Alternative reagent to mercuric nitrate catalyst for dissolution of aluminum-clad nuclear fuels in nitric acid
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Alternative reagent to mercuric nitrate catalyst for dissolution of aluminum-clad nuclear fuels in nitric acid Philip A. Anderson and Jerry D. Christian Lockheed Martin Idaho Technologies Company, Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho 83415 (Received 15 January 1997; accepted 25 July 1997)
A substitute additive, HBF4 , has been discovered that will replace Hg(NO3 )2 catalyst 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 mgycm2 -h can be achieved in a continuous dissolution process at 100 ±C using a dissolvent of 0.15 M HBF4 in 7 M HNO3 that 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 mmymo. 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 HBF4 and complexed aluminum fluoride species in nitric acid, which provide a large semi-buffered supply of HF. This allows the HNO3 to attack the aluminum metal. The small concentration of HF does not compete favorably with HNO3 for reaction with and consumption by Al.
I. INTRODUCTION
Aluminum-clad nuclear fuels highly enriched in uranium-235 are used for research purposes, production of power, production of radionuclides for medical and commercial uses, and production of nuclear weapons materials. The fuel matrix inside the cladding may consist of uranium metal, a uranium-aluminum alloy, or uranium oxide. When such fuels have been irradiated to the end of useful life in the reactor, they are discharged and dissolved to separate and recover the product isotopes and/or the remaining uranium for further use. The dissolvent must be compatible with the solvent extraction separations process and the solidification processes for the resultant high-level radioactive waste that remains. Nitric acid has been the dissolvent used for this purpose since the 1950s. It has the added advantage over other decladding dissolvents, such as a sodium hydroxide solution, in that its use enables dissolution of the uranium fuel in a semi-continuous process of high throughput rate. In addition, nitric acid solutions are generally acceptable with respect to the corrosion performance of the process vessels, transfer lines, waste tanks, etc., which are usually made of stainless steel or high-nickel alloys. Aluminum metal and its alloys readily form an oxide film on the surface that reacts very slowly with nitric acid. The dissolution rate is impracticably slow without the use of a mercuric nitrate catalyst, which was discovered and developed in the 1950s1–4 and which has been utilized since that time5–8 to dissol
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