Patent Number: H00009202
Section: description

A clearer understanding of the present invention will be obtained from the disclosure which follows. DESCRIPTION OF THE PREFERRED EMBODIMENTS Safety experiments were conducted in which there was a need to prevent fission-product cesium from spreading into unshielded but confined portions of the experimental hardware. The experiments were water-reactor safety experiments in which preirradiated, zircaloy-clad, uranium oxide fuel elements were intentionally caused to fail in a steam environment at 750.degree. F. in order to study the migration of fission-product aerosols following cladding failure. Only part of the experimental apparatus was intended to become contaminated, and the remainder was to remain clean and be reusable. After cladding failure induced in Tests No. 1 and 2, the radioactive cesium thus released to the high temperature steam was carried into the reusable section of the system, thereby severely complicating the post-test handling of the experimental hardware. In these first two experiments, stainless steel wire mesh filters (demisters) were provided to hold back the fission-product aerosols. However, they failed to retain all of the fission products carried by the high temperature steam, particularly the cesium isotopes. Thus, post-test disassembly had to be conducted from behind heavy lead shielding. Accordingly, the wire mesh demisters alone did not do the job, even though the cesium which was most likely present in the steam was CsOH, which is a liquid at the operating conditions of the experiment (750.degree. F. steam). Following the first experiment with its extensive spread of radioactive cesium within the closed experimental system, it was recognized that a material must be discovered which would be capable of removing the cesium isotopes from the steam system during test work either by chemical combination or physical combination, or both. When found, it was intended to insert this material, in suitable form, within the circulating system where it could make contact with the 750.degree. F. steam before it entered the reusable components. Accordingly, it was necessary to examine experimentally a number of materials which on theoretical considerations might meet certain necessary criteria, as defined by the objects set forth hereinabove. Testing was done in an apparatus which was believed to be fairly prototypic of a conventional nuclear reactor system. A number of materials were tested, including stainless steel demisting cloth, silicon carbide, crushed quartz, vitreous carbon, crushed borosilicate glass (Pyrex), borosilicate glass beads, glass wool, and a zeolite. All of these materials were found to be capable of removing some of the cesium from the steam. Of these materials, borosilicate glass in the form of solid spherical beads best met the requirements, but only when the beads were heated to about 700.degree. F., and preferably to about 800.degree. F. and higher. The excellent behavior of the borosilicate glass is believed to arise because the cesium forms a stable silicate by combining with the elements in the glass under the test conditions, and also because the cesium and/or its compounds condenses on the exposed glass surfaces. Based upon these results, it was decided to deploy glass beads in the main filter of stainless steel wire mesh (the demister) in the primary steam exit line in the hardware which was utilized in the Tests No. 1 and 2. To make the demister more effective, it was modified to include a bed of 3 mm diameter Pyrex balls or beads which surrounded the stainless steel wire mesh demister. The steel demister element was exactly the same as that used in the two earlier tests. The borosilicate balls were installed so that the effluent steam passed first through the balls and then through the wire mesh demister element. This test work indicated that the removal of the radioactive cesium from the hot steam approaches 90 to 100%. It is believed that this method of stripping fission products from a high temperature vapor medium, such as steam, is unique. Certainly, the use of borosilicate glass, a dense, solid substance, as an agent that acts to combine chemically is unique and not obvious, since glass is normally used because of its inertness to the materials with which it comes in contact. Thus, the practice of the present invention exploits a rare instance where the glass is used deliberately as a chemical reactant. Although the chemical reaction between the radioactive cesium and the silica in the glass is fairly rapid at 700.degree. F., it is preferable that the temperature be at least about 800.degree. F., or even higher, in order to provide for enhanced penetration of the cesium into the borosilicate glass spherical beads in order to achieve a more complete removal of the cesium from the high temperature steam. Clearly, however, the temperature must not be allowed to reach the melting point of the glass beads. It is felt that the method of this invention has application to the steam systems of commercial light water cooled power reactors, particularly those in which the system steam can become contaminated with radioactive cesium due to the breaching of the fuel element cladding. It is contemplated that the inventive method can be utilized as a means for continually cleaning the circulating high temperature steam of radioactive cesium by locating a filter apparatus containing the silica glass spheres at one or more locations in the plant piping of the circulating high temperature steam. Thus, the use of this invention can maintain or improve safety at a nuclear reactor plant by keeping the radioactivity within the circulating high temperature steam at a low level. In light of the foregoing disclosure, further alternative embodiments of the inventive method for the removal of radioactive cesium from high temperature vapors will undoubtedly suggest themselves to those skilled in the art. It is thus intended that the disclosure be taken as illustrative only, and that it not be construed in any limiting sense. Modifications and variations may be resorted to without departing from the spirit and the scope of this invention, and such modifications and variations are considered to be within the purview and the scope of the appended claims.