Level, distribution and sources of Np, Pu and Am isotopes in Peter the Great Bay of Japan sea

Abstract

Transuranium elements such as Np, Pu and Am, are considered to be the most important radioactive elements in view of their biological toxicity and environmental impact. Concentrations of ²³⁷Np, Pu isotopes and ²⁴¹Am in two sediment cores collected from Peter the Great Bay of Japan Sea were determined using radiochemical separation combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. The ^239,240Pu and ²⁴¹Am concentrations in all sediment samples range from 0.01 Bq/kg to 2.02 Bq/kg and from 0.01 Bq/kg to 1.11 Bq/kg, respectively, which are comparable to reported values in the investigated area. The average atomic ratios of ²⁴⁰Pu/²³⁹Pu (0.20 ± 0.02 and 0.21 ± 0.01) and ²⁴¹Am/^239+240Pu activity ratios (3.32 ± 2.76 and 0.45 ± 0.17) in the two sediment cores indicated that the sources of Pu and Am in this area are global fallout and the Pacific Proving Grounds through the movement of prevailing ocean currents, and no measurable release of Np, Pu and Am from the local K-431 nuclear submarine incident was observed. The extremely low ²³⁷Np/²³⁹Pu atomic ratios ((2.0-2.5) × 10^-4) in this area are mainly attributed to the discrepancy of their different chemical behaviors in the ocean due to the relatively higher solubility of ²³⁷Np compared to particle active plutonium isotopes. It was estimated using two end members model that 23% ± 6% of transuranium radionuclides originated from the Pacific Proving Grounds tests, and the rest (ca. 77%) from global fallout.

Keywords: Environmental radioactivity; Japan sea; Migration behaviors; Sources; Transuranium radionuclides.