Stephen J. Tobin

The TCS Rotating Magnetic Field FRC Current-Drive Experiment

Field-reversed configurations (FRCs) have extremely attractive reactor attributes because of their singly connected geometry. They have been created in theta-pinch devices, but being compact toroids and lacking a center hole, their toroidal current cannot be sustained by transformer action as in other toroidal configurations. A new device, the Translation, Confinement, and Sustainment (TCS) facility has been constructed to use rotating magnetic fields (RMFs) to build up and sustain the flux of hot FRCs formed by the normal theta-pinch method. RMF formation and sustainment of similar, but cold, pure poloidal field configurations have been demonstrated in devices called rotamaks, and RMF formation, but not sustainment, has been achieved in a smaller FRC facility called the Star Thrust Experiment (STX). Initial formation and sustainment have now been achieved in TCS, albeit still with cold (Te ~ 50 eV) plasmas. Both the formation and final steady-state conditions are found to agree with newly developed analytic and numerical models for RMF flux buildup and sustainment inside a standard cylindrical flux conserver. The required plasma conditions (mainly resistivity but also density) can now be determined for the planned hot FRC, RMF flux buildup experiments and for eventual reactor conditions.

Integrated Nondestructive Assay Systems to Estimate Plutonium in Spent Fuel Assemblies

Abstract An integrated nondestructive assay (NDA) system combining active (neutron generator) and passive neutron detection and passive gamma (PG) detection is being analyzed in order to estimate the amount of plutonium, verify initial enrichment, burnup, and cooling time, and detect partial defects in a spent fuel assembly (SFA). Active signals are measured using the differential die-away (DDA), delayed neutron (DN), and delayed gamma (DG) techniques. Passive signals are measured using total neutron (TN) counts and both gross and spectral resolved gamma counts. To quantify how a system of several NDA techniques is expected to perform, all of the relevant NDA techniques listed above were simulated as a function of various reactor conditions such as initial enrichment, burnup, cooling time, assembly shuffling pattern, reactor operating conditions (including temperature, pressure, and the presence of burnable poisons) by simulating the NDA response for five sets of light water reactor assemblies. This paper compares the performance of several exploratory model-fitting options (including neural networks, adaptive regression with splines, iterative bias reduction smoothing, projection pursuit regression, and regression with quadratic terms and interaction terms) to relate data simulated with measurement and model error effects from various subsets of the NDA techniques to the total Pu mass. Isotope masses for SFAs and expected detector responses (DRs) for several NDA techniques are simulated using MCNP, and the DRs become inputs to the fitting process. Such responses include eight signals from DDA, one from DN, one from TN, and up to seven from PG; the DG signal will be examined separately. Results are summarized using the root-mean-squared estimation error for plutonium mass in held-out subsets of the data for a range of model and measurement error variances. Different simulation assumptions lead to different spent fuel libraries relating DRs to Pu mass. Some results for training with one library and testing with another library are also given.

Nondestructive Assay Options for Spent Fuel Encapsulation

This report describes the role that nondestructive assay (NDA) techniques and systems of NDA techniques may have in the context of an encapsulation and deep geological repository. The potential NDA ...

AFCI Safeguards Enhancement Study: Technology Development Roadmap

The Advanced Fuel Cycle Initiative (AFCI) Safeguards Campaign aims to develop safeguards technologies and processes that will significantly reduce the risk of proliferation in the U.S. nuclear fuel cycle of tomorrow. The Safeguards Enhancement Study was chartered with identifying promising research and development (R&D) directions over timescales both near-term and long-term, and under safeguards oversight both domestic and international. This technology development roadmap documents recognized gaps and needs in the safeguarding of nuclear fuel cycles, and outlines corresponding performance targets for each of those needs. Drawing on the collective expertise of technologists and user-representatives, a list of over 30 technologies that have the potential to meet those needs was developed, along with brief summaries of each candidate technology. Each summary describes the potential impact of that technology, key research questions to be addressed, and prospective development milestones that could lead to a definitive viability or performance assessment. Important programmatic linkages between U.S. agencies and offices are also described, reflecting the emergence of several safeguards R&D programs in the U.S. and the reinvigoration of nuclear fuel cycles across the globe.

