Relative Fission Product Yield Determination in the Usgs
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RELATIVE FISSION PRODUCT YIELD DETERMINATION IN THE USGS TRIGA MARK I REACTOR by Michael A. Koehl © Copyright by Michael A. Koehl, 2016 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Nuclear Engineering). Golden, Colorado Date: ____________________ Signed: ________________________ Michael A. Koehl Signed: ________________________ Dr. Jenifer C. Braley Thesis Advisor Golden, Colorado Date: ____________________ Signed: ________________________ Dr. Mark P. Jensen Professor and Director Nuclear Science and Engineering Program ii ABSTRACT Fission product yield data sets are one of the most important and fundamental compilations of basic information in the nuclear industry. This data has a wide range of applications which include nuclear fuel burnup and nonproliferation safeguards. Relative fission yields constitute a major fraction of the reported yield data and reduce the number of required absolute measurements. Radiochemical separations of fission products reduce interferences, facilitate the measurement of low level radionuclides, and are instrumental in the analysis of low-yielding symmetrical fission products. It is especially useful in the measurement of the valley nuclides and those on the extreme wings of the mass yield curve, including lanthanides, where absolute yields have high errors. This overall project was conducted in three stages: characterization of the neutron flux in irradiation positions within the U.S. Geological Survey TRIGA Mark I Reactor (GSTR), determining the mass attenuation coefficients of precipitates used in radiochemical separations, and measuring the relative fission products in the GSTR. Using the Westcott convention, the Westcott flux, ; modified spectral index, ; neutron temperature, ; and gold-based cadmium ratiosφ were determined for various sampling√⁄ positions in the USGS TRIGA Mark I reactor. The differential neutron energy spectrum measurement was obtained using the computer iterative code SAND-II-SNL. The mass attenuation coefficients for molecular precipitates were determined through experiment and compared to results using the EGS5 Monte Carlo computer code. Difficulties associated with sufficient production of fission product isotopes in research reactors limits the ability to complete a direct, experimental assessment of mass attenuation coefficients for these isotopes. Experimental attenuation coefficients of radioisotopes produced through neutron activation agree well with the EGS5 calculated results. This suggests mass attenuation coefficients of molecular precipitates can be approximated using EGS5, especially in the instance of radioisotopes produced predominantly through uranium fission. Relative fission product yields were determined for three sampling positions in the USGS TRIGA Mark I reactor through radiochemical analysis. The relative mass yield distribution for valley nuclides decreases with epithermal neutrons compared to thermal neutrons. Additionally, a proportionality constant which related the measured beta activity of a fission product to the number of fissions that occur in a sample of irradiated uranium was determined for the detector iii used in this study and used to determine the thermal and epithermal flux. These values agree well with a previous study which used activation foils to determine the flux. The results of this project clearly demonstrate that R-values can be measured in the GSTR. iv TABLE OF CONTENTS ABSTRACT……. .......................................................................................................................... iii LIST OF FIGURES ..................................................................................................................... viii LIST OF TABLES .......................................................................................................................... x ACKNOWLEDGMENTS ............................................................................................................. xi DEDICATION……. ..................................................................................................................... xii CHAPTER 1 INTRODUCTION ............................................................................................ 1 1.1 Executive Summary: ......................................................................................... 1 1.2 Motivation ......................................................................................................... 2 1.3 Thesis Organization ........................................................................................... 3 CHAPTER 2 BACKGROUND AND RATIONAL FOR STUDIES ...................................... 4 2.1 K-value method fission basis and the R-value .................................................. 4 2.2 Previous Efforts using R-values ........................................................................ 7 2.3 Fission Product Yield Energy Dependence of Epithermal Neutrons ................ 8 2.4 Water Boiler & Omega West Reactor Thermal Calibrations .......................... 12 2.4 References Cited .............................................................................................. 13 CHAPTER 3 MEASURED NEUTRON FLUX PARAMETERS IN THE USGS TRIGA MARK I REACTOR....................................................................................... 16 3.1 Abstract ........................................................................................................... 16 3.2 Introduction ..................................................................................................... 16 3.3 Experimental ................................................................................................... 18 3.3.1 Foil Irradiations ................................................................................ 18 3.3.2 Application of Westcott Convention ............................................... 21 3.3.3 SAND-II-SNL Code Operations ...................................................... 23 3.4 Results ............................................................................................................. 23 3.4.1 Westcott Convention ......................................................................... 23 3.4.2 SAND-II-SNL Code ........................................................................ 25 3.4.2 Comparison to Similar TRIGA Reactors ......................................... 27 3.5 Discussion ....................................................................................................... 29 3.5.1 Westcott Convention ........................................................................ 29 3.5.2 SAND-II-SNL Code ........................................................................ 29 3.5.3 Comparison to Similar TRIGA Reactors ......................................... 30 v 3.6 Conclusion ....................................................................................................... 31 3.7 Acknowledgements ......................................................................................... 31 3.8 References Cited .............................................................................................. 31 CHAPTER 4 EXPERIMENTAL AND MONTE CARLO INVESTIGATION OF MASS ATTENUATION COEFFICIENTS OF FISSION PRODUCT ISOTOPES IN MOLECULAR PRECIPITATES .............................................................. 33 4.1 Introduction ..................................................................................................... 33 4.2 Experimental ................................................................................................... 35 4.2.1 Gas Proportional Counting System .................................................. 35 4.2.2 Source Preparation ........................................................................... 35 4.2.3 Gas Proportional Counter Efficiency ............................................... 37 4.2.4 EGS5 Monte Carlo Simulation ........................................................ 37 4.2.5 Empirical Mass Attenuation Formula .............................................. 38 4.3 Results ............................................................................................................. 39 4.3.1 Detector Resolution ......................................................................... 39 4.3.2 Mass Attenuation Coefficients ......................................................... 39 4.4 Discussion ....................................................................................................... 42 4.4.1 Detector Resolution ......................................................................... 42 4.4.2 Mass Attenuation Coefficients ......................................................... 43 4.5 Conclusions ..................................................................................................... 44 4.6 Acknowledgements ......................................................................................... 44 4.7 References Cited .............................................................................................. 44 CHAPTER 5 RELATIVE FISSION PRODUCT YIELDS IN THE USGS TRIGA MARK I REACTOR....................................................................................... 46 5.1 Introduction ..................................................................................................... 46 5.2 Experimental ..................................................................................................