Mercury Reduction and Removal During High-Level Radioactive Waste Processing and Vitrification
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MERCURY REDUCTION AND REMOVAL DURING HIGH-LEVEL RADIOACTIVE WASTE PROCESSING AND VITRIFICATION
RUSSELL E. EIBLING AND JOHN R. FOWLER E. I. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, South Carolina, USA 29808-0001
ABSTRACT A reference process for immobilizing the high-level radioactive waste in borosilicate glass has been developed at the Savannah River Plant. This waste contains a substantial amount of mercury from separations processing. Because mercury will not remain in borosilicate glass at the processing temperature, mercury must be removed before vitrification or must be handled in the off-gas system. A process has been developed to remove mercury by reduction with formic acid prior to vitrification. Additional benefits of formic acid treatment include improved sludge handling and glass melter redox control.
INTRODUCTION As a byproduct of the production of nuclear defense materials at the Savannah River Plant (SRP) since 1953, approximately 23 million gallons of alkaline highlevel waste (HLW) is stored in large underground tanks onsite. The reference process for immobilization of this waste is vitrification in borosilicate glass. The alkaline HLW is composed of two major components: 1) soluble compounds which are present as a salt solution or a cry'stallized salt phase and 2) insoluble compounds, referred to as sludge, which are mostly hydrated transition metal oxides and hydroxides. Mercuric oxide, present in the sludge at a few percent level, would be volatilized from the glass melter at the processing temperature of 1150°C. A process has been developed at the Savannah River Laboratory (SRL) that will recover the mercury prior to vitrification by chemical reduction to mercury metal by using formic acid. This new process avoids introducing large quantities of mercury into the melter and replaces the more complex mercury recovery from the melter off-gas system. BACKGROUND The mercury present in SRP sludge is due to the use of mercuric nitrate as a catalyst during nitric acid dissolution of spent fuel rods. After separation of the products, the resultant nitric acid solution containing both mercury and fission products is rendered alkaline with 50% NaOH, thereby producing a precipitate of metal hydroxides and oxides. The resulting sludge slurry is sent to one million gallon, water-cooled tanks where the sludge settles out of the bulk liquid. The bulk liquid is then transferred to an evaporator to concentrate the soluble salt solids. The resulting sludge has the composition given in Table I based upon analyses of samples taken from SRP tanks. The variations in sludge composition are due to differences in the types of waste being stored in various tanks.
618 TABLE
I
Composition of Washed,
Dried Radioactive SRP Sludges
Element
Weight %
Observed Range of Weight %
Fe Mn Al Hg Ni
15.3 3.1 17.6 1.3 1.2
3-34 0.8-10 3-38 0-9 0.3-7
A reference process is currently being developed at SRL to immobilize the sludge into a borosilicate glass matrix for ultimate disposal in a Federal
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