Uranium Metal Dissolution in the Presence of Fluoride and Boron Page: 4 of 22
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WSRC-TR-2003-00500, Rev. 0
Page 3 of 21
SUMMARY
H-Area Operations is planning to process plutonium-contaminated (Pu-contaminated)
uranium metal scrap in its efforts to de-inventory excess nuclear materials. The use of H-
Canyon and HB-Line was evaluated for feasibility in meeting processing targets and
requirements. SRTC performed flowsheet development to support the decision about how
and where the scrap will be processed. Specifically, experimental work was performed to
(1) gather reaction rate data at a range of processing conditions, (2) generate data for
calculating hydrogen and total gas generation rates, (3) propose a process flowsheet, and
(4) demonstrate that the proposed flowsheet does not pose a criticality hazard.
Uranium metal dissolution experiments have shown that acceptable dissolution rates can
be achieved for the Pu-contaminated scrap program using either nitric acid (HNO3)
concentrations above 7M or low HNO3 concentrations (1-4M) in the presence of fluoride
and boron. At low acid concentrations in the absence of fluoride, the reaction rates are
unacceptably slow for the Pu-contaminated scrap program. The observed behavior of
dissolution rates as a function of acid concentration and temperature are in general
agreement with what is expected based on the literature.
Gas generation tests have demonstrated that hydrogen generation is not an issue at the
conditions being proposed for plant operations. At HNO3 concentrations above 2M, the
hydrogen component of the offgas is less that 0.1% by volume. The total amount of gas
generation will be approximately 18.6 mL/hr per square centimeter of exposed metal
surface area.
Mixing studies have shown that criticality is not a likely event in the dissolver insert
either at room temperature or at 1000C. In 2M HNO3/0.025M potassium fluoride (KF)
and 2 g/L boron (B) at room temperature, a steady gas stream is generated from the
surface of uranium metal. The gas generation rate is sufficient to mix the contents of the
dissolver insert. In 4M HNO3/0.025M KF and 2 g/L boron, there is insufficient gas
generation to disperse the dissolved uranium. Instead of mixing, the denser uranium
solution drops down out of the dissolver insert and away from the metal being dissolved.
Variations in acid concentration away from 2M HNO3 should not be a problem since
dissolution in both 1M or 3M HNO3 (with KF and boron) at room temperature produces
gas generation comparable to that of 2M HNO3. When the temperature is raised to
100 C, the gas generation at 1-4M HNO3 in 0.025M KF and 2 g/L boron exceeds that of
2M HNO3 at room temperature, and thus will provide excellent mixing in the dissolver
insert.
Based on reactions at room temperature and boiling, and the rates of reaction at 1000C,
SRTC recommends the use of 2M HNO3/0.025M KF and 2 g/L boron at boiling in H-
Canyon to process the Pu-contaminated scrap material.
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Pierce, Robert A. Uranium Metal Dissolution in the Presence of Fluoride and Boron, report, February 2, 2004; South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc739747/m1/4/?rotate=90: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.