Generation of the¹H in H₂O neutron thermal scattering law covariance matrix of the CAB model

Abstract

The thermal scattering law (TSL) of 1H in H2O describes the interaction of the neutron with the hydrogen bound to light water. No recommended procedure exists for computing covariances of TSLs available in the international evaluated nuclear data libraries. This work presents an analytic methodology to produce such a covariance matrix-associated to the water model developed at the Atomic Center of Bariloche (Centro Atomico Bariloche, CAB, Argentina). This model is called as CAB model, it calculates the TSL of hydrogen bound to light water from molecular dynamic simulations. The performance of the obtained covariance matrix has been quantified on integral calculations at “cold” reactor conditions between 20 and 80∘ C. For UOX fuel, the uncertainty on the calculated reactivity ranges from ±71 to ±155 pcm. For MOX fuel, it ranges from ±110 to ±203 pcm.

Impact of H in H2O thermal scattering data on criticality calculation: uncertainty and adjustment

“…It is worth noticing that for skewed distributions with a strong kurtosis, as the majority of cases presented here, one standard deviation does not refer to 68% of the population: for lct8.5, 68.3% of the cases are within the mean ±1σ, but for lst2.1, 73% are within the mean ±1σ. One can also compare the present results for the lct and pst benchmarks with the values presented in reference [27]. In this reference, uncertainties due to TSL for the MISTRAL-1 experiment, similar to the lct benchmarks, range from 70 to 150 pcm.…”

“…The uncertainty propagation method is the Total Monte Carlo method [25], leading to k eff uncertainties (later called ∆k eff ), corresponding to one standard deviation of the observed probability density functions (pdf), as well as higher moments. Similar works on uncertainty propagation and covariance generation for TSL can be found in references [26,27]. In the first reference, the TMC method (and fast TMC) was applied, whereas the second reference defines TSL covariance matrices, with an application to UO 2 and MOX fuels.…”

CONRAD – a code for nuclear data modeling and evaluation

“…Improved results were obtained by using the CAB model for water established at the nuclear center of Bariloche (Argentina) thanks to Molecular Dynamic simulations. The correlation matrix between the Molecular Dynamic parameters obtained from the least-squares fit of neutron data are discussed in reference [45].…”

Methodology for Generating Covariance Data of Thermal Neutron Scattering Cross Sections

“…2) parameters to data, 3,4 or analytical fitting of molecular dynamics (MD) model parameters to data. 5 The latter two studies explicitly state that although they provided covariances for their respective parameters, they did not change the values of their parameters.…”

“…This contrasts with previous works that constrained the fit to the phonon density of states 4 or the average cosine of the scattering angle and total cross section. 5 Although the method can generate covariance matrix data for any property used in the evaluation procedure [including Sðα; βÞ], this work focuses on generating covariance data for the phonon density of states. While there are several groups looking into thermal scattering covariances [e.g., the General Nuclear Database Structure and Working Party on International Nuclear Data Evaluation Cooperation (WPEC) subgroup 42/44/48], there is no agreedupon method for generating or storing thermal scattering covariance data.…”