D.R. Dickinson

Much attention has been focused on the Hanford Site radioactive waste storage tanks as a results of problems that have been envisioned for them. One problem is the potential chemical reaction between ferrocyanide precipitate particles and nitrates in the absence of water. This report addresses the question of whether dryout of a portion of ferrocyanide sludge would render it potentially reactive. Various sludge dryout mechanisms were examined to determine if any of them could occur. The mechanisms are: (1) bulk heating of the entire sludge inventory to its boiling point; (2) loss of liquid to the atmosphere via sludge surface evaporation; (3) local drying by boiling in a hot spot region; (4) sludge drainage through a leak in the tank wall; and (5) local drying by evaporation from a warm segment of surface sludge. From the simple analyses presented in this report and more detailed published analyses, it is evident that global loss of water from bulk heating of the sludge to its boiling point or from surface evaporation and vapor transport to the outside air is not credible. Also, from the analyses presented in this report and experimental and analytical work presented elsewhere, it is evident that formation of a dry local or global region of sludge as a result of tank leakage (draining of interstitial liquid) is not possible. Finally, and most importantly, it is concluded that formation of dry local regions in the ferrocyanide sludge by local hot spots or warm surface regions is not possible. The conclusion that local or global dryout is incredible is consistent with four decades of waste storage history, during which sludge temperature have gradually decreased or remained constant and the sludge moisture content has been retained. 54 refs.

Effect of aluminum oxide dissolution on aluminum corrosion in high temperature deionized water is discussed. Such dissolution promotes corrosion by removing protective oxide and is especially significant in nonisothermal systems, such as aluminum-clad fuel elements in nuclear reactors cooled by pressurized water, where the water cannot be kept saturated with dissolved oxide. Hypothesis is advanced that resistance to dissolution lies principally in mass transfer from oxide-water surface into bulk water; dissolution rate can be estimated by use of general equations for mass transfer, if oxide solubility is known as a function of temperature. Dissolution rates thus calculated were in good agreement with in-reactor corrosion rates of aluminum-clad fuel elements in high temperature deionized water, which had been previously measured at Hanford. Fair agreement was obtained with measurements in recirculating out-of-reactor loops. It is concluded that mass transfer of dissolved oxide into the water may be the principal factor controlling corrosion rate of aluminum-clad fuel elements in many nuclear reactor systems using high temperature pressurized deionized water as coolant.

The single-shell waste tanks at the Hanford Site that contain significant quantities of ferrocyanide have been considered a possible hazard, since under certain conditions the ferrocyanide in the waste tanks could undergo an exothermic chemical reaction with the nitrates and nitrites that are also present in the tanks. The purpose of this report is to assess the credibility of local dryout of ferrocyanide due to a hotspot. This report considers the following: What amount of decay heat generation within what volume would be necessary to raise the temperature of the liquid in the sludge to its boiling point? What mechanisms could produce a significant local concentration of heat sources? Is it credible that a waste tank heat concentration could be as large as that required to reach the dryout temperatures? This report also provides a recommendation as to whether infrared scanning of the ferrocyanide tanks is needed. From the analyses presented in this report it is evident that formation of dry, and thus chemically reactive, regions in the ferrocyanide sludge by local hotspots is not credible. This conclusion is subject to reevaluation if future analyses of tank core samples show much higher {sup 137}Cs or {sup 90}Sr concentrations than expected.more » Since hotspots of concern are not credible, infrared scanning to detect such hotspots is not required for safe storage of tank waste.« less