JAERI-Conf 95-015 3-2 TECHNICAL ISSUES RELATING TO THE RECYCLE OF CONTAMINATED SCRAP METAL by Stephen Warren, U.S.Department of Energy, Office of Environmental Restoration (EM-43), Quince Orchard, 19901 Germantown Road, Germantown, MD 20784 Donald E. Clark, Westinghouse Hanford Co., 601 Williams Blvd., Suite 2A, Richland, WA 99352 ABSTRACT A review was made of the literature on melting of radioactive metals that was published in the 1980s and 1990s with attention to the resultant partitioning of radioactivities. Various factors influencing the transfer of radionuclides from the melted ingot phase to other phases such as the slag layer need to be considered both in optimizing the partitioning of radioactivities and in assessing the radiation exposures received by workers. Important technical issues relating to the recycle of radioactive scrap metal (RSM) have been identified and will be discussed in this presentation. Of particular interest is the modeling of radiation doses resulting from operations involving RSM and with recycled materials resulting from the melting of RSM, with emphasis on radioactively contaminated ferrous metals. Such doses result from exposure to the radioactively contaminated metals as well as secondary wastes (i.e., slag, dust, and aerosols/filter media) produced as byproducts of the melting operations. Summary and Conclusions The reported results for partitioning of radioactivities that are achievable by melting of contaminated ferrous metals have been reviewed. The resultant redistribution and stabilization of radioactivities is important for possible recycling of these materials. For dose calculations of the various recycling options, it is essential to have credible partitioning data for each treatment scenario. Such data exist for only a few radionuclides (e.g., of the elements uranium, plutonium, and cobalt); the need for new or additional data has been identified. There are a variety of contaminated metals some of which require treatment with different slags and different melting processes. Control of thermochemistry of such processes is essential. Optimal sizing of melting heats is also required for cost-effectiveness. In general, most of the reported melting work was not done under controlled conditions and no useful thermodynamic data were obtained. There is a need for such data on contaminants in iron and various slags. At the very least, the partition ratios for key radionuclides in given slags must be determined. - 135- JAERI-Cont 95-015 Data on microscopic distribution and speciation of radionuclides are lacking. Also, the effects of subsequent treatments (e.g., rolling, milling, welding) on radioactivity remaining in the remelted metals have not been investigated. Pretreatment methods applied to radioactively contaminated metals prior to melting should include decontamination of surfaces, segregation according to base metal, segregation according to radionuclide contaminant chemistry, and size reduction/compaction as required for furnace loading. Studies culminating in the design of an integrated process for feed pretreatment/preparation and control of gaseous effluents should be initiated. These studies must include attention to mechanical methods capable of operation within containment to assure adherence to the as- low-as-reasonably-achievable (ALARA) principle of radiological safety. The key issues to be addressed in implementing a recycle program of any magnitude for radioactively contami'-ated metals include the following: establishment of a credible database concerning materials to be recycled, including chemical and radioactive characteristics determination of radioactive partitioning between the metal and slag phases assured operability of the process, subject to widely varying feed chemistry and conditions demonstrated ability to seal the candidate process to prevent the release of hazardous species effective modeling of radiation exposures to workers throughout the recycling process An integrated program for recycling radioactively contaminated metals is being developed which will focus resources and address these issues in the near future. Introduction In the United States (U.S.), very large quantities of radioactively contaminated metals have been generated as by-products of nuclear weapon materials production and the associated research and development activities at federal sites operated by the U.S. Department of Energy (DOE). When no longer available or useful for intended purposes, or when they are the result of decommissioning of facilities, such metals are referred to as radioactive scrap metal (RSM). In addition to the RSM produced at DOE sites, significant quantities of RSM have arisen or will be generated in the future from activities in the private