NSUF 17-1116: Investigation of irradiation-induced recrystallization in U-Mo fuel
Microstructural changes, characterized by grain subdivision and increased porosity, are commonly observed in nuclear fuels at high burnup, such as the U-Mo alloys irradiated to a fission density > 3×1021 fissions/cm3 [1]-[3] and the radial peripheral region of high-burnup UO2 fuel rods [4]. This restructuring process, termed “recrystallization” or “high burn-up structure (HBS)”, caused accelerated fuel swelling and imposed uncertainties on fuel performance at high burnup. To achieve a predictable and stable fuel swelling behavior, it is important to understand the driving mechanisms of the recrystallization process. This study focuses on the recrystallization mechanism of U-Mo fuel. U-Mo samples irradiated with high-energy heavy ions at various doses will be systematically characterized using Transmission Electronic Microscopy (TEM).
A previous TEM study [5], which was performed at the Center for Advanced Energy Studies (CAES), has revealed that recrystallization occurred in U-Mo irradiated with 80-MeV Xe ions. The samples characterized were U-7wt%Mo dispersion fuel particles. Figure 1 shows the overview of a sample irradiated to a dose of 2.6×1017 ions/cm2. Many small grains delineated with large bubbles were observed within the Xe deposition range (Zone B). Compared to the original cell/grain size (~ 4 µm on average), these newly-formed grains are much smaller (< 200nm), demonstrating the occurrence of irradiation-induced recrystallization. The typical microstructure of cell/grain boundaries before irradiation is shown in figure 2(a). Colonies of a-U precipitates (two examples are indicated with arrows in figure 2(a)) cluster around grain boundaries. These precipitates were generated from thermally-induced ?-phase uranium decomposition during fuel plate fabrication [5]. After irradiation, they dissolved in the ?-U matrix (Figure 2(b)) due to irradiation-induced phase reversion [6]. At the same time, sub-grain-like structures formed at the prior grain-boundary areas. Three sub-grains were outlined and labeled in figure 2(b) to show the possible recrystallized structure in the irradiated grain boundary area. Their boundaries have a slightly lighter contrast than their interiors. This observation leads to the speculation that recrystallization is related to irradiation-induced phase reversion in U-Mo fuel particles. Phase transformation is usually accompanied with substantial lattice distortions in uranium metals [7], and lattice stresses increase significantly in this process. Since the formation of a new set of defect-free small grains is an effective approach to release the lattice stresses [8], it is reasonable to deduce that the new grains observed at the prior grain boundary areas after irradiation is a microstructural response to the lattice stresses accumulated during irradiation-induced phase reversion.
(Please see the figures in the attached file.)
[1] Y.S. Kim, G.L. Hofman, J.S. Cheon, J. Nucl. Mater. 436 (2013) 14-22. [2] A. Leenaers, W. Van Renterghem, S. Van den Berghe, J. Nucl. Mater. 476 (2016) 218-230. [3] G.L. Hofman, Y.S. Kim, Nucl. Eng. Tech. 37 (2005) 299-308. [4] V.V. Rondinella, T. Wiss, Mater. Today 13 (2010) 24-32. [5] B. Ye, L. Jamison, Y. Miao, S. Bhattacharya, G.L. Hofman, A.M. Yacout, J. Nucl. Mater. 488 (2017) 134-142. [6] M.L. Bleiberg, L.J. Jones, B. Lustman, J. Appl. Phys. 27 (1956) 1270-1283. [7] H.H. Klepfer, P. Chiotti, Characteristics of the solid state transformation in uranium, Ames laboratory ISC technical reports, paper 163, 1957. [8] R.D. Doherty, D.A. Hughes, F.J. Humphreys, J.J. Jonas, D.J. Jensen, M.E. Kassner, W.E. King, T.R. McNelley, H.J. McQueen, A.D. Rollett, Mater. Sci. Eng. A238 (1997) 219-274.
