Experimental Gamma Ray Burnup Analysis of Penn State TRIGA Reactor Spent Fuel Rods

Effective material control and accountability of reprocessing facilities requires the verification of operator declared burnup, cooling time, and irradiation history before fuel dissolution for safeguards and process control purposes. To perform this task rapidly, inspectors utilize nondestructive radiometric methods relying on the buildup of fission products during the burning of fuel. Many of these burnup metrics are derived from nuclides that are not direct fission products and arise from fission, capture, and decay pathways. Research reactor fuel poses an even more challenging problem, since their fuel has a more complex and irregular irradiation history that complicates the traditional calculation of burnup using the paired isotope analysis method. The Radiation Science and Engineering Center at The Pennsylvania State University houses the Penn State Breazeale Reactor (PSBR), a TRIGA Mark-III research reactor, and an inventory of irradiated research reactor fuel spanning 48 years of operation. Direct gamma ray measurements of the nuclide composition of irradiated fuel removed from the core were performed to calculate the incremental and average burnup of several spent fuel elements from the PSBR. An HPGe detector was used to collect gamma ray spectra at several intervals over the vertical length of the fuel elements using a collimated beam port to determine the segmented burnup across the axial length of the fuel rod. Predicted burnup values came from the highly specialized, Monte Carlo-coupled depletion simulation code TRIGSIMS developed for the PSBR. Comparison of the two methods resulted in fuel element average errors ranging from 5.49% to 32.37% between the burnups based on measurements and the TRIGSIM simulated values. A correction factor for the years of irradiation history data to account for periods of shutdown and variable power was attempted, but the handwritten daily operator logbook requires further investigation as the approximated value of the irradiation history correction factor did not improve burnup values.