Absolute Measurement of 137Cs and 134Cs/137Cs and154Eu/137Cs Ratios to Verify University of Texas TRIGA Reactor Spent Fuel Burnup

The absolute measurement of fission product gamma activity or the ratio of fission product activities (such as 134Cs/137Cs or 154Eu/137Cs) has been used previously to measure or verify burnup (BU) of a commercial reactor spent fuel with a known reactor power history. If a cooling time of a spent fuel is less than a year, BU can be measured by measuring 140La, 95Zr, 144Ce, or 106Ru absolute gamma-ray activity. If a cooling time is greater than 1 year, then absolute measurement of 137Cs or the ratios discussed above could be used. A reactor power history is one of the most important variables for a BU calculation. The amount of 137Cs isn’t affected by the BU history but the amount of other short-lived fission products or fission products with large absorption cross sections depend on the most recent reactor power levels. Therefore, a lack of knowledge about a reactor power history can make verification difficult or even infeasible. In this work, the ratio of fission product activities was used to infer BU of a spent fuel rod with an unknown power history at the University of Texas TRIGA reactor. A fuel rod holder with a 12-feet long collimator was fabricated for measurements at three axial locations along the fuel rod using a high purity germanium detector (HPGe) to obtain an axial BU profile. The fuel BU was simulated in MCNP6 to estimate expected fission product activities assuming a trial power history. Results showed that research reactor fuel BU can be inferred from these measurements with sufficient accuracy for safeguards verification purposes.