Ferrocyanide safety project ferrocyanide aging studies. Final report

Abstract

SummaryThis final report gives the results of the work conducted by Pacific Northwest National Laboratory (PNNL) from FY 1992 to FY 1996 on the Ferrocyanide Aging Studies, part of the Ferrocyanide Safety Project. The Ferrocyanide Safety Project was initiated as a result of concern raised about the safe storage of ferrocyanide waste intermixed with oxidants, such as nitrate and nitrite salts, in Hanford Site single-shell tanks (SSTs). In Numerous studies conducted by the Task Team defmed safety criteria and resulted in closure of the USQ in March 1994. The potential for ferrocyanide reactions in Hanford Site SSTs was evaluated, and the energy released during these reactions was quantified. Dynamic X-ray diffraction was also used to identify specific reactions and to quantify reaction rates. In addition, a number of experimental and theoretical studies were conducted in an effort to analyze the thermal characteristics of the tanks and to investigate the likelihood of "hot spots" forming as a result of radiolytic heating.Resolving the Ferrocyanide Safety Issue requires that tank contents meet the safety criteria and operations be conducted such that waste conditions fall within the criteria limits. Because the ferrocyanide sludge has been exposed for many years to other highly caustic wastes, as well as to elevated temperatures and both gamma and beta radiation, ferrocyanide decomposition has occurred in the tanks. As a result, the concentration of ferrocyanide is less than that predicted by tank inventory records. As such, C-Farm tanks have been re-categorized as safe, allowing resolution of the Ferrocyanide Safety Issue for these tanks. In the PNNL studies, ferrocyanide waste simulants were used instead of actual waste. The simulants used in these studies were primarily In-Farm simulants, prepared by WHC to mimic the waste generated when the In-Farm flowsheet was used to remove radiocesium from waste supernates in the SSTs. In the In-Farm flowsheet, nickel ion and ferrocyanide anion were added to waste supernate to precipitate sodium nickel ferrocyanide and co-precipitate radiocesium. Once the radiocesium was removed, supernates were pumped from the tanks and disposed to the ground via cribs. Later, new wastes from cladding removal processes and/or from evaporators were added.These new wastes were typically highly caustic, having hydroxide ion concentrations of over 1 M and as high as 4 M. The Aging Studies investigated possible reactions between this caustic waste and the precipitated ferrocyanide waste in a radiation field.Over a 5-year period, these studies demonstrated and characterized the aging of ferrocyanide waste simulants through dissolution and hydrolysis processes. The experiments show the ultimate products are ammonia (which is slowly radiolyzed), carbonate, and oxides or hydroxides of iron and nickel. Thus, most of the fuel value in the solid phase is consumed in the process. Any ferrocyanide tank waste that contacted highly caustic waste also would undergo similar aging processes. The rate of agi...

Quarterly report on the ferrocyanide safety program for the period ending June 30, 1996

“…Experiments on waste simulants show that the rate of aging was strongly affected by temperature (Lilga et al 1996). As shown in Table 3-2, the rate of aging increased dramatically with increasing temperature.…”

“…The final aging report (PNNL-11211) was issued on June 28, 1996 and summarizes all work completed to date on aging studies (Lilga et al 1996). The information was key to completing the technical basis safety document, WHC-SD-WM-SARR-038, Rev.…”

“…The experiments showed that the rate of thermal ferrocyanide aging does not increase at higher pH when the pH exceeds nine (Lilga et al 1996). The precipitation of sodium nickel ferrocyanide was done at slightly alkaline conditions @H = 8.0 -10).…”