Additional Info
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| Abstract | Microstructural changes, characterized by grain subdivision and increased porosity, are commonly observed in nuclear fuels at high burnup, such as the U-Mo alloys irradiated to a fission density > 3×1021 fissions/cm3 [1]-[3] and the radial peripheral region of high-burnup UO2 fuel rods [4]. This restructuring process, termed “recrystallization” or “high burn-up structure (HBS)”, caused accelerated fuel swelling and imposed uncertainties on fuel performance at high burnup. To achieve a predictable and stable fuel swelling behavior, it is important to understand the driving mechanisms of the recrystallization process. This study focuses on the recrystallization mechanism of U-Mo fuel. U-Mo samples irradiated with high-energy heavy ions at various doses will be systematically characterized using Transmission Electronic Microscopy (TEM). A previous TEM study [5], which was performed at the Center for Advanced Energy Studies (CAES), has revealed that recrystallization occurred in U-Mo irradiated with 80-MeV Xe ions. The samples characterized were U-7wt%Mo dispersion fuel particles. Figure 1 shows the overview of a sample irradiated to a dose of 2.6×1017 ions/cm2. Many small grains delineated with large bubbles were observed within the Xe deposition range (Zone B). Compared to the original cell/grain size (~ 4 µm on average), these newly-formed grains are much smaller (< 200nm), demonstrating the occurrence of irradiation-induced recrystallization. The typical microstructure of cell/grain boundaries before irradiation is shown in figure 2(a). Colonies of a-U precipitates (two examples are indicated with arrows in figure 2(a)) cluster around grain boundaries. These precipitates were generated from thermally-induced ?-phase uranium decomposition during fuel plate fabrication [5]. After irradiation, they dissolved in the ?-U matrix (Figure 2(b)) due to irradiation-induced phase reversion [6]. At the same time, sub-grain-like structures formed at the prior grain-boundary areas. Three sub-grains were outlined and labeled in figure 2(b) to show the possible recrystallized structure in the irradiated grain boundary area. Their boundaries have a slightly lighter contrast than their interiors. This observation leads to the speculation that recrystallization is related to irradiation-induced phase reversion in U-Mo fuel particles. Phase transformation is usually accompanied with substantial lattice distortions in uranium metals [7], and lattice stresses increase significantly in this process. Since the formation of a new set of defect-free small grains is an effective approach to release the lattice stresses [8], it is reasonable to deduce that the new grains observed at the prior grain boundary areas after irradiation is a microstructural response to the lattice stresses accumulated during irradiation-induced phase reversion. (Please see the figures in the attached file.) [1] Y.S. Kim, G.L. Hofman, J.S. Cheon, J. Nucl. Mater. 436 (2013) 14-22. [2] A. Leenaers, W. Van Renterghem, S. Van den Berghe, J. Nucl. Mater. 476 (2016) 218-230. [3] G.L. Hofman, Y.S. Kim, Nucl. Eng. Tech. 37 (2005) 299-308. [4] V.V. Rondinella, T. Wiss, Mater. Today 13 (2010) 24-32. [5] B. Ye, L. Jamison, Y. Miao, S. Bhattacharya, G.L. Hofman, A.M. Yacout, J. Nucl. Mater. 488 (2017) 134-142. [6] M.L. Bleiberg, L.J. Jones, B. Lustman, J. Appl. Phys. 27 (1956) 1270-1283. [7] H.H. Klepfer, P. Chiotti, Characteristics of the solid state transformation in uranium, Ames laboratory ISC technical reports, paper 163, 1957. [8] R.D. Doherty, D.A. Hughes, F.J. Humphreys, J.J. Jonas, D.J. Jensen, M.E. Kassner, W.E. King, T.R. McNelley, H.J. McQueen, A.D. Rollett, Mater. Sci. Eng. A238 (1997) 219-274. |
| Award Announced Date | 2017-09-20T12:30:01.96 |
| Awarded Institution | Center for Advanced Energy Studies |
| Facility | Microscopy and Characterization Suite |
| Facility Tech Lead | Yaqiao Wu |
| Irradiation Facility | None |
| PI | Bei Ye |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 1116 |