Patent Number: 047028629
Section: description

DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, a method of the kind mentioned at the outset is characterized by the feature that radioactive thermoplastic synthetic material is used as the thermoplastic matrix material. This radioactive thermoplastic synthetic material may itself be active waste. Although using it as the matrix material increases the specific radioactivity of the obtained synthetic material blocks to be finally stored, this can be done within permissible limits, such that no additional measures need be taken in the final storage of the synthetic-material blocks obtained. On the other hand, however, an increased amount of radioactive waste can be taken up by the volume occupied by a plastic block. It is known to burn radioactive waste to reduce the volume, and to bind the ash, for instance in concrete, and to store it. However, this method cannot be used if the radioactive waste contains radioactive halogen-containing plastic such as polyvinylchloride, since hydrochloric-acid gas would be produced in burning such a plastic. This acid gas would have to be neutralized and would result as a neutralization product in the form of a radioactive salt solution which in turn would have to be converted into a solid product capable of final storage. It is advantageous to melt the waste in an electromagnetic high-frequency field to radioactive thermoplastic synthetic material. Thereby, the halogen-containing plastic in the radioactive waste is heated uniformly, so that the formation of hydrochloric acid gas by local overheating of the plastic waste mixture is prevented. A typical heterogeneous radioactive waste mixture produced in the manufacture of plutonium-containing nuclear reactor fuel assemblies may contain the following organic components, in addition to inorganic components: 70% by weight radioactive polyvinylchloride, 15% by weight radioactive sythetic rubber, 10% by weight radioactive cellulose, and 5% by weight various other radioactive synthetic materials. These radioactive organic waste components are comminuted in an impeller breaker to an average grain size of 5 mm. The comminuted radioactive organic waste components are then optionally mixed with inorganic radioactive waste components, for instance, small metal scrap or broken glass and the mixture is filled into a container of polytetrafluoroethylene. After closing the container with a lid which also consists of polytetrafluoroethylene, the container is arranged in an electromagnetic high-frequency field, and the polyvinylchloride contained in the container is heated to a temperature of 150.degree. C. Thereby, not only the radioactive polyvinylchloride in the container, but also the other radioactive thermoplastic synthetic materials are made to flow. These radioactive thermoplastic synthetic materials which have been made to flow become effective as matrix material. They enclose from all sides the waste components that have remained solid, and all the radioactive substances contained in the waste. After the electromagnetic high-frequency field has been turned off, the matter contained in the container of polytetrafluoroethylene solidifies to a void-less radioactive synthetic material block which is placed, together with the polytetrafluoroethylene container, in a barrel of alloy steel, which is capable of final storage after being closed with an alloy steel lid. Advantageously, a substance which binds HC1 without gas development is added to the radioactive plastic in the radioactive waste mixture prior to the heating in the electromagnetic high-frequency field and thus, prior to the fusing. Especially well suited as such a substance is calcium oxide in powder form which is uniformly admixed to the waste in an amount of less than 1% by weight based on the radioactive halogen-containing plastic content of this waste. It is further of advantage, for better utilizatin of the capaciaty of the container of polytetrafluoroethylene, to fill and fuse into this container radioactive waste mixture repeatedly one after the other. It is of advantage here to pre-densify the respectively filled-in mixture by means of a compacting tool and to thereby reduce the number of fusing processes. It is also advantageous to perform the constraining of the radioactive waste mixture in the container of polytetrafluoroethylene in an atmosphere above the waste mixture in the container, which has a working pressure of less than 1 bar. Thereby, the formation of voids in the radioactive plastic block by gas bubbles in the thermoplastic phase is more effectively prevented. This working pressure can be produced by means of a suction pump which is connected to a suction nozzle of the lid of the container of polytetrafluoroethylene. With this method for the final conditioning, a density of the organic component of the radioactive waste to be finally stored can be obtained which is nearly the same as the theoretical density of 1.4 g/cm.sup.3 of polyvinylchloride and which means a densification by a factor of 3 to 5 over the organic component of the waste mixture prior to the fusion. The foregoing is a description corresponding to German Application No. P 31 42 356.6, dated Oct. 26, 1981, international priority of which is being claimed for the instant application, and which is hereby made part of this application. Any discrepancies between the foregoing specification and the aforementioned corresponding German application are to be resolved in favor of the latter.