Substantiation of the admissable concentration of radionuclides in the utilization of concrete from disassembled reactors

Abstract

The lifetime of a nuclear reactor is limited for technical and economic reasons, and after its service life has been exhausted, a reactor must be decommissioned. When a reactor is disassembled, a large volume of radioactive wastes is produced. The radioactivity of the materials from the disassembled reactor is determined by the induced activity (activation of the materials and equipment irradiated with neutron fluxes in the course of operation) and radioactive contamination by activated products of corrosion and fission products (radioactive deposits on the surface of the equipment, constructional, protective, and building materials) as a result of rupture and accidents in the technological-loop system of the reactor. Estimates of the radioactivity and volume of the materials which are freed when a reactor is decommissioned are presented in, for example, [1][2][3][4][5].The complete or partial disassembly of the buildings and technological systems of modern reactors results in the formation of hundreds of thousands of tons of wastes [4, 5]. More than 90% of this mass consists of reinforced concrete. Only a small fraction of these materials is contaminated and (or) activated up to a high level and must be treated as radioactive wastes that must be buried. Most of the constructional and building materials removed from the operation of the reactors have very little residual radioactivity (of the order of 1 Bq/g and less) or is nonradioactive.For this reason, it is helpful to consider a variant of recycling of some of the wastes, and secondary utilization of these wastes in different industrial production processes or burial as nonradioactive wastes. Of course, all of this work must be performed with a specially organized, strict radiation monitoring of their residual radioactivity. The peculiarity of this type of monitoring is that the dose rate of the ionizing radiation which is due to the residual radioactivity of the wastes is extremely low -a fraction of the natural background.It is obvious that when these materials are further utilized in different industrial processes, the individual and collective irradiation dose to the population should not exceed the admissable limit. The limiting criterion for the individual effective equivalent dose is commonly taken to be 0.01 of the natural background (10 /zZv/yr); for skin the limit is 50 times higher; and, for the collective equivalent dose the limit is I man-Zv/yr from an individual source [6, 7].Investigations are now being conducted abroad on the development of standards in order to do away with the regulatory control of materials with low residual radioactivity [6][7][8][9][10].The lack of a general government standardized base in our country is an impediment to repeated widespread use of materials with low residual radioactivity and to achieving a significant economic savings by reuse of and decrease in the volume of wastes buried as radioactive wastes.A relation between the degree of contamination and the irradiation dose is customarily determined by means of a...