Extrapolation Lengths in Pulsed Neutron Diffusion Measurements

Sjöstrand, N.G.

doi:10.1007/978-1-4613-9913-1_2

Cited by 10 publications

(3 citation statements)

References 43 publications

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“…Where; λ is the fundamental mode decay constant, Σ a is the neutron macroscopic absorption cross-section, D is the diffusion coefficient which is defined as D ≈ Σ a /(3 * Σ 2 t ), B g is geometric buckling, C is the neutron cooling coefficient, and v is the neutron velocity [5]. This equation is an extension of diffusion theory but can reasonably calculate the time-dependent rate of change of the flux leaking from a moderator.…”

Section: Pnda Theorymentioning

confidence: 99%

Validation of S(α, β) thermal neutron scattering libraries using pulsed-neutron die-away experiments

et al. 2023

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Validation of the accuracy of S(α, β) thermal scattering law (TSL) evaluations for moderator materials is an important task for the development of high-performance nuclear engineering systems. Many recent thermal neutron scattering evaluations have had limited experimental validation. Like other nuclear data, validation of TSL libraries has historically been by integral criticality benchmarks. While sufficient for general study, these benchmarks often have limited sensitivity to the tested TSLs, and compounding uncertainties from other nuclear data can make validation ambiguous. In some cases, no criticality benchmarks exist that are sensitive to the TSLs of interest. With the development of high-performance next-generation thermal nuclear reactors, alternative validation of applicable TSLs is of high importance. By performing thermal neutron measurements via pulsed-neutron die-away (PNDA) experiments, along with parallel simulations, the integral performance of various TSL evaluations can be compared to measured experimental data. An experimental testbed using a D-T neutron generator, moderator sample, and thermal neutron detector was assembled at Rensselaer Polytechnic Institute. A Thermo Scientific D211 Deuterium-Tritium Neutron generator is used to generate 10 μs neutron pulses. Various targets of different sizes and geometries are used to moderate the neutrons. Multiple detector types and configurations were tested to optimize the experiment. The room-temperature polyethylene TSL evaluation is well vetted and is similar across different evaluations. This makes it an ideal evaluation to compare with the experimental results.

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Section: Pnda Theorymentioning

confidence: 99%

Validation of S(α, β) thermal neutron scattering libraries using pulsed-neutron die-away experiments

et al. 2023

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“…Generally, any extrapolated dimension (to the point where the asymptotic neutron flux is assumed to vanish) can be expressed as = g + d , where g is the geometric dimension and d the extrapolation distance related to this dimension. The extrapolation distance is proportional to the transport mean free path l tr for thermal neutrons in the medium, d = k l tr , but the proportionality coefficient k depends on the theoretical model assumed (here a hydrogenous medium is a particular case) and on the size and curvature of the surface of the investigated volume (Williams 1964, Sjöstrand 1977, Woźnicka 1997. Therefore, the measured thermal neutron parameters are influenced by various factors, purely experimental as well as interpretational.…”

Section: Introductionmentioning

confidence: 99%

The diffusion cooling coefficient for thermal neutrons in Plexiglas

Drozdowicz¹

1998

J. Phys. D: Appl. Phys.

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The thermal neutron diffusion cooling coefficient is a macroscopic material parameter which is needed for a description of the decay of the thermal neutron pulse in a medium and gives information on the diffusion cooling of the thermal neutron spectrum in a bounded volume. Experimental results from various measurements for Plexiglas (a hydrogenous substance) are overviewed in the paper. A method for a theoretical calculation of the parameter is presented. The formula used utilizes other thermal neutron parameters and a cooling function which describes the deviation of the neutron spectrum in a bounded system from the distribution in an infinite one. The energy dependence of the function is obtained numerically from relations which result from the eigenvalue problem of the scattering operator when both the decay constant and the spectrum of the thermal neutron flux are developed in powers of the geometrical buckling. The calculation utilizes Granada's synthetic scattering function. The obtained value of the diffusion cooling coefficient for Plexiglas is at the temperature of C. The uncertainty is estimated to be within the physical model of the scattering kernel used. Results obtained using the same type of kernel in other theoretical methods are also included.

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“…Therefore, the "correct" value for ‫ܤ‬ ଶ is the value that reproduces the physical ߙ in a real homogeneous system with material properties ܽ ୣ , ‫ܦ‬ ୣ , ‫,ܥ‬ etc. Likewise, the "correct" ‫ݖ‬ in any standard-geometry ‫ܤ‬ ଶ formula is that which yields the "correct" ‫ܤ‬ ଶ[54].The treatment of ‫ݖ‬ and ‫ܤ‬ ଶ in PNDA experiments have been the subject of considerable study[24,[54][55][56][57][58], including for light water and for spherical systems. The appropriate ‫ݖ‬ is a function of material density, material scattering and absorption properties, and geometry.…”

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confidence: 99%

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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Extrapolation Lengths in Pulsed Neutron Diffusion Measurements

Cited by 10 publications

References 43 publications

Validation of S(α, β) thermal neutron scattering libraries using pulsed-neutron die-away experiments

Validation of S(α, β) thermal neutron scattering libraries using pulsed-neutron die-away experiments

The diffusion cooling coefficient for thermal neutrons in Plexiglas

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

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