Ferrocyanide safety project ferrocyanide aging studies FY 1995 annual report

Abstract

SummaryThis annual report gives the results of the work conducted by the Pacific Northwest Laboratory in FY 1995 on Task 3 of the Ferrocyanide Safety Project, Ferrocyanide Aging Studies. Aging refers to the dissolution and hydrolysis of simulated Hanford ferrocyanide waste in alkaline aqueous solutions by radiolytic and chemical means. The ferrocyanide simulant primarily used in these studies was driedIn-Farm-lB, Rev. 7, prepared by Westinghouse Hanford Company to simulate the waste generated when the In-Farm flowsheet was used to remove radiocesium from waste supernates in single-shell tanks at the Hanford Site. In the In-Farm flowsheet, nickel ion A d ferrocyanide anion were added to waste supernates to precipitate sodium nickel ferrocyanide, Na,NiFe(CN),, and co-precipitate radiocesium. Once the radiocesium was removed, supernates were pumped from the tanks, and new wastes from cladding removal processes 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 task is investigating reactions this caustic waste may have had with the precipitated ferrocyanide waste in a radiation field.In previous Aging Studies research, N%NiFe(CN), in simulants was shown to dissolve in basic solutions, forming insoluble Ni(OH), and soluble Na,Fe(CN),. The influence on solubility of base strength, sodium ion concentration, anions, -and temperature was previously investigated. Destruction of ferrocyanide anion by hydrolysis to form ammonia and formate ion was found to be promoted by gamma irradiation. Increasing temperature and gamma dose rate increased the rate of hydrolysis, as indicated by the amount of ammonia formed with time. Ammonia was found to be radiolyzed in the gamma field. The change in concentration of formate ion generally paralleled that of ammonia but was three to four times less concentrated. Total soluble iron concentrations were found to decrease in what appeared to be a pseudo-first-order fashion. The nickel concentration tended to increase, sometimes to 90% redissolution. Aluminum added to the hydrolysis solution had relatively little effect and may have slightly promoted hydrolysis.. In FY 1995 Aging Studies work, temperature and gamma dose rate effects were further investigated. Consistent with prior work, as the temperature or the gamma dose rate increased, the rate of hydrolysis also increased. As an approximation, ammonia production with time was described as being linear. Rate constants for ammonia production followed a linear Arrhenius relationship and also increased linearly with increasing applied gamma dose rate. Rate constants for ammonia production at other conditions could be predicted with use of the temperature and dose rate relationships.Ammonia was found to be destroyed in the gamma environment. Rate constants of 1.1 x Mlday at an applied dose rate of 1.07 x l@ Rad/h, and 2.1 x lo4 Mlday at 8.91 x 103 Rad/h were measured for this process. Formate ion destruction was found to occur at...

Washing and caustic leaching of Hanford tank sludge: Results of FY 1997 studies

“…Interestingly, wastes in Tanks BY-lOS, C-I0S, C-109, and C-112 were scavenged with ferrocyanide to precipitate the J37Cs as NaCsNiFe(CN)6 or Cs 2 NiFe(CNk The Cs being present would explain the 137CS behavior during washing and caustic leaching as the nickel ferrocyanide salts in these wastes. Recent studies have shown that Na2NiFe(CN)6 decomposes by both radio lytic and hydrolytic mechanisms under conditions that would be expected in the Hanford tanks, and it has been suggested that these pathways would have led to a reduction in the overall ferrocyanide concentrations in the sludges (Lilga et al 1995). However the cesium nickel ferrocyanide salts are less soluble in aqueous NaOH (Bryan et al 1993;Lilga et al 1995), so their decomposition might be slower than the Na analog.…”

“…Recent studies have shown that Na2NiFe(CN)6 decomposes by both radio lytic and hydrolytic mechanisms under conditions that would be expected in the Hanford tanks, and it has been suggested that these pathways would have led to a reduction in the overall ferrocyanide concentrations in the sludges (Lilga et al 1995). However the cesium nickel ferrocyanide salts are less soluble in aqueous NaOH (Bryan et al 1993;Lilga et al 1995), so their decomposition might be slower than the Na analog. Indeed, ferrocyanide has been detected in actual C-109 and C-l12 sludges at levels close to 1 molo (Bryan et al 1995).…”

Headspace gas and vapor characterization summary for the 43 vapor program suspect tanks

“…Hydrogen cyanide is an unlikely product of the degradation of complex metal ferrocyanides under tank conditions. High ammonia concentrations are also associated with the aging of ferrocyanide wastes (Lilga 1992(Lilga , 1993(Lilga , 1994(Lilga , 1995. As expected, none of the Ferrocyanide Watch List tanks sampled for hydrogen cyanide contained any credible indications of its presence.…”

Tank characterization report for single-shell tank 241-BY-106

“…Recent aging studies of ferrocyanide waste show that the combined effects of temperature, radiation, and pH during 38 years or more of storage would have destroyed most of the ferrocyanide originally added to the tanks (Babad et al 1993;Lilga et al 1993Lilga et al , 1994Lilga et al , 1995Lilga et al and 1996. This prediction has been confirmed by the tank samples analyzed to date.…”