Uncertainty Quantification for New Approaches to Spent Fuel Assay

Abstract Estimating plutonium (Pu) mass in spent nuclear fuel assemblies (SFAs) helps inspectors ensure that no Pu is diverted. Therefore, nondestructive assay (NDA) methods are being developed to assay Pu mass in SFAs. Uncertainty quantification is an important task in most assay methods, and particularly for SFA assay. A computer model (MCNPX) is being used to predict isotope masses and the spatial distribution of masses in virtual SFAs for 64 combinations of initial fuel enrichment (IE), fuel utilization [burnup (BU)], and cooling time (CT) values. Additional MCNPX modeling for the same 64 virtual SFAs provided the expected detector responses (DRs) for several NDA techniques such as the passive neutron albedo reactivity method and the 252Cf interrogation with prompt neutrons method. A previous paper describes one uncertainty quantification approach involving Monte Carlo (MC) simulation using individually any of six new NDA options together with IE, BU, and CT. This paper provides an interpretation of the MC approach that is suited for a numerical Bayesian alternative, separately assesses the impact of MCNPX interpolation error, and compares several options to use subsets of IE, BU, CT, and one DR.

On Using Code Emulators and Monte Carlo Estimation to Predict Assembly Attributes of Spent Fuel Assemblies for Safeguards Applications

Abstract The quantification of the plutonium mass in spent nuclear fuel assemblies is an important measurement for nuclear safeguards practitioners. A program is well underway to develop nondestructive assay instruments that, when combined, will be able to quantify the plutonium content of a spent nuclear fuel assembly. Each instrument will quantify a specific attribute of the spent fuel assembly, e.g., the fissile content. In this paper, we present a Monte Carlo-based method of estimating the mean and distribution of some assembly attributes. An MCNPX model of each instrument has been created, and the response of the instrument was simulated for a range of spent fuel assemblies with discrete parameters (e.g., burnup, initial enrichment, and cooling time). The Monte Carlo-based method interpolates between the modeled results for an instrument to emulate a response for parameters not explicitly modeled. We demonstrate the usefulness of this technique in applying the technique to six different instruments under investigation. The results show that this Monte Carlo-based method can be used to estimate the assembly attributes of a spent fuel assembly based upon the measured response from the instrument.

Robotic Spent Fuel Monitoring – It is time to improve old approaches and old techniques!

This report describes various approaches and techniques associated with robotic spent fuel. The purpose of this description is to improve the quality of measured signatures, reduce the inspection burden on the IAEA, and to provide frequent verification.

Spectral Shifting in Nondestructive Assay Instrumentation

This project involves spectrum tailoring research that endeavors to better distinguish energies of gamma rays using different spectral material thicknesses and determine neutron energies by coating detectors with various materials.

Nondestructive Assay Data Integration with the SKB-50 Assemblies - FY16 Update

A project to research the application of non-destructive assay (NDA) techniques for spent fuel assemblies is underway at the Central Interim Storage Facility for Spent Nuclear Fuel (for which the S ...

Near Real-Time Nondestructive Active Inspection Technologies Utilizing Delayed γ-Rays and Neutrons for Advanced Safeguards

In this two year project, the research team investigated how delayed γ-rays from short-lived fission fragments detected in the short interval between irradiating pulses can be exploited for advanced safeguards technologies. This program contained experimental and modeling efforts. The experimental effort measured the emitted spectra, time histories and correlations of the delayed γ-rays from aqueous solutions and solid targets containing fissionable isotopes. The modeling effort first developed and benchmarked a hybrid Monte Carlo simulation technique based on these experiments. The benchmarked simulations were then extended to other safeguards scenarios, allowing comparisons to other advanced safeguards technologies and to investigate combined techniques. Ultimately, the experiments demonstrated the possible utility of actively induced delayed γ-ray spectroscopy for fissionable material assay.