MCNP Model of the Hybrid K-Edge Densitometer System Installed at ORNL

摘要

A hybrid K-edge densitometer/X-ray fluorescence (HKED) system is being installed in the Radiochemical Engineering Development Center at Oak Ridge National Laboratory (ORNL) for training purposes and algorithm development. The International Atomic Energy Agency uses HKED systems for verification measurements at nuclear fuel reprocessing plants throughout the world, including those in France, Japan, and the United Kingdom. Following installation of the system at ORNL, operations will begin on measurements of calibration standards containing varying concentrations of uranium and plutonium in nitrate solutions. These measurements are to be used to validate system accuracy while also ensuring reliable operation over periods of continuous use. A Monte Carlo N-Particle (MCNP) code-based model of the ORNL HKED system is being developed, and the computer simulations potentially will improve understanding of measurement uncertainties and further validate the accuracy of the system. Eventually, the MCNP models may be used to decrease counting and calibration times and to study candidate improvements to the system. Issues were encountered in the development of the MCNP model of the HKED system, including long runtimes and high uncertainties. This paper will discuss actions taken to overcome these issues, which involved importance function biasing, importing a source distribution obtained through other software, and exploring the use of alternative data libraries. Also, this paper will compare real example spectra to results of MCNP code-based simulations.