During the past few decades, the U.S. Department of Energy (DOE) has made significant investments to improve nuclear data libraries, as the quality of this data impacts nearly all analyses of nuclear systems. Nuclear fission product yields are one component of these libraries, and are important for the analysis of nuclear fissioning systems. In accident scenario simulations, for example, gamma-ray branching ratio data impacts inventory calculation of isotopes. At present, a number of fission product yield data sets are available from a number of different facilities. There are non-trivial variations between these data sets which can significantly impact simulation results. These variations may stem from inconsistencies in experimental setup and nuclear data uncertainties. The goal of this research is to improve nuclear fission product yields by employing a consistent experimental setup and data analysis procedure for a wide variety of actinides.
To this end, an experiment was conducted at the Godiva-IV critical assembly in New Mexico in collaboration with Lawrence Livermore National Laboratory (LLNL) and Pacific Northwest National Laboratory (PNNL). The Godiva facility provides an excellent source of fast neutrons to irradiate the target actinides at a high neutron flux. Fission product yields were analyzed from an irradiated sample by performing gamma-ray spectroscopy. The targets were irradiated, then gamma-rays were counted for 7 days.
The actinides analyzed in this thesis are ²³⁵U and ²³⁸U. A comparison between the nuclear fission product yields found as part of this work and previous data sets is also presented.
One result of this research is the creation of a standardized experimental setup to analyze fission product yields. A significant outcome is that some isotopes with no past experimental fission product yield data now have measurement data. One limitation of our approach is that it relies on the availability of accurate branching ratio data. Inaccurate branching ratios effect the quality of the predicted fission yield data, and the results show that analyzing different gamma-rays from the same isotope may lead to different fission product yields.
Funding Statement (additional comments about funding)
This work was funded by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy's National Nuclear Security Administration by Lawrence Livermore National Laboratory under Contract No DE-AC52-07NA27344. The U.S. Department of Energy's Nuclear Criticality Safety Programs National Criticality Experiments Research Center (NCERC), utilized in this work, is supported by the National Nuclear Security Administration's Office of the Chief of Defense Nuclear Safety, NA-511.