Results of studying the structure and dynamics of radioactive contamination of bottom sediments in the zone of the severe nuclear accident of 1985 in the Chazhma Bight (in the framework of the Federal Goal Program 'Treatment of Radwastes for 1996-2005' are presented. The spontaneous chain reaction (SCR) parameters are evaluated with the Nordheim-Fux model (the maximum release of 137 Cs and 90 Sr is equal to 2 · 10 9 and 1.3 · 10 9 Bq, respectively, which is 10 7 times less than their release from the Chernobyl NPP accident). The analysis of bottom sediment samples has shown that the distribution of contamination (more than 95% of which is caused by 60 Co) is inhomogeneous both in the area and in the vertical. Most of the contaminated ground is located at a depth of 15-20 cm from the bottom surface. With the help of magnetic separation of high level radioactive samples of silt the grains of weight 1-3 mg with radioactivity of the order of 1 μ Ci 60 Co and density 4.9 g/cm 3 were found. These grains as porous fused 'hot' particles from the fuel assembly with embedded fragments of induced 60 Co activity provide a 50% contribution to the total radioactivity of the sample. An integral stock of 60 Co in epicenter of the accident as of August 1997 was 2 · 10 11 Bq, i.e., 20 Ci for August 1985, or 10 times higher than estimates of the Pacific Fleet [1, 2]. Based on a survey expedition, sedimentological studies, and numerical modeling of transport processes in the Chazhma Bight an explanation is given to a tenfold growth of the exposure doze rate (EDR) in the epicenter of the accident since the early 1990s. This is caused by the erosion of a sediment cover overlapping a layer with products of the accident that remained untouched until 1991.