Patent Number: 047088222
Section: description

cl DETAILED DESCRIPTION OF THE INVENTION In a solidified package 3 shown in FIG. 1, radioactive solid waste 1 assumes a spherical pelletized shape and is embedded in a solidifying material 2. If an external pressure P is applied to the solidified package 3, stress concentrates in the solidified package and particularly at the boundary between the solidifying material 2 and the radioactive solid waste 1, and tangential stress .sigma. which is a cause of cracking reaches a maximum. In this case, the intensity of the tangential stress is given as a function of the external pressure P, modulus of elasticity E.sub.1 of the radioactive solid waste, and modulus of elasticity E.sub.2 of the solidifying material. FIG. 2 shows the dependency of the internal stress .sigma./P, normalized by external pressure, on the ratio E.sub.2 /E.sub.1, from which it will be understood that when the modulus of elasticity E.sub.1 of the radioactive solid waste is smaller than that E.sub.2 of the solidifying material (E.sub.1 &lt;E.sub.2), the stress .sigma. at the boundary therebetween is greater than the external pressure P. Therefore, if the safety factor is set simply by comparing the compressive strength of the solidifying material with the external pressure P, a sufficient durability is not often maintained under practical conditions. The intensity of the stress concentrated at the boundary between the solid waste and the solidifying material is in inverse proportion to the radius of curvature of the surface of the solid waste. In practice, the radioactive waste consists of an aggregate of conduit pieces, waste cloth, plastic materials, as well as materials which have been dried, granulated, and pelletized, having a coarse surface and various radii of curvature. Therefore, the stress concentrates unevenly, unlike in the completely spherical representation of FIG. 1; i.e., the stress concentrates locally. With an actual solidified package, therefore, the inclination of the curve becomes steeper than that of FIG. 2, and the safety factor decreases greatly. This curve always passes through the point [.sigma./P, E.sub.2 /E.sub.1 ]=[1, 1]. When the modulus of elasticity E.sub.2 of the solidifying material is smaller than the modulus of elasticity E.sub.1 of the radioactive solid waste, therefore, the stress does not become greater than the external pressure, and the safety factor does not decrease. Steel material such as conduit pieces have a modulus of elasticity of 10.sup.6 kg/cm.sup.2, waste cloth and plastic materials have moduli of elasticity in the range of 10.sup.2 to 10.sup.3 kg/cm.sup.2, and materials obtained by drying concentrated liquid waste or ion-exchange resins, followed by pulverization and pelletization, have a modulus of elasticity of about 10.sup.3 kg/cm.sup.2. Though it is not possible to adjust the modulus of elasticity E.sub.1 freely, the modulus of elasticity E.sub.2 of the solidifying material can be adjusted so that the ratio E.sub.2 /E.sub.1 of moduli of elasticity becomes smaller than 1, in order to maintain the desired safety factor and to prevent the solidified package from being destroyed. There now follows a description of an embodiment for solidifying radioactive solid waste according to the present invention wherein mirabilite pellets are embedded in a polyester resin, the mirabilite pellets being obtained by pelletizing a powder obtained by drying concentrated liquid waste from a boiling-water reactor. The mirabilite pellets employed in this embodiment had an almond shape, measure about 3 cm long, about 2 cm wide, and 1.3 cm thick, and were prepared according to a known process, i.e., the process disclosed in Japanese Patent Laid-Open No. 15078/1980. The modulus of elasticity of the mirabilite pellets as 3.times.10.sup.3 kg/cm.sup.2. For the solidifying material, a polyester resin was used, having properties as shown in Table 1, that was formed by the radical polymarization reaction of an unsaturated polymer with a crosslinked monomer. FIG. 3 is a schematic diagram illustrating the crosslinking polymerization reaction, in which the unsaturated polyester polymer consists of ester bonds of glycol G and unsaturated acid M. The distance between an unsaturated acid M and a neighboring unsaturated acid M across a glycol G is called the distance between crosslinking points. Therefore the distance between crosslinking points can be increased by using a glycol with a large molecular weight and a long chain. By using a polybutadiene glycol instead of the traditionally-used propylene glycol, the inventors have succeeded in increasing the distance between crosslinking points 7-fold and in reducing the modulus of elasticity to one-fiftieth the original value i.e., to 5.times.10.sup.2 kg/cm.sup.2). 250 kg of the mirabilite pellets were placed into a cage within a 200-liter drum, and the solidifying material was poured into fill the space between the drum wall and the mirabilite pellets with the solidifying material. The drum was left to stand and harden, thereby obtaining a solidified package. The solidified package was subjected to an sea disposal test simulating a depth of 6,500 meters (pressure of 650 kg/cm.sup.2). The solidified package was not destroyed and no cracks developed. In this embodiment, the ratio E.sub.2 /E.sub.1 of the modulus of elasticity of mirability pellets to the modulus of elasticity of polyester is 0.2 and, hence, it is considered that stress does not concentrate. As a comparative example, a solidified package was also prepared using a customarily employed plastic material (details are shown in Table 1) with a high modulus of elasticity, and was subjected to the same test. In this case cracks developed, and the solidified package was partly destroyed. The ratio E.sub.2 /E.sub.1 of the modulus of elasticity of the plastic material to the modulus of elasticity of the mirabilite pellets was about 10. That is, tangential stresses of 5 to 10 times as great concentrated at the boundaries between the plastic material and the mirabilite pellets if an external pressure of 500 kg/cm.sup.2 was applied (which corresponds to a sea depth of 5,000 meters). The plastic material used as the solidifying material broke under a static water pressure of about 2,500 kg/cm.sup.2. Therefore, the solidified package developed cracks, and was destroyed as the worst case. TABLE 1 ______________________________________ Plastic solidifying material used in the Plastic solidifying embodiment of material used in the the invention comparative example ______________________________________ Unsaturated Unsaturated alkyl con- Unsaturated alkyl con- polyester taining polybutadiene taining propylene monomer glycol glycol Crosslinking Styrene Styrene monomer Features Long distance between Short distance between crosslinking points crosslinking points (molecular weight of up (molecular weight of up to 2,000), and small to 300), and large modulus of elasticity modulus of elasticity (5 .times. 10.sup.2 kg/cm.sup.2) (3 .times. 10.sup.4 kg/cm.sup.2) ______________________________________ According to the present invention, the solidifying material is not limited to a plastic but could also be cement. In this case, the cement may have natural rubber or synthetic rubber latex mixed therewith to adjust the modulus of elasticity of the cement to be within the range of about 10.sup.4 kg/cm.sup.2 to 10.sup.2 kg/cm.sup.2, so that the modulus of elasticity is smaller than that of the radioactive solid waste. When more than one kind of radioactive solid waste are to be treated, the modulus of elasticity of the solidifying material should, of course, be based upon the smallest modulus of elasticity of the wastes.