APPLICATION OF MONTE CARLO MODELING OF COMPTON SUPPRESSION SPECTROSCOPY TO SPENT FUEL MATERIAL ACCOUNTANCY

Open Access
Author:
Bender, Sarah Elizabeth
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Kenan Unlu, Thesis Advisor
Keywords:
  • Compton Suppression
  • Safeguards
Abstract:
Gamma ray spectroscopy is the quantitative use of photons emitted during the decay of unstable nuclides to identify a sample. Compton suppression is a technique used to reduce the contribution of scattered gamma ray photons to the detector response allowing small peaks to be better resolved. Often, Compton suppression is coupled with neutron activation analysis (NAA) for nondestructive trace isotope identification. In this study, the application of Compton suppression technique is evaluated for deployment in a measurement system used for nuclear safeguards activities at reprocessing facilities. Material accountancy and process monitoring in a nuclear fuel reprocessing facility is uniquely challenging due to the complexity of the highly radioactive process stream. The experiments described here are an investigation of the feasibility of incorporating a Compton suppression system into a novel safeguards detection system called the Multi-Isotope Process (MIP) Monitor. The objective of the project is to build a system that detects subtle changes in the distribution of elements in a reprocessing stream using measured gamma ray spectra coupled with multivariate analysis techniques autonomously in near-real time. The high concentration of 137Cs in the aqueous phase process stream makes the concept difficult to implement in this stage of the separation because of the dominant 661keV peak and subsequent Compton scatter. The Compton suppression technique was developed to reduce the scatter which causes a Compton continuum. A Compton suppression spectrometer is generally comprised of a primary, high resolution detector and is surrounded by lower resolution, high stopping power guard detectors. The detectors operate in anticoincidence with one another whereby if a signal is generated in both detectors within a set time window, the pulse will not be added to the spectrum. The Pennsylvania State University Radiation Science and Engineering Center (RSIC) is equipped with a custom-designed Canberra build Compton suppression system (CSS). The system is comprised of an HPGe primary detector surrounded by NaI annulus and plug detectors. The CSS system has a measured Peak-to-Compton ratio of 1000:1 compared to the standalone HPGe Peak-to-Compton ratio of 58:1. Measurements described here investigate the feasibility of utilizing a Compton suppression system into a novel reprocessing safeguards detection system called the Multi-Isotope Process (MIP) monitor. The goal of the Multi-Isotope Process (MIP) project is to detect subtle changes in the elements and their distribution in a reprocessing stream using the gamma ray spectra. This distribution can be correlated to process variables such as burnup, acid concentration, organic ligand concentration, temperature, etc. The high concentration of 137Cs in the aqueous phase makes the concept difficult to implement in this stage of the separation because the dominant 661keV peak and subsequent Compton scatter. The existing PSU CSS system was utilized to evaluate the efficacy of a Compton suppression spectrometer for applications involving spent nuclear fuel. Monte Carlo modeling of a Compton suppressed detection system was also performed using the high energy particle physics transport code Geant4. The PSU CSS system was used to validate the model results so that the same methodology could be applied to other detector geometries. The CSS was calibrated using NIST Standard sources to determine the energy and FWHM as a function of energy to apply during modeling. The model was tested with several standard button sources to evaluate summing and Compton reduction algorithms as well as more complex surrogate sources which mimic spent nuclear fuel. The surrogate sources were designed to simulate detection challenges posed by fuel samples such as high 137Cs activity and weak low energy features. Source definition inputs characterizing actual spent fuel were produced using the burnup code ORIGEN-ARP. The Geant-4 model of the Penn State Compton Suppression System is in development with which to verify the ability to model anticoincidence spectral gating. Due to the high cost and limited availability of spent fuel samples, once validated, the model will be used to predict spectra from a range of spent fuel samples for system testing purposes. Though initially successful using MCNPX, deficiencies in handling complex source terms led to a change to using Geant-4. The geometry of the Penn State Compton Suppression System has been modeled optimized. The dead layer thickness will be optimized using the measured efficiency calibration of the Penn State system.