Monte Carlo characterization of the cosmic ray muon flux in shallow subsurface geological repositories intended for disposal of radioactive materials.

Recent challenges in monitoring subsurface geological repositories intended for disposal of radioactive materials such as spent nuclear fuel call for new, innovative concepts that are facility independent, cost-effective, passive, and reliable. Once nuclear material is in place at these facilities, reverifying the inventory may no longer be feasible if continuity of knowledge is lost or unavailable to the inspectors. Using cosmic ray muons may present several potential advantages over conventional photon/neutron signatures, and their use in safeguards applications have only received attention in the past decade. However, there have been limited efforts to explore the integration of cosmic ray muons into repository safeguards and study potential gains, risks, and costs. This paper presents a Monte Carlo-based methodology to characterize the cosmic ray muon flux, including muon angular and energy differential distributions at depths representative of subsurface geological repositories. Since there have been limited measurements at these sites and a measurement made in one site is not always transferable to another site, the objective is to develop an efficient simulation method and useful parametrizations to provide a convenient tool for enabling muon simulations at any geological repository site. It is expected these results will provide a better understanding of how muons can be integrated into an existing geological repository safeguards framework.

A stilbene - CdZnTe based radioxenon detection system.

Atmospheric monitoring of radioxenon is one of the most widely used methods by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to detect elevated levels of 131mXe, 133/133mXe, and 135Xe. The ratios of these radionuclides help discriminate between peaceful use of nuclear technology and nuclear weapon explosions. Radioxenon detection systems often use plastic scintillators in the capacity of an electron detector and a gas cell, plastic gas cells are responsible for introducing high memory effect in these systems. This work presents the design of a new detection system for radioxenon monitoring that utilizes silicon photomultipliers, a stilbene gas cell, and a CdZnTe detector. This detector was evaluated using xenon radioisotope samples produced in the TRIGA reactor at Oregon State University. A 48-h background was collected and calculations of the Minimum Detectable Concentration (MDC) were carried out using the Region of Interest (ROI) approach. An MDC of less than 1 mBq/m3 was obtained for 131mXe, 133Xe, and 133mXe in accordance with the sensitivity limits set by the CTBTO and performs respectably when compared to state-of-the-art radioxenon detection systems. Using 131mXe, this study indicates that the stilbene gas cell exhibits a memory effect of 0.045 ±â€¯0.017%, this is almost a two-order magnitude improvement compared to plastic scintillators. The primary purpose of this work is to explore the use of new stilbene detection media for radioxenon application and addressing the problem of memory effect.