U, Pu, and Am nuclear signatures of the Thule hydrogen bomb debris

This study concerns an arctic marine environment that was contaminated by actinide elements after a nuclear accident in 1968, the so-called Thule accident. In this study we have analyzed five isolated hot particles as well as sediment samples containing particles from the weapon material for the determination of the nuclear fingerprint of the accident. We report that the fissile material in the hydrogen weapons involved in the Thule accident was a mixture of highly enriched uranium and weapon-grade plutonium and that the main fissile material was U-235 (about 4 times more than the mass of Pu-239). In the five hot particles examined, the measured uranium atomic ratio was U-235/U-238 = 1.02 +/- 0.16 and the Pu-isotopic ratios were as follows: Pu-240/Pu-239 0.0551 +/- 0.0008 (atom ratio), Pu-238/Pu239+240 = 0.0161 +/- 0.0005 (activity ratio), Pu-241/Pu239+240 = 0.87 +/- 0.12 (activity ratio), and Am-241/Pu239+240 = 0.169 +/- 0.005 (activity ratio) (reference date 2001-10-01). From the activity ratios of Pu-241/Am-241, we estimated the time of production of this weapon material to be from the late 1950s to the early 1960s. The results from reanalyzed bulk sediment samples showed the presence of more than one Pu source involved in the accident, confirming earlier studies. The Pu-238/Pu239+240 activity ratio and the Pu-240/Pu-239 atomic ratio were divided into at least two Pu-isotopic ratio groups. For both Pu-isotopic ratios, one ratio group had identical ratios as the five hot particles described above and for the other groups the Pu isotopic ratios were lower (Pu-238/Pu239+240 activity ratio similar to 0.01 and the Pu-240/Pu-239 atomic ratio 0.03). On the studied particles we observed that the U/Pu ratio decreased as a function of the time these particles were present in the sediment. We hypothesis that the decrease in the ratio is due to a preferential leaching of U relative to Pu from the particle matrix.