Measurement and Calculation of Neutron Diffusion Parameters in Water

Kobayashi, Keisuke; Seki, Y.; Mizoo, Nobutatsu; Watanabe, Tetsuyou; Watanabe, Takashi; Nishihara, Hiroshi

doi:10.3327/jnst.3.275

Cited by 3 publications

(2 citation statements)

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“…By determining ߙ over a range of ‫ܤ‬ ଶ (or for samples of varying dimension), the parameters ܽ ୣ , ‫ܦ‬ ୣ , and ‫ܥ‬ can be calculated with a quadratic fit of ߙ to ‫ܤ‬ ଶ (or higher-order fits may be applied as appropriate). PNDA experiments have been used extensively to study the diffusion properties of water [23,[25][26][27][28][29][30][31][32][33][34]. If an external source makes up for losses in the flux such that it is constant in time, this is a stationary case.…”

Section: Theoretical and Experimental Aspects Of Neutron Diffusionmentioning

confidence: 99%

“…Considering that the PNDA-based experimental ‫ܮ‬ data used in the work is biased about 1% lower than the stationary-method ‫ܮ‬ data (see Section 2.1), it is possible that the experimental ‫ܦ‬ ୣ average is biased too low (since ‫ܮ‬ ≈ ඥ‫ܦ‬ ୣ /ܽ ୣ and ܽ ୣ can be determined more accurately than ‫ܦ‬ ୣ ). The uncertainty in experimentally calculated ‫ܥ‬ (and in higher-order terms) is always large [23,29,30,33,34]. This is because higher-order diffusion effects are very weak in light water (i.e., the absorption cross section is very small compared to the scattering cross section, and differential scattering probabilities have near-linear dependence on the scattering angle).…”

Section: Uncertaintiesmentioning

confidence: 99%

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Validation of Thermal Scattering Laws for Light Water at Elevated Temperatures With Diffusion Experiments

2021

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Light water is the most important neutron moderator in many reactor applications. Thermal neutron scattering kernels for H bound in H2O are represented by ENDF-format thermal scattering laws (TSLs) evaluated at discrete temperatures T. Nuclear data evaluations are conventionally validated utilizing a combination of critical benchmarks and experimental cross section data. Existing public critical benchmarks may be inadequate for testing water TSLs over the full range of T of interest in reactor applications, and experimental thermal scattering cross section data for water is sparse at elevated T. In this work, MC21 is used to simulate the decay of the equilibrium thermal neutron flux in light-water spheres of several radii. The fundamental-mode time eigenvalue α is calculated at 22 °C and 227 °C (at saturation pressure) for each sphere using three different H-H2O TSLs. Fitting α to a polynomial function of geometric buckling allows calculation of the thermal neutron diffusion length L. Extrapolation lengths are treated as a function of the transport mean free path and geometry. Experimental L data from 25 publications at 49 temperatures (from 10 °C to 295 °C), computed by several different time-dependent and space-dependent decay methods, is used to develop an empirical fit of L vs. T. The MC21-calculated L for the TSLs tested are within ±1% of the predicted values at 22 °C and 227 °C. This validation approach, which may be repeated at arbitrary T, constitutes an integral benchmark specific to and characterizing the detailed physics of the thermal scattering kernel applied.

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