Dissolution Flowsheet for High Flux Isotope Reactor Fuel

Technical Report · Tue Sep 27 00:00:00 EDT 2016

DOI:https://doi.org/10.2172/1327788· OSTI ID:1327788

Daniel, W. E. ^[1]; Rudisill, T. S. ^[1]; O'Rourke, P. E. ^[1]; Karay, N. S ^[1]

---

1. Savannah River Site (SRS), Aiken, SC (United States). Savannah River National Lab. (SRNL)

As part of the Spent Nuclear Fuel (SNF) processing campaign, H-Canyon is planning to begin dissolving High Flux Isotope Reactor (HFIR) fuel in late FY17 or early FY18. Each HFIR fuel core contains inner and outer fuel elements which were fabricated from uranium oxide (U₃O₈) dispersed in a continuous Al phase using traditional powder metallurgy techniques. Fuels fabricated in this manner, like other SNF’s processed in H-Canyon, dissolve by the same general mechanisms with similar gas generation rates and the production of H₂. The HFIR fuel cores will be dissolved and the recovered U will be down-blended into low-enriched U. HFIR fuel was previously processed in H-Canyon using a unique insert in both the 6.1D and 6.4D dissolvers. Multiple cores will be charged to the same dissolver solution maximizing the concentration of dissolved Al. The objective of this study was to identify flowsheet conditions through literature review and laboratory experimentation to safely and efficiently dissolve the HFIR fuel in H-Canyon. Laboratory-scale experiments were performed to evaluate the dissolution of HFIR fuel using both Al 1100 and Al 6061 T6 alloy coupons. The Al 1100 alloy was considered a representative surrogate which provided an upper bound on the generation of flammable (i.e., H₂) gas during the dissolution process. The dissolution of the Al 6061 T6 alloy proceeded at a slower rate than the Al 1100 alloy, and was used to verify that the target Al concentration in solution could be achieved for the selected Hg concentration. Mass spectrometry and Raman spectroscopy were used to provide continuous monitoring of the concentration of H₂ and other permanent gases in the dissolution offgas, allowing the development of H₂ generation rate profiles. The H₂ generation rates were subsequently used to evaluate if a full HFIR core could be dissolved in an H-Canyon dissolver without exceeding 60% of the calculated lower flammability limit (LFL) for H₂ at a given Hg concentration. Complete dissolution of the Al 1100 and Al 6061 T6 alloys up to a final Al concentration of 2 M was obtained using a 7 M HNO₃ solution containing a 0.002 M Hg catalyst. However, following the dissolutions, solids were observed in the solution. The solids were amorphous, but likely originated from the Si present in the alloys. No crystalline materials, such as Al(NO₃)₃ were observed. During the course of the dissolution experiments, it was determined that delaying the addition of Hg once the HNO₃ solution reached the boiling point can reduce the total offgas and H₂ generation rates. The delay in starting the Hg addition is not necessary for HFIR fuel dissolution, but could be useful in other research reactor dissolution campaigns. The potential to generate flammable concentrations of H₂ in the offgas during a HFIR fuel dissolution was evaluated using the experimental data. The predicted H₂ concentration in the dissolver offgas stream was compared with 60% of the calculated H₂ LFL at 200 °C using several prototypical experiments. The calculations showed that a full HFIR core can be dissolved using nominally 0.002 M Hg to catalyze the dissolution. The margin between the predicted H₂ concentration and the calculated LFL was greater when the solution was allowed to boil for 45 min prior to initiating the Hg addition. When the Hg was increased to 0.004 M, the predicted H₂ concentration exceeded the calculated LFL early in the dissolution. The dissolution experiments also demonstrated that additional Hg (beyond the initial 0.002 M) could be added as the Al concentration increases. The ability to add more Hg during a HFIR fuel dissolution could be beneficial if slow dissolution rates are observed at high Al concentrations. Experimental data were used to demonstrate that the predicted H₂ concentration in a dissolver was below 60% of the calculated LFL at 200 °C when 0.004 M Hg was used to catalyze the dissolution if the Al concentration is conservatively greater than 0.5 M. Data also show that the Hg concentration during a HFIR fuel dissolution can be increased from 0.002 to 0.008 M at an Al concentration of 1.3 M.

• Research Organization: Savannah River Site (SRS), Aiken, SC (United States). Savannah River National Lab. (SRNL)
• Sponsoring Organization: USDOE Office of Environmental Management (EM)
• DOE Contract Number: AC09-08SR22470
• OSTI ID: 1327788
• Report Number(s): SRNL-STI-2016-00485; TRN: US1700321
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
SPENT FUELS
TEMPERATURE RANGE 0400-1000 K
HFIR REACTOR
HYDROGEN
CONCENTRATION RATIO
SOLUTIONS
URANIUM OXIDES U3O8
DISSOLUTION
EXPERIMENTAL DATA
DISSOLVERS
FLOWSHEETS
NITRIC ACID
ALUMINIUM BASE ALLOYS
COMPARATIVE EVALUATIONS
FUEL ELEMENTS
BOILING
FLAMMABILITY
BENCH-SCALE EXPERIMENTS
CATALYSTS
MERCURY
REPROCESSING
LIMITING VALUES
SAVANNAH RIVER PLANT
HFIR
Spent Fuel
H-Canyon