Alternative reagent to mercuric nitrate catalyst for dissolution of aluminum-clad nuclear fuels in nitric acid

Abstract

A substitute additive, HBF4, has been discovered that will replace Hg(NO3)2catalyst for dissolving aluminum spent nuclear fuels in nitric acid for recovery of usable materials. The catalyst or substitute is necessary to penetrate a protective oxide film that continuously forms on the Al surface in the oxidizing acid. A penetration rate of alloy Al-6061 T6 of 40 mg/cm2-h can be achieved in a continuous dissolution process at 100 °C using a dissolvent of 0.15 M HBF4in 7 M HNO3that achieves a steady-state composition of 1.0 M Al3+and 3.3 M HNO3, while maintaining a corrosion rate of a type 304L stainless steel dissolver vessel of 0.015 mm/mo. The penetration rate of aluminum is correlated with the equilibrium concentration of HF in the system. The postulated mechanism involves dissolution of the alumina film by approximately 0.006 M HF in equilibrium with the HBF4and complexed aluminum fluoride species in nitric acid, which provide a large semi-buffered supply of HF. This allows the HNO3to attack the aluminum metal. The small concentration of HF does not compete favorably with HNO3for reaction with and consumption by Al.