Polymer composite matrices for disposal of radionuclide sources of ionizing radiation

Abstract

Radionuclide sources of ionizing radiation are used in industry, medicine, agriculture, and scientific research [1]. They have a wide range of radionuclide compositions: 60Co, 137Cs, 192Ir, 17~ 75Se are often used and many old sources contain 226Ra. Small individual sources can have radioactivity of -1015 Bq and higher. Spent sources are highlevel wastes. Old and spent sources are enormously hazardous to the environment and man [2]. The most suitable matrix materials for treating these sources are metals [3], though cement is still used in many countries [2]. Insertion of the sources into metal matrices, which, as a rule, consist of lead or lead alloys, makes it possible to isolate the sources reliably from the environment, though the corresponding technological support is required. Mobile setups for treating wastes using metal matrices are now used only in Russia [3, 4]. Apparently, there are no alternatives to metal matrices for disposal of high-level sources. But, for not very intense sources other matrices, which do not require complicated equipment, can be used. This is desirable for small users of sources who cannot operate a complicated technology.Our objective in the present paper is to describe the possibility of treating sources using polymer composite materials: matrices based on epoxy resins and inorganic cement or bentonite fillers.Polymer Composite Materials. In contrast to single-phase materials, the performance characteristics of composite materials with two or more components can exceed those of each component separately. Thus, due to the relative low radiation resistance of polymers, only a limited quantity of radioactive wastes can be embedded in polymer materials. The amount of wastes that can be embedded in cement matrices is limited by the water permeability of cement block, whose porosity can reach 20%, and correspondingly by the possibility of radionuclides escaping. Composite materials based on a polymer with a cement powder or bentonite filler withstand tens of times higher doses than the initial polymer, and they remain chemically highly stable.Composite materials based on I~D-20 epoxy resin (the hardening agent is polyethylene polyamine) and a M-400 and -500 cement filler were chosen as the matrix material for treating the sources. The epoxy matrix was chosen due to its high chemical stability and strength as well as due to the good adhesion of the solidified resin to the materials of the source capsules. As a rule, these are different metals, but glass, ceramics, and polymers can also be used. Filler in the form of cement powder was chosen in order to absorb water in the case when microcracks appear in the monolithic block during long-term storage of the sources. Bentonite filler also has the same properties.The polymer composite materials for the tests were prepared as follows. Filler was added in amounts of 40 to 80% by mass to the epoxy resin, which was mixed with a two-blade stainless-steel mixer until a uniform mixture was obtained, the hardening agent was added in amounts from...