Publications

The Nuclear Criticality Safety Program at Sandia National Laboratories has developed a suite of benchmark problems to be utilized for validation of MCNP6 Version 1.0 with ENDF/B-VII Release 1 cross sections. The benchmark suite covers a broad range of fissile material types, material forms, moderators, reflectors, and neutron energy spectra. It is anticipated that this benchmark suite will cover the vast majority of critical safety applications at SNL. The benchmark suite establishes a Bias and Bias Uncertainty for use in criticality safety analyses. In addition, the Bias and Bias Uncertainty value derived from the benchmark suite using the traditional SNL NCS methodology is demonstrated to be equivalent to the more robust statistical techniques used at many DOE sites.

Abstract not provided.

The process nuclear criticality accident that occurred at the Mayak Production Association (Chelyabinsk-40) on January 2, 1958 involving a vessel of uranyl nitrate solution claimed the lives of three workers and left a fourth worker with continuing health problems. There are a myriad of uncertain parameters involved with this accident: What was the molarity of the solution? How much solution was in the vessel at the time of the accident? In what position was the vessel and the solution when it went critical? How important was the impact of reflection due to the workers and/or the floor? These uncertain parameters have made this accident particularly difficult to analyze in the past. This work aims to lower the uncertainty on some of these parameters. A most-probable solution composition is determined by comparing literature on the physical properties of uranyl nitrate solutions to those presented in LA-13638 [1], which describes the accident in question. Using this most-probable solution, the main contributions to the reactivity of the system and hence the eventual accident, are identified through Serpent 2 and OpenFOAM analyses. Serpent 2, a Monte Carlo software tool, is used to perform calculations of the reactivity effects of lowering the vessel toward the floor and the reactivity added by the close proximity of workers. OpenFOAM, a C++ partial differential equation solver toolkit, is used to simulate the fluid inside the vessel as the vessel is tipped. This is done by treating the solution and air inside the vessel as two incompressible, isothermal, and immiscible fluids using a volume of fluid (VoF) approach. The goal of this approach is simply to track the interface between the two fluids, and hence give an accurate description of the geometrical structure of the solution as the vessel is tipped. These two unique tools are then coupled to provide a time-dependent flow simulation to study the effect that the changing geometrical structure had on the criticality of the system, which is novel to the criticality safety field. This work provides a more accurate picture of the accident going forward. Key Words: Serpent 2, OpenFOAM, multi-physics, prompt neutron excursion, nuclear criticality safety accident, process condition change.