Montana Tech THE UNIVERSITY OF MONTANA |
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Research 97 In This Issue Montana Decontamination Research Activity ________________ |
Decontamination
and Decarburization For many years, the Department of Energy (DOE) has recognized the need to dispose or re-use materials originally used in the construction of experimental and production reactor facilities. As decommissioning of major DOE facilities became closer to reality, the need for reliable techniques to reprocess radioactive scrap metal (RSM) was recognized. While estimates of metallic scrap from decommissioning vary, more than one million tons will likely be available within the next twenty years. In 1992, the Idaho National Engineering Laboratories (INEL) recognized this need and established a metal recycle group to recommend recycle technologies. During the review of available technologies, a group of INEL engineers visited Montana Tech to present their program and solicit ideas. During that visit, Montana Tech presented an approach in which stainless steel ingots, containing known quantities of rare earth (lanthanide group) elements representing radioactive contaminants, were to be melted and characterized. These surrogate "contaminated" materials were then to be melted using various combinations of fluxing agents, oxidizers, and melting techniques to demonstrate removal of these contaminants. Later, Montana Tech wrote a proposal giving Tech INEL team member status which enabled it to contract several services. Initial Montana Tech tasks were to:
Listed below are the major participants in the decontamination program who provided key services in accomplishing the RSM recycle effort. The valuable services provided by each participant are hereby acknowledged. AFFCO, Anaconda, MT—Scrap consolidation, pilot-scale induction melting , mold preparation; Ashe Analytics, Butte, MT—X-Ray fluorescence analysis; Colorado School of Mines, Golden, CO—Dual compartment electroslag melting; Newport-News Shipbuilding, Inc., Newport-News, VA—Pilot-scale induction melting; Oregon Graduate Institute for Science and Technology, Beaverton, OR—Electroslag melting, vacuum arc melting, and product fabrication; Oregon State University, Radiation Laboratory, Corvallis, OR—Neutron activation analysis; Retech, Ukiah, CA—Cold hearth plasma melting and vacuum induction melting; Schlosser Forge, Cucamonga, CA—Forging of surrogate ingots; Teledyne Wah Chang, Albany, OR—Analytical services. Montana Tech’s RSM recycle program provided students with hands-on experience in the melting field. During the three year program, Todd Weldon, Dan Paolini, and Darryl Webber, completed their Masters degrees in Metallurgical Engineering. Each student not only received library search and laboratory experience, but also witnessed industrial-scale melting experiments in facilities throughout the U. S. Initially, this program provided starting material which contained surrogate
contaminants representing radioactive materials contained in the structural materials and
components from decommissioned nuclear facilities. This program removed or reduced the
concentration of the contaminants to acceptable levels. The approach proposed by Montana
Tech and accepted by INEL was to represent radioactive contaminants using rare earth
elements in their place, to incorporate these elements into stain- less steel ingots, and
to demonstrate removal of these contaminants by induction melting. An extensive literature
search revealed large scale decontamination efforts dating back to the 1950’s and
continuing through the present time. However, the search also confirmed the lack of data
available on removal rates of radioactive constituents during melting processes. |
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involved in the project, incorporation of the lanthanides (cerium, lanthanum, and
neodymium) could not be accomplished in conventional melting facilities. These elements
have a propensity to oxidize during melting. In order to successfully incorporate these
elements into stainless steel scrap, Montana Tech devised a method to encapsulate the
lanthanide metals under argon into stainless steel capsules. These were then incorporated
into a stainless steel feed stock which was subsequently cold hearth plasma melted at the
Retech facility. In such a furnace, feed material is melted into a water-cooled
copper hearth and then continuously cast into a water cooled copper mold. This method
eliminates the potential for reac- tion with and contamination from refractory materials.
Typical 8-inch ingots resulting from this melting process weighed approximately 500 pounds
and contained 0.15% of the surrogate elements. During the program, more than 10,000 pounds
of surrogate material were prepared in this manner. During the initial phases of the
effort, several flux compositions were selected, based on applicability to steel melting
and oxidation potential. Small heats (1.5 pound) were air induction melted in Montana
Tech’s induction melting facilities by using selected slags, temperatures, and times.
These experiments demonstrated that air induction melting could reduce contaminant levels
to less than 0.0001% (<1 ppm). As a way of confirming these laboratory results, several
2,000 pound heats were induction melted at AFFCO and at Newport News Shipbuilding, Inc.
These large pilot scale melts confirmed the laboratory results, removing individual
surrogates to less than 0.0001% . |
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With these positive results on surrogate removal, Tech shifted emphasis to removal of carbon contamination. Reprocessed stainless steel tends to be high in carbon. Since elevated carbon has a detrimental effect on the corrosion resistance of stainless steel, carbon levels are commonly held to less than 0.03% for corrosion resistant applications. In later stages of the program, Montana Tech demonstrated removal of surro- gates to less than 0.0001% and reduction of carbon to less than 0.03% by vacuum induction melting (VIM) of 200 pound heats at the Retech facility in Ukiah, CA. During the last phase of the effort, conversion of carbon steel to low carbon stainless steel was also demonstrated in the Retech VIM. Figure 1 shows the VIM with the access door open, a mold box in place, and the crucible (on the door) in the pouring posi- tion. Figure 2 shows Sam Worcester, Larry Twidwell, and Darryl Webber on the operating deck of the VIM. |
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Results of this innovative research were presented at ICMR ’94 in Akita, Japan, at Spectrum ’96 in Seattle, and at the American Vacuum Society Vacuum Melting Symposia in Sante Fe in 1994 and in 1997. Both Tech’s program and other programs spon- sored by the DOE have resulted in a recent DOE directive to require recycle rather than direct burial of RSM for future decommissioning. RSM will be melt decontaminated and possibly fabricated into low level waste burial boxes and high level waste storage casks. Successful appli- cation of developed technology is encouraging for any investigator, but the additional benefit of providing hands-on pilot scale experience for graduate students at Tech is icing on the cake. |
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Figure 1. View of a 200-lb vacuum induction
furnace with the mold box in place in the vacuum chamber. The melting crucible in the coil
box (shown in the pouring position) is attached to the access door for easy accessibility
during melt preparation. When the door is closed, the crucible (containing the charge)
will be in the upright position above the mold.
Figure 2. Operating deck of the vacuum
induction melting furnace. The control console is pictured on the left of the operator,
with the vacuum chamber on the right. The triangle-shaped structure on top of the chamber
allows feeding of alloy additions while under vacuum. Also visible on top of the chamber
are two sight ports and the two-color pyrometer used for temperature measurement.
