We acknowledge M. R. Vejar, J. N. Wacker and R. J. Abergel for their feedback on this work. NextGen is supported by the National Nuclear Security Administration’s (NNSA) Office of Defense Nuclear Nonproliferation Research and Development (NA-22) through the Nonproliferation Stewardship Program (NSP). This manuscript was assembled with support from the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program at the Lawrence Berkeley National Laboratory (LBNL) under contract DE-AC02-05CH11231.