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Swelling Mechanisms of UO2 Lattices with Defect Ingrowths.

Günay SD - PLoS ONE (2015)

Bottom Line: In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method.Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects.Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects.

View Article: PubMed Central - PubMed

Affiliation: Yıldız Technical University, Department of Physics, Faculty of Science, Esenler, Istanbul, Turkey.

ABSTRACT
The swelling that occurs in uranium dioxide as a result of radiation-induced defect ingrowth is not fully understood. Experimental and theoretical groups have attempted to explain this phenomenon with various complex theories. In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method. Based on their resemblance to experimental data, the simulation results presented here show that fission induces only oxygen Frenkel pairs while alpha particle irradiation results in both oxygen and uranium Frenkel pair defects. Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects. It is shown that obstruction type Frenkel pairs are responsible for both fission- and alpha-particle-induced lattice swelling. Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects. Additionally, at high concentrations, some of the obstruction type uranium Frenkel pairs formed diatomic and triatomic structures with oxygen ions in their octahedral cages, increasing the slope of the linear dependence.

No MeSH data available.


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Relative lattice expansions.(a) Total number of defects after equilibration and (b) initial number of uranium FPs. The inset was taken from Ref. 21 and presents experimental lattice expansion data as a function of cumulative dose.
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pone.0134500.g008: Relative lattice expansions.(a) Total number of defects after equilibration and (b) initial number of uranium FPs. The inset was taken from Ref. 21 and presents experimental lattice expansion data as a function of cumulative dose.

Mentions: Figs 7 and 8 present plots of lattice expansion, Δa/a0, versus number of defects. There is no clear functional relationship between Δa/a0 and obstruction type oxygen defects or distortion type uranium defects. Obstruction type oxygen and distortion type uranium defects contribute almost nothing to the total number of defects (Figs 5 and 6) when present as IFPs in quantities of less than ~15 and ~10, respectively. Therefore, these defects have not been included in Fig 7. At lower than saturation levels, lattice expansion has an exponential dependence on the number of distortion type oxygen (see Fig 7(a)), distortion + obstruction types of oxygen (see Fig 7(c)), distortion + obstruction types of oxygen + uranium (see Fig 8(a)), and initial uranium FP (see Fig 8(b)) defects. The maximum lattice expansion observed was about 1.4% using the Yakub potential and 0.5% using the Günay potential. These values correspond to volume changes of 4.2 and 1.5%, respectively. For comparison, the experimentally determined relationship between lattice expansion and alpha dose, as determined by Weber [21], is shown as inset in Fig 8(b). Fitting the data obtained from the damage ingrowth model of Weber [21] yielded the expression Δa/a0 = 8.4×10−3[1-exp(-0.85Dα×10−16)], which predicts the lattice expansion to be 0.84% under saturation conditions [21]. Inspiring with Weber’s equation, we have also fitted the data to the same equation and in all cases the saturation value of the lattice expansion Δa/a0 is estimated between 0.44%-0.48% for Günay and 1.3%-1.8% for Yakub potentials. The reason lower lattice expansion values were obtained using the Günay potential could be that it uses a stronger attractive interaction than the Yakub model does. Weber [21] has estimated that one defect pair for every 3–4 unit cells occurs in the saturation region and that such a defect concentration is indicative of isolated defects and negligible clustering. Both potentials resulted in very similar numbers of obstruction type defects to those determined by Weber. It is of interest that, as shown in Fig 7(b), lattice expansion varies linearly with the number of surviving obstruction type uranium defects. Evidently, such uranium defects, which are coordinated to six uranium ions, are responsible for lattice expansion. According to Eq 2, the gradient of the plot in Fig 7(b) indicates the volume increments per obstruction type uranium Frenkel pair, ΔvF, to be 39.56 and 49.47 Å3, using the Günay and Yakub potentials, respectively.


Swelling Mechanisms of UO2 Lattices with Defect Ingrowths.

Günay SD - PLoS ONE (2015)

Relative lattice expansions.(a) Total number of defects after equilibration and (b) initial number of uranium FPs. The inset was taken from Ref. 21 and presents experimental lattice expansion data as a function of cumulative dose.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4526661&req=5

pone.0134500.g008: Relative lattice expansions.(a) Total number of defects after equilibration and (b) initial number of uranium FPs. The inset was taken from Ref. 21 and presents experimental lattice expansion data as a function of cumulative dose.
Mentions: Figs 7 and 8 present plots of lattice expansion, Δa/a0, versus number of defects. There is no clear functional relationship between Δa/a0 and obstruction type oxygen defects or distortion type uranium defects. Obstruction type oxygen and distortion type uranium defects contribute almost nothing to the total number of defects (Figs 5 and 6) when present as IFPs in quantities of less than ~15 and ~10, respectively. Therefore, these defects have not been included in Fig 7. At lower than saturation levels, lattice expansion has an exponential dependence on the number of distortion type oxygen (see Fig 7(a)), distortion + obstruction types of oxygen (see Fig 7(c)), distortion + obstruction types of oxygen + uranium (see Fig 8(a)), and initial uranium FP (see Fig 8(b)) defects. The maximum lattice expansion observed was about 1.4% using the Yakub potential and 0.5% using the Günay potential. These values correspond to volume changes of 4.2 and 1.5%, respectively. For comparison, the experimentally determined relationship between lattice expansion and alpha dose, as determined by Weber [21], is shown as inset in Fig 8(b). Fitting the data obtained from the damage ingrowth model of Weber [21] yielded the expression Δa/a0 = 8.4×10−3[1-exp(-0.85Dα×10−16)], which predicts the lattice expansion to be 0.84% under saturation conditions [21]. Inspiring with Weber’s equation, we have also fitted the data to the same equation and in all cases the saturation value of the lattice expansion Δa/a0 is estimated between 0.44%-0.48% for Günay and 1.3%-1.8% for Yakub potentials. The reason lower lattice expansion values were obtained using the Günay potential could be that it uses a stronger attractive interaction than the Yakub model does. Weber [21] has estimated that one defect pair for every 3–4 unit cells occurs in the saturation region and that such a defect concentration is indicative of isolated defects and negligible clustering. Both potentials resulted in very similar numbers of obstruction type defects to those determined by Weber. It is of interest that, as shown in Fig 7(b), lattice expansion varies linearly with the number of surviving obstruction type uranium defects. Evidently, such uranium defects, which are coordinated to six uranium ions, are responsible for lattice expansion. According to Eq 2, the gradient of the plot in Fig 7(b) indicates the volume increments per obstruction type uranium Frenkel pair, ΔvF, to be 39.56 and 49.47 Å3, using the Günay and Yakub potentials, respectively.

Bottom Line: In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method.Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects.Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects.

View Article: PubMed Central - PubMed

Affiliation: Yıldız Technical University, Department of Physics, Faculty of Science, Esenler, Istanbul, Turkey.

ABSTRACT
The swelling that occurs in uranium dioxide as a result of radiation-induced defect ingrowth is not fully understood. Experimental and theoretical groups have attempted to explain this phenomenon with various complex theories. In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method. Based on their resemblance to experimental data, the simulation results presented here show that fission induces only oxygen Frenkel pairs while alpha particle irradiation results in both oxygen and uranium Frenkel pair defects. Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects. It is shown that obstruction type Frenkel pairs are responsible for both fission- and alpha-particle-induced lattice swelling. Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects. Additionally, at high concentrations, some of the obstruction type uranium Frenkel pairs formed diatomic and triatomic structures with oxygen ions in their octahedral cages, increasing the slope of the linear dependence.

No MeSH data available.


Related in: MedlinePlus