-neutron activation was also noted in an independent review of the new dosimetry system performed by the National Research Council (NRC 1987~.
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From page 61... ...
As the spectrum of neutrons released from the nuclear explosion traverses those various environments, it is constantly changing because of the interaction of the neutrons with the elements in their path. The calculation of those interactions depends not only on the elements encountered between the nuclear explosion and the point of measurement, but also on the neutron cross sections of those materials, the abundance of the materials in the path of the neutrons, and how well the neutron cross sections of the materials are known.
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From page 62... ...
By the time they create thermal-neutron activation, the neutron energy is measured in electron volts. The final dose to a person and the production of isotopes by thermal neutrons are determined not only by the neutrons released in the explosion and their transport through air, but also by their interactions with terrain and structures that act as shielding, which further dissipate neutron energy and reduce the quality and magnitude of the radiation exposure at any point of interest.
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From page 63... ...
The results were achieved with the use of new nitrogen and oxygen cross sections for neutrons over a broad energy range, an increase in the number of neutron groups used in the calculation, and the use of different Sn techniques. However, it was possible to confirm agreement in Nagasaki only after new and better measurements were made at greater distances there and a benchmark activation-calculation experiment was completed at APRF (Straume and others
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From page 64... ...
First, there was an examination of various hypotheses about unexpected ways in which the Hiroshima bomb might have disassembled during explosion. Second, a composite source-term sensitivity analysis was conducted to determine whether any plausible combination of bomb source term and fission-spectrum neutrons could be found that would match all the in situ measured data sets for detonations at a number of bomb burst heights.
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From page 65... ...
_ 0 .~ ~ ~ W~ _ _ _ ~ . 1000 1500 Slant Range (m)
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From page 66... ...
, changes in the size and details of the air-overground geometry of the calculation, the use of newer discrete ordinates calculation and transport codes, as well as changes in the time-dependent source and geometry (see Table 4-1) , reduced the calculated thermal activation near the Nagasaki hypocenter by nearly a factor of two and brought the discrete-ordinates calculation into closer agreement with Monte Carlo calculations (Whalen 1994~.
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From page 67... ...
RADIATION TRANSPORT CALCULATIONS TABLE 4-1 Feature-by-Feature Companson of DS86 and Parameters Suggested in 1993 67 DS86 Sources - LANL 1983 sources - 27 neutron energy groups - 20 angle bins Geometry - Seven-zone air density profile - maximal radius 2800 m - maximal height 1500 m - maximal radial mesh 25 m Cross sections - ENDF/B-S - 46 neutron and 23 gamma-ray energy groups - scattering order P3 - custom weighted by region Code - DOT-4, 2-D discrete ordinates - first collision source - 240-direction angular quadrature - negative source fixup - convergence criterion 1 x 10-2 - weighted-difference flux calculation Delayed-Radiation Methodology DS86 1993 Time-dependent source - neutrons: augmented Maxwellian spectra, E to 2.5 MeV - gamma ray: empirical spectra Time-dependent geometry - line-of-sight optical depth (g/cm3) from 2-D air density contours - contours from STLAMB hydrodynamic code Transport codes - ANISN: transport in uniform air, 300 time steps - ANISN: transport in hydrodynamically perturbed air - morse: fluence perturbation due to the air-ground interference Prompt-Radiation Methodology 1993 Sources - LANL 1990 sources - 46 neutron-energy groups, 20 angles - reformatted to 174 energy groups and continuous angle distribution Geometry -DS86 material compositions and profiles - continuous vertical density variation - maximum radius 3000 m - maximal height 2000 m - maximal radial mesh 25 m Cross sections - ENDF/B-6.2 - 174 neutron - 38 gamma-ray energy groups - Scattering order P3 Code - DORT, 2-D discrete ordinates - first collision source - 240-direction angular quadrature - no negative source fixup - convergence criterion 1 x 10-3 - theta-weighted flux calculation Time-dependent source - neutrons: ENDF/B-6 spectra, E to 8 MeV - gamma rays: ENDF/B-6 spectra Time-dependent geometry - 2-D air density contours - contours from STLAMB hydrodynamic code Transport codes - DORT: transport in 2-D air-over-ground, 12 time steps
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From page 68... ...
THE HIROSHIMA BOMB AND POSSIBLE OTHER SOURCE TERMS The combination of the changes in calculation methods, the new chlorine measurement, and the definition and implementation of new oxygen and nitrogen cross sections were all necessary to achieve agreement between the calculations and in situ measurements in Nagasaki. None of those improvements has been implemented in DS86, which has remained essentially unchanged since its inception in 1986.
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From page 69... ...
2000 2500 FIGURE 4-2 Nagasaki thermal-neutron activation (1993) revised calculation to measurement ratio as a function of slant range from the hypocenter placed beside Hiroshima thermal-neutron activation (1993)
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From page 70... ...
to suggest that the Hiroshima bomb could have split from the shock of the high explosive before the bomb reached peak nuclear power providing a portal for the release of fissionspectrum neutrons. They suggested that adding such high-energy neutrons might result in a bomb output spectrum that would better match the in situ neutronactivation measurements.
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From page 71... ...
The result from this analysis is that no combination of composite source and height-of-burst could be identified that could simultaneously reconcile the best available 1993 calculation with the TLD, sulfur activation, and thermal-neutron activation data (Kaul and others 1994~. The overall implication of all the work done on alternative disassembly hypotheses is that they do not reconcile the Hiroshima calculations and measurements.
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From page 72... ...
Using the APRF at about one-third of the total neutron leakage calculated for the weapons produced an excellent fit to within a few percent of thermal-neutron activation data. That solution failed for two reasons: it produced a discrepancy of a factor of 10 with the sulfur data under the bomb, and it required the elimination of most of the conventional leakage (which is a softer neutron spectrum)
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From page 73... ...
It will include a late-time output spectrum that incorporates new iron cross sections and transmission through the bomb case, a new Monte Carlo source term as a function of energy and angle, and a Monte Carlo transport of the new source to the ground accounting for the tilt and heading of the bomb, and the transport of delayed neutrons over time through air created in a new spherical air blast calculation. The new calculations, which will use the latest ENDF/B-6 cross sections, will be compared with source and DORT calculations.
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