A buried stainless steel nuclear waste storage tank contains significant amounts

Question

A buried stainless steel nuclear waste storage tank contains significant amounts of dissolved radioactive Cr3+, Al3+, Fe3+, and Ni2+ at pH2. It is proposed to neutralize this acid tank solution to pH7 in the tank with concentrated NaOH. The neutralized solution will then be pumped out of the tank for transport to other facilities for further processing. Explain the advantages and disadvantages of the neutralization step using appropriate Pourbaix diagrams. On the basis of your evaluation, would you recommend neutralization ?

Explanation / Answer

In all cases, from the Pourbaix diagrams for Cr, Al, Fe and Ni, the component metal cations would precipitate as the respective hydoxides at pH 7, leaving a sludge at the bottom of the tank. If neutralization would not be effected, the issue now becomes whether these reducible metal cations can contribute to the corrosion of the stainless steel tank, even whether this stainless steel can remain passive at this pH.

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