Conditions Conducive to Forming Crystalline Uranyl Silicates in High Caustic Nuclear Waste Evaporators

The laboratory conditions used to synthesize the uranyl silicate minerals are almost identical to the evaporator conditions under which high caustic nuclear wastes are processed to reduce total liquid waste volume. The only significant difference is in the sodium ion concentration in such caustic nuclear wastes, which typically averages ~5.6 M Na⁺. The goal of this study was to experimentally determine whether uranium silicate minerals can be produced under nuclear waste evaporator conditions. If the formation of these uranium minerals is possible, it may not only lead to the clogging of the evaporators but also result in the accumulation of fissile ²³⁵U and thus present a criticality problem.

In this investigation, synthetic uranyl silicate minerals (sodium weeksite, sodium boltwoodite, and uranophane) were produced only under low Na⁺ concentration (<0.02 M), while attempts to synthesize these same uranyl silicate minerals in the presence of high Na⁺ concentration (high ionic strength reacting media), which is typical of caustic nuclear waste evaporator processing conditions, proved unfruitful. In the presence of high Na⁺ concentration, the main product for the same soluble silica-uranium reaction mixture shifts toward the formation of mainly clarkeite (Na[(UO₂)O(OH)](H₂O)_0-1), a hydrated sodium uranate, and not toward the formation of uranyl silicates.

Thus, the presence of high Na⁺ concentration in the reaction mixture of dissolved uranium and silica inhibits or suppresses the formation of crystalline uranyl silicates. The conclusion is therefore made that evaporator fouling by uranyl silicate minerals is not easily attained under nuclear waste processing conditions because of the high Na⁺ concentration in the liquid wastes.