Hot particles of the Yenisei River flood plain, Russia.

Some high-activity hot particles (HP) were found in the flood plain of the Yenisei River, near the Krasnoyarsk Mining-and-Chemical Combine (MCC), and their radionuclide compositions were determined. The ratios of plutonium and caesium isotopes in the particles are indicative of their reactor origin. The 137Cs activity of the particles amounts to 29,200 kBq/particle, which is higher than the corresponding activities of the fuel particles that formed as a result of the accident at the Chernobyl NPP. All the particles have been divided into two major groups according to the 137Cs/34Cs ratio: in the first group, the 137Cs/134Cs ratio is more than or equal to 3000, and in the second the 137Cs/134Cs ratio is less than or equal to 1000. The particles of the first and the second groups were preliminarily estimated to be formed 30 and 20 years ago, which suggests that there must have been at least two accidents at the MCC reactors, with part of the fuel released into the Yenisei River.