Evolution of porosity in the high-burnup fuel structure

Romano, Antonino Davide; Horvath, Matthias; Restani, Renato

doi:10.1016/j.jnucmat.2006.09.016

Cited by 51 publications

(18 citation statements)

References 10 publications

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“…For average burnup 10 MW·days/kg the energy release in a 100 µm thick peripheral layer is 30% higher than the average value over the pellet. These results agree with the experimental data of [8], where it is shown that for average burnup 105 MW·days/kg the local burnup in a thin layer near the surface of a fuel pellet is three times higher than the average burnup.…”

supporting

confidence: 91%

Simulation of neutron-physical processes in the surface layer of a fuel kernel

et al. 2008

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A large change in the structure, density, and chemical and phase composition occurs in nuclear fuel with deep burnup, and an edge zone is formed. Simulation of the formation of an edge zone in a fuel kernel of thermal reactors will make it possible to suggest ways to decrease its influence on the characteristics of a fuel element. In the present work, the neutron-physical processes occurring in the peripheral layer of a fuel kernel are simulated. The distribution of nuclear reaction rates along the radius of a fuel pellet is calculated using the SCALE-4.3, MCNP-4B, and UNK computer programs. The radial dependence of the local breeding ratio is calculated. It is shown that for fresh fuel BR > 1 for fissile nuclei in a 100 µm thick layer, while the initial BR averaged over a pellet is no more than 0.5. The volume energy distribution in a 100 µm thick peripheral layer is 30% higher than the average value over a pellet. A combined pellet, where the central part possesses the standard enrichment (4-5% 235 U) and the peripheral layer contains less than 0.7% 235 U, is proposed to decrease the influence of the edge zone on the properties of fuel.The main goal of scientific and technical development work in fuel-cycle power reactors is increasing fuel burnup and the reliability of fuel assemblies under operating conditions. This is achieved mainly by increasing the initial fuel enrichment and by using consumable absorbers. However, an increase of burnup leads to the development of processes that affect the integral characteristics of fuel elements. One such process gives rise to an edge zone with an easily distinguishable altered microstructure on the periphery of the fuel pellets. The structure of the edge zone is characterized by the presence of many fine gas bubbles, vanishing of the initial grain structure, and formation of new, much smaller subgrains (<1 µm) [1,2].It has been found experimentally that an edge zone starts to form in uranium dioxide fuel pellets in thermal reactors at burnup higher than 45-50 MW·days/kg. This layer is 100-200 µm thick and is characterized by much higher burnup than the main part of the fuel pellet. Thus, for average burnup 50 MW·days/kg the burnup in a 100 µm thick peripheral layer will reach 80 MW·days/kg, and the burnup directly on the surface of a pellet will reach 150 MW·days/kg.The formation and development of an edge zone where the porosity is 3-5 times higher than the initial porosity creates a barrier for heat flow from fuel to cladding and degrades the thermal conductivity of the fuel-cladding gap with emission of gaseous fission products. An increase of the fuel temperature and the emission of gaseous fission products from the edge zone will increase the pressure on the cladding of a fuel element, which can have a large effect on the characteristics of the fuel element. Simulation of the processes resulting in the formation of an edge zone will make it possible to suggest ways to decrease the effect of such a zone on the characteristics of a fuel element.

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confidence: 91%

Simulation of neutron-physical processes in the surface layer of a fuel kernel

et al. 2008

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“…Formation of fission-gas bubbles all over the surface of the pellet. When compared with other high burnup fuel results [6][7][8], the state and extent of the HBS is far less advanced here. The thickness of the (completely developed) HBS, for instance, is about 92 µm at 83 GWd/tU, whereas more than 350 µm has been reported elsewhere [7].…”

Section: Resultsmentioning

confidence: 74%

HIGH BURNUP CHANGES IN UO₂FUELS IRRADIATED UP TO 83 GWD/T IN M5^(R)CLADDINGS

Noirot

Aubrun²,

Desgranges³

et al. 2009

Nuclear Engineering and Technology

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“…The radial extent of this lower density peripheral region ($1 mm) seems to correspond to the high-burnup-structure region, with high porosity, as reported elsewhere. 23) Furthermore, because transmission tomography can be used in combination with emission tomography, a linear attenuation coefficient matrix was derived, to be employed in the emission reconstruction process, so as to increase the final image accuracy. This work has been already carried out 14) and will be presented separately.…”

Section: Discussionmentioning

confidence: 99%

“…The radial extent of the lower density peripheral region ($1 mm) appears to correspond to a high-burnup-structure region with a much higher level of porosity such that it yields a lower overall density. 23) The uncertainty (1) in the central region is <0:7%, whilst the periphery is characterised by higher values (1.2%-2.5%, depending on the sample). These uncertainties, for the two distinct regions, central and peripheral, were derived from the results of a sensitivity study carried out as follows: First, noise was introduced into the input sinograms, so as to derive new sinograms using the relation…”

Section: Density Distributionmentioning

confidence: 95%

Nondestructive Determination of Fresh and Spent Nuclear Fuel Rod Density Distributions through Computerised Gamma-Ray Transmission Tomography

Caruso¹,

Murphy²,

Jatuff³

et al. 2008

Journal of Nuclear Science and Technology

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The morphology of single PWR UO 2 fuel rods has been investigated by employing computerised gamma-ray transmission tomography. Four highly burnt fuel rods of different burnups (from 52 GWd/t to 126 GWd/t) and a fresh rod have been investigated. This paper describes the tomographic station built to acquire the projections together with the related experimental procedure, followed by the description of the image reconstruction techniques implemented. The principal results obtained are presented and analysed at three levels. First, the derived linear attenuation coefficient matrix, which is very useful for the implementation of emission tomography, is discussed. Second, the variation of density as a function of rod radius is presented, showing an interesting morphological change of the fuel with burnup. Particularly for the highest burnup sample, the periphery is characterised by a lower density (a steep decrease of almost 10% from the adjacent region) characteristic of an ultrahigh-burnup high-porosity structure. Finally, the nondestructive derivation of the volume-averaged fuel density was performed, compared with other experimental data, and plotted as a function of burnup, so that an interesting linear relationship between density and burnup was established, providing a potential indicator for the noninvasive determination of nuclear fuel rod burnup.

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Evolution of porosity in the high-burnup fuel structure

Cited by 51 publications

References 10 publications

Simulation of neutron-physical processes in the surface layer of a fuel kernel

Simulation of neutron-physical processes in the surface layer of a fuel kernel

HIGH BURNUP CHANGES IN UO₂FUELS IRRADIATED UP TO 83 GWD/T IN M5^(R)CLADDINGS

Nondestructive Determination of Fresh and Spent Nuclear Fuel Rod Density Distributions through Computerised Gamma-Ray Transmission Tomography

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Evolution of porosity in the high-burnup fuel structure

Cited by 51 publications

References 10 publications

Simulation of neutron-physical processes in the surface layer of a fuel kernel

Simulation of neutron-physical processes in the surface layer of a fuel kernel

HIGH BURNUP CHANGES IN UO2FUELS IRRADIATED UP TO 83 GWD/T IN M5(R)CLADDINGS

Nondestructive Determination of Fresh and Spent Nuclear Fuel Rod Density Distributions through Computerised Gamma-Ray Transmission Tomography

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HIGH BURNUP CHANGES IN UO₂FUELS IRRADIATED UP TO 83 GWD/T IN M5^(R)CLADDINGS