We are grateful to Dr. Richard N. Wright of the Idaho National Laboratory for the support of this work. We thank Mr. Masanori Ando of the Japan Atomic Energy Agency for providing the data for Fig. 3. The research was sponsored by the Office of Nuclear Energy, the U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.