sector including nuclear power production. For economic and safety reasons, the recycling and reuse of RSM - 136- JAERl-Conf 95-015 is now receiving serious consideration by U.S. nuclear managers in their planning for waste management, facility and site decommissioning, and environmental remediation activities. In the case of some radioactively contaminated metals, where only the contamination of accessible surfaces has occurred, decontamination or removal of radioactivity by chemical or mechanical methods is readily achievable. Within the restrictions of existing regulations, such decontaminated materials may then be reused or recycled into other products. However, where volumetric or persistent contamination and inaccessible surfaces are involved, melting is recognized as a desirable option that can effect volume reduction; production of useful product sizes, shapes, and volumes; homogeni2ation of radioactivities; reduced radiation exposures; and partitioning of radioactivities, including an effective removal of radionuclides for certain RSM. Over the past two decades or so, many studies and demonstration tests of the melting of radioactively contaminated metals have been conducted and reported on, particularly in the U. S. Germany, France, United Kingdom, and Japan. Worcester and co-workers (Worcester. 1993) reported that they had identified nearly 300 publications related to the application of melting to RSM. A summary of large-scale melting programs for ferrous RSM is given in Table 1. The present review is focused on the reported work with melting of ferrous metals, and especially on the results obtained for partitioning of radioactivities. Table 1. Summaiy of Large-Scale Mejong Programs for Radioactively Contaminated Ferrous Metals' Reference/Country Type/Size of Furnace (Ion) Total Weight Melted (ton) GomerfWyUK Induction/0.5 1 diluted to 24, Arc/5 and 160 then rediluttd 300 Basic Oi Menon (!990ySweden Induction/1.6 210 Peulue(1992VFrancc Arc/16 2.800 Sappok (1992yGermany Induction/22 2.200 35 2.80O Mies (199iyGermany Induction/22 600 35 ThomaOMOyGtrmatty lnduction/2.2 160 Nakamara(I992)/Japan Inducu'on/Oj 5 Maub(1975yusA Arc/10 29,200 Large, SEC (I993J/USA Induction/20 2.200 Echols, SEG(l993yUS A Induction/20 2.740 Larsen(t965)/USA Induction/0 8 80 1 Adapted from Worcester < 1993). - 137 - JAERl-Conf 95-015 The radioactive contaminants include fission and activation products, transuranic and uranium nuclides, and radioactive daughters. From a DOE perspective and considering the principal facilities involved in environmental cleanup operations, important radionuclides include the following elements: hydrogen, carbon, manganese, iron, cobalt, nickel, zinc, strontium, cesium, technetium, cerium, zirconium, ruthenium, tellurium, uranium, plutonium, and americium. For considering the viability of melting RSM for recycling, the potential radiation doses that would be received by workers and others exposed to the scrap, primary and secondary wastes, and the final products (if radioactive) must be known. An early study of radiation exposures resulting from recycle of smelted radioactive metals was reported by O'Donnell and co­ workers (O'Donnell, 1978). Their generic methodology provides a framework that is applicable with modification to the melting of RSM. The dose calculations needed for recycling decision making will be accomplished through computer modeling and use of reported partitioning ratios for the radionuclides of interest. Radiological safety concerns could then be addressed through development of appropriate specifications for the recycled RSM. This review is intended to assess the present state of understanding and to identify data needs that should be addressed to evaluate the recycling of RSM for containers. Melting of Radioactive Scrap Metal CRSM') The melting of RSM for recycling into containers involves heating the metal to a molten state and permitting phase separation to occur. Depending on the thermochemical conditions, during the melting process, radioactivities will be redistributed in the melt, the insoluble (oxide) slag, the ceramic lining, dust, and vapor phase, as well as on other accessible surfaces. On cooling and separation from the other phases, the metal ingot is available for manufacture of the recycled metal products (e.g., a waste container). The ingot will contain lower concentrations (if any) of the initially present radionuclides since they would be homogeneously distributed throughout the metallic mass and would serve to attenuate any remaining radiation. The slag is the residue of the smelting procedure (typically, a few weight-percent of the total) and may contain larger or smaller concentrations of the nuclides than those present initially in the RSM, depending on the