Adrienne M. LaFleur

Development of Self-Interrogation Neutron Resonance Densitometry to Improve Detection of Possible Diversions for PWR Spent Fuel Assemblies

A new nondestructive assay technique called self-interrogation neutron resonance densitometry (SINRD) is currently being developed at Los Alamos National Laboratory to improve existing nuclear safeguards and material accountability measurements for light water reactor fuel assemblies. The viability of using SINRD to improve the detection of possible diversion scenarios for pressurized water reactor 17 × 17 spent low-enriched uranium (LEU) and mixed oxide (MOX) fuel assemblies was investigated via Monte Carlo N-Particle eXtended transport code (MCNPX) simulations. The following capabilities were assessed: (a) verification of the burnup of a spent fuel assembly, (b) ability to distinguish fresh and one-cycle spent MOX fuel from three- and four-cycle spent LEU fuel, and (c) sensitivity and penetrability to the removal of fuel pins. SINRD utilizes 244Cm spontaneous-fission neutrons to self-interrogate the spent fuel pins. The amount of resonance absorption of these neutrons in the fuel can be quantified using a set of fission chambers (FCs) placed adjacent to the assembly. The sensitivity of SINRD is based on using the same fissile materials in the FCs as are present in the fuel because the effect of resonance absorption lines in the transmitted flux is amplified by the corresponding (n, f) reaction peaks in the FC. SINRD requires calibration with a reference assembly of similar geometry in a similar measurement configuration with the same surrounding moderator as the spent fuel assemblies. However, this densitometry method uses ratios of different detectors so that several systematic errors related to calibration and positioning cancel in the ratios.

Development of Self-Interrogation Neutron Resonance Densitometry to Quantify the Fissile Content in PWR Spent LEU and MOX Assemblies

Abstract A new nondestructive assay technique called self-interrogation neutron resonance densitometry (SINRD) is currently being developed at Los Alamos National Laboratory to improve existing nuclear safeguards and material accountability measurements for light water reactor fuel assemblies. The viability of using SINRD to quantify the fissile content (235U and 239Pu) in pressurized water reactor 17 × 17 spent low-enriched uranium and mixed-oxide fuel assemblies in water was investigated via Monte Carlo N-particle extended transport code simulations. SINRD utilizes 244Cm spontaneous fission neutrons to self-interrogate the fuel pins. The amount of resonance absorption of these neutrons in the fuel can be quantified using 235U and 239Pu fission chambers placed adjacent to the assembly. The sensitivity of this technique is based on using the same fissile materials in the fission chambers as are present in the fuel because the effect of resonance absorption lines in the transmitted flux is amplified by the corresponding (n,f) reaction peaks in the fission chamber. SINRD requires calibration with a reference assembly of similar geometry. However, this densitometry method uses ratios of different fission chambers so that most systematic errors related to calibration and positioning cancel in the ratios.

Development of prototype induced-fission-based Pu accountancy instrument for safeguards applications

Prototype safeguards instrument for nuclear material accountancy (NMA) of uranium/transuranic (U/TRU) products that could be produced in a future advanced PWR fuel processing facility has been developed and characterized. This is a new, hybrid neutron measurement system based on fast neutron energy multiplication (FNEM) and passive neutron albedo reactivity (PNAR) methods. The FNEM method is sensitive to the induced fission rate by fast neutrons, while the PNAR method is sensitive to the induced fission rate by thermal neutrons in the sample to be measured. The induced fission rate is proportional to the total amount of fissile material, especially plutonium (Pu), in the U/TRU product; hence, the Pu amount can be calibrated as a function of the induced fission rate, which can be measured using either the FNEM or PNAR method. In the present study, the prototype system was built using six (3)He tubes, and its performance was evaluated for various detector parameters including high-voltage (HV) plateau, efficiency profiles, dead time, and stability. The systems capability to measure the difference in the average neutron energy for the FNEM signature also was evaluated, using AmLi, PuBe, (252)Cf, as well as four Pu-oxide sources each with a different impurity (Al, F, Mg, and B) and producing (α,n) neutrons with different average energies. Future work will measure the hybrid signature (i.e., FNEM×PNAR) for a Pu source with an external interrogating neutron source after enlarging the cavity size of the prototype system to accommodate a large-size Pu source (~600g Pu).

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.

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.