Description
Authors: 1: Simon Jerome, NMBU, Norway; 2: Justin Gwynn, DSA, Norway; 3: Hilde Elise Heldal, HI, Norway; 4: Ole Christian Lind, NMBU, Norway; 5: Hans Christian Teien, NMBU, Norway; 6: Steve Tims, ANU, Australia; 7: Andrey Volynkin, HI, Norway
Affiliations:
1 Norges miljø- og biovitenskapelige universitet, Ås, Norway, 2 Direktoratet for Strålevern og Atomsikkerhet, Tromsø, Norway, 3 Havforskningsinstituttet, Bergen, Norway, 4 Australian National University, Canberra, Australia
In 1989, the Komsomolets submarine of the Soviet Navy sank in the Norwegian sea, approximately 180 kilometres south-west of Bear Island, with the tragic loss of 42 submariners. The Komsomolets was powered by an OK-650B PWR reactor that delivered ~190 MWth, fuelled with enriched U-235. In addition, the Komsomolets carried plutonium warheads. The inventory at the time of sinking has been estimated to be ~29 PBq fission products, U-235 and U-238 contained in the reactor fuel, and ~16 TBq Pu-239 and Pu-240 in two torpedoes. In 2019, the fission product inventory was estimated to be ~3 PBq, of which ~95% was Sr-90 and Cs-137. Although the submarine lies 1 700 metres deep, and is relatively intact, there is an ongoing release of radioactive material through corrosion and aging of the components of the reactor and torpedoes. The site is therefore monitored regularly for stable elements, fission products and actinides.
In this work, we present the results of uranium and plutonium measurements in the environs of the Komsomolets, based on chemical separation and measurement by accelerator mass spectrometry (AMS). Samples of water, sediments and biota in and around the submarine were recovered during the most recent sampling mission in 2019 and returned to NMBU for actinide analysis. Preparation of water samples consisted of addition of U-233 and Pu-242 isotope dilution tracers, precipitation of uranium and plutonium on ferrous hydroxide, and sediment and biota samples were prepared by drying and dissolution using high temperature digestion in nitric and hydrofluoric acids. After preparation, the oxidation state of plutonium was adjusted to Pu4+, and separation, isolation and purification of the uranium and plutonium fractions were carried out by tandem anion exchange chromatography and extraction chromatography. Targets for AMS measurement were prepared by baking the separated uranium and plutonium with ferric oxide and compaction into a suitable form for measurement. Isotope dilution tracers were prepared from material supplied by NPL (UK) at activity concentrations of 14 mBq/g for U-233 and 5.8 mBq/g for Pu-242; maintenance of traceability at these concentrations was achieved by a combination of mass spectrometry and liquid scintillation counting, which imposed an additional 0.39% uncertainty for U-233 and 0.55% for Pu-242. Measured concentration ranges were:
Seawater Biota Sediment
U-236 Minimum 0.16 µBq/kg 110 µBq/kg 68 µBq/kg
Maximum 3.5 µBq/kg 180 µBq/kg 250 µBq/kg
Pu-239+ Minimum 3.3 µBq/kg 83 mBq/kg 140 mBq/kg
Pu-240 Maximum 790 µBq/kg 210 mBq/kg 21 Bq/kg
Measurement uncertainty was dominated by the uncertainty arising from counting of analyte ions in the AMS, although an additional relative uncertainty of 0.81% is imposed on plutonium measurements in conversion from activity to mass arising from uncertainty on the Pu-242 half-life; the corresponding value for U-233 is 0.13%.
