Normalization of residual-radioactivity nuclides and their mixtures in ferrous metals

Abstract

A large volume of wastes is formed when buildings and technological systems in reactor plants are partially or completely disassembled. For example, a power-generating unit of a nuclear power plant with a BBI~R-1000 reactor contains approximately 10000 ton, of steel, of which only some is contaminated and (or) activated to a high level and must be regarded as radioactive wastes to be buried. Most of the constructional, building, and shielctmg materials removed from reactor plants have a low residual radioactivity (-I Bq/g and less) or they are not radioactive at all. For a BBI~R reactor approximately 50% of the steel wastes are suitable for reprocessing.The following variants, which are most often considered, of returning wastes into the production cycle are possible. I. The metals are freed of control immediately after disassembly, and the possibility of reprocessing wastes and using the finished products by any available method is allowed (uncontrollable reprocessing and unlimited use).2. Metals are freed for further uncontrollable use after remelting (volume contamination) at a special plant (controllable reprocessing). In this case, the measurement of the activity can be greatly simplified and the secondary products of remelting can be removed as radioactive wastes [1].3. Remelted metal (controllable reprocessing) is used to prepare a specific product, for example, for the nuclear cycle (limited use).In the present paper, which is an elaboration of the investigations in [2], we analyze the radiation consequences of freeing scrap metal for the example of removing from operation a BBI~R-1000 reactor. It is assumed that in the process of disassembling a single power-generating unit, 5000 tons of metal will be suitable for reprocessing. The admissible concentration of long-lived radionuclides for freeing the scrap metal in accordance with the first and second variants is calculated. It is assumed that over the time necessary for utilizing 5000 tons of wastes, as assumed for the calculations, the operator obtains an individual dose of I0 ~Zv/yr.We examine below eight scenarios characterizing the level of irradiation for the most general stages of production of steel from scrap metal. The parameters of the scenarios for calculating the admissible concentration are presented in Table 1, where the following notation has been adopted: t -irradiation time; c -dust concentration in air; c~ -dose coefficient of external irradiation for a given scenario and photon energy; Kex t and Kin h are the coefficients employed for calculating external and inhalation irradiation, respectively, and account for the change in the specific activity of a given nuclide for the chosen scenario as compared with the specific activity of scrap metal at the moment it is freed.I. The scrap metal is shipped mainly by rail and truck. The highest individual dose is associated with the truck shipment; this is due to the long time required to ship the wastes.In the scenarios it was assumed that scrap metal with a bulk density of 0.9 ton/m 3 is trans...