Int. J. Environ. Res. Public Health 2009, 6 176 utilising water from Lake Kyzyltash (R2). The Mayak PA radioisotope plant was originally known as Plant BB. It now incorporates several additional laboratories and buildings, and is known as Plant 45. It is reportedly a major supplier of radionuclides, radiation sources and radionuclide preparations, having supplied companies in Europe and the USA with a large variety of products including α, β, γ and X-ray sources, fast neutron radiation sources, heat sources (90Sr and 238Pu based) and a wide range of radioactive isotopes such as 14C, 137Cs, 60Co, 241Am, 238Pu, 237Np, 193Ir and 147Pm (with specific activities up to ~11 TBq g-1 and ~30 TBq g-1 for 60Co and 193Ir, respectively: TBq = 1012 Bq). The first facility at the Mayak PA for reprocessing irradiated fuel, plant (RT-1), was completed in 1977 on the site of the original Facility B. Since then, Mayak has reprocessed spent nuclear fuel (SNF) from VVER-440 PWR, BN-350 and BN-600 fast breeder reactors (FBR), research reactors and nuclear vessel power units. Most reprocessing has been of VVER-440 SNF. RT-1 is equipped with a storage pond for spent fuel, two lines for reprocessing, using a version of the PUREX (purification/extraction) solvent extraction process and one line used for cutting and dissolving fuel. After delivery in specially built railway wagons, SNF is stored in the cooling pond for three years or longer before being chemically processed to separate fuel-grade plutonium and uranium from other waste products, which are destined for vitrification. The facility’s capacity (based on VVER-440 SNF) is projected as 400 t yr-1 (t = metric tonne uranium in fuel assemblies). According to Rosatom website data, RT-1 had reprocessed about 3500 t of SNF by 2001. The Mayak PA also serves as a repository for radioactive materials. Storage can be divided into storage of fissile and/or reactor-grade radioactive materials and the storage of waste products, both “historical” from weapons production and “contemporary” from reprocessing activities. According to inventory estimates made in 1990[1], some 30,000 PBq (PBq = 1015 Bq) of solid and liquid waste had been accumulated at the Mayak PA. 1.2. Contamination History Drainage of the area is primarily via the Techa River (Figure 1). In addition, a number of natural lakes and ponds on the plant site have been employed as reservoirs to manage intermediate and low- level radioactive effluents. These include Lake Karachay and Lake Kyzyltash, Reservoirs 3, 4 and 17 (originally local ponds) and artificial Reservoirs 10 and 11 which were created by damming the Techa River. Since the early 1990s, confirmation of nuclear activities at Mayak and information relating to environmental releases of radionuclides has become available in a number of publications [eg., 1-5]. Three significant contamination events have occurred at Mayak PA: • Direct releases of radionuclides to the Techa (Reservoir 3) between 1949 and 1956. • The Kyshtym accident – a thermal explosion in a high-level radioactive waste tank in 1957. • Dispersal of radionuclides from the dried-out bed of Lake Karachay in 1967.