Determining nuclear weapons yields from bomb debris

Whether for treaty monitoring or historical interest, sometimes it is necessary to evaluate the yield of a nuclear weapons test long after the fact. A new method that involves measuring stable molybdenum isotopes could get around the time sensitivity of current approaches, which include monitoring seismic signals and measuring short-lived radionuclides (Proc. Natl. Acad. Sci.USA 2016, DOI: 10.1073/pnas.1602792113). Researchers from Los Alamos National Laboratory used mass spectrometry to measure the amounts of different molybdenum isotopes in debris samples from the 1945 Trinity nuclear weapons test, the world’s first. ⁹⁵Mo and ⁹⁷Mo are stable radionuclide daughters in the decay chains from plutonium fission, therefore the test debris has elevated ⁹⁵Mo/⁹⁶Mo and ⁹⁷Mo/⁹⁶Mo levels compared with natural abundances. The team used the molybdenum ratios and the amount of plutonium in the debris to calculate that the yield of the Trinity test was equivalent to 22,100 metric tons of TNT; the official Department of Energy estimate is 21,000 metric tons of TNT. Although the paper implies that the approach could be used to evaluate modern-era tests, the researchers declined to answer whether it could be used to evaluate underground tests, such as those done as recently as January by North Korea.