BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//CERN//INDICO//EN
BEGIN:VEVENT
SUMMARY:Development and assessment of a mechanistic model describing high
burnup structure behavior in oxide nuclear fuel
DTSTART;VALUE=DATE-TIME:20191106T104000Z
DTEND;VALUE=DATE-TIME:20191106T110000Z
DTSTAMP;VALUE=DATE-TIME:20201125T203950Z
UID:indico-contribution-4352-19127@indico.psi.ch
DESCRIPTION:Speakers: Tommaso Barani (Politecnico di Milano)\nThe combinat
ion of high burnups and low temperatures occurring in the rim of oxide fue
ls\, or in plutonium-rich islands of heterogenous mixed-oxide fuels\, lead
s to a re-crystallization of the as-fabricated microstructure. The resulti
ng structure\, known as high burnup structure (HBS)\, is featured by nanom
etric-size grains and may develop substantial porosity as burnup proceeds.
These aspects strongly affect the performance of oxide fuels in normal op
erating conditions and may jeopardize the integrity of the fuel rods durin
g accidental transients (e.g.\, LOCA) at high burnups. Thus\, developing m
odels to be included in fuel performance codes is of the utter importance
for a more reliable simulation of fuel rod behavior at high burnups. In th
is work\, we present a mechanistic model describing the formation and evol
ution of the HBS in oxide fuels\, featured by a level of complexity suitab
le for application in fuel performance codes. The model includes a semi-em
pirical modelling approach to HBS formation\, expressing the volumetric po
rtion of restructured fuel as a function of local effective burnup (i.e.\,
the burnup accumulated below 1000°C) by the Kolmogorov-Johnson-Mel-Avram
i (KJMA) relationship for phase transitions. The KJMA parameters were fitt
ed combining experimental data on restructured volumes extracted from [1\,
2] with TRANSURANUS simulations for the estimation of local effective bur
nup. We used the ImageJ image analysis software to derive novel data from
[2]. The evolution of the HBS intergranular porosity is accounted for in a
mechanistic fashion. Starting from the cluster-dynamics master equations
governing the problem\, we apply a Fokker-Planck expansion of the master e
quations in the cluster size space. In doing so\, we derive a single-size
model that describes not only the time-evolution of HBS pores number densi
ty\, but also the first (mean size) and second (variance) moments of the p
ore size distribution. The model accounts for HBS pore growth due to the c
apture of fission gas atoms diffusing from the restructured HBS grains\, a
nd for pore-size-dependent irradiation re-solution. Moreover\, pores are a
ssumed not to be in mechanical equilibrium and can thus relax overpressure
by absorbing vacancies. Pores interconnection by impingement during growt
h is accounted for\, assuming pair interactions of HBS pores treated as mo
nodispersed hard spheres [3]. We implemented the model in the SCIANTIX gas
behavior code [4] and compared the calculation results to available exper
imental data in terms of xenon depletion in the fuel matrix [5]\, to model
s available in the open literature\, and to data on HBS pore number densit
y\, average radius\, and resulting fuel swelling [6]. Model results demons
trate a good agreement of model predictions with the experimental data and
considered models from the open literature\, on both xenon depletion and
porosity evolution\, together with computational burdens in-line with fuel
performance codes requirements.\n[1] J. Noirot et al.\, J. Nucl. Mater. 4
62 (2015) 77–84.\n[2] T. Gerczak et al.\, J. Nucl. Mater. 509 (2018) 245
–259.\n[3] S. Torquato\, Phys. Rev. E\, 51 (1995) 3170–3182.\n[4] Scia
ntix\, https://gitlab.com/poliminrg/sciantix\n[5] C.T. Walker\, J. Nucl. M
ater. 275 (1999) 56–62.\n[6] F. Cappia et al.\, J. Nucl. Mater. 480 (201
6) 138–149\n\nhttps://indico.psi.ch/event/6941/contributions/19127/
LOCATION:PSI Auditorium WHGA/001
URL:https://indico.psi.ch/event/6941/contributions/19127/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Mesoscale fission gas percolation simulations of heterogeneous and
irregular grain networks
DTSTART;VALUE=DATE-TIME:20191106T093000Z
DTEND;VALUE=DATE-TIME:20191106T100000Z
DTSTAMP;VALUE=DATE-TIME:20201125T203950Z
UID:indico-contribution-4352-19116@indico.psi.ch
DESCRIPTION:Speakers: Andrew Albert Prudil (Canadian Nuclear Laboratories)
\nThe two-species Included Phase Model (IPM) has been utilized to simulate
percolation of grain boundary fission gas bubbles on two-dimensional hexa
gonal grids representing (U\,Pu)O2 fuel. Simulations were performed in wh
ich transport of vacancies and fission gas were coupled with minimization
of internal\, elastic and interfacial energies on networks of 15-300 grai
ns with a computationally efficient mesoscale technique. An open surface a
t a boundary allows gas venting and subsequent tunnel collapse. The impact
of the stochastic nature of compositional heterogeneity and irregular gra
in size on macroscopic fuel performance behaviour was explored statistical
ly by analyzing the results of a series of randomly generated cases. The m
acroscopic fuel performance properties: grain boundary concentration\, fis
sion gas release fraction\, vacancy fraction\, and fractional coverage wer
e analyzed for each case.\nCompositional heterogeneity in Mixed Oxide fuel
was simulated by randomly distributing plutonium-rich\, high fission dens
ity grains on a regular hexagonal grid\, the proximity of which to the fre
e surface was found to dominate the percolation behavior. Irregular grain
size was simulated on equiangular hexagonal grains with irregular edge len
gth and grain area. An analysis of individual edges showed discrete growth
modes corresponding to the number of fission gas bubbles which nucleate o
n each edge. For edges in the bulk\, percolation time is most strongly cor
related to the edge length\, then to a lesser degree grain area and proxim
ity to the open surface.\n\nhttps://indico.psi.ch/event/6941/contributions
/19116/
LOCATION:PSI Auditorium WHGA/001
URL:https://indico.psi.ch/event/6941/contributions/19116/
END:VEVENT
BEGIN:VEVENT
SUMMARY:A DEM/FFT approach to compute the effective thermal conductivity o
f fragmented fuels under accident conditions
DTSTART;VALUE=DATE-TIME:20191106T102000Z
DTEND;VALUE=DATE-TIME:20191106T104000Z
DTSTAMP;VALUE=DATE-TIME:20201125T203950Z
UID:indico-contribution-4352-19112@indico.psi.ch
DESCRIPTION:Speakers: Jean-Mathieu Vanson (CEA Cadarache)\nSimulation of t
he effective properties of granular media such as thermal conductivity is
of a great interest for many applications. Under accident conditions the n
uclear fuel may crack and relocate inside its cladding. In this case\, the
relocation leads to a degraded conductivity of the fuel. Thermal conducti
vity evaluation is interesting to evaluate the temperature in the fuel rod
. \nWe developped a chained method involving Discrete Element Method (DEM)
[1] and Fast Fourier Transform (FFT)[2] : We first compute the packing mic
rostructure\,\naccording to the grains kinetic\, with DEM. The microstruct
ure is voxelised and finally the FFT method is employed to compute the eff
ective thermal conductivity of the granular media. This method allows us t
o compute accurately the effective thermal conductivity of a granular medi
a taking its real microstructure into account.\nWe also performed a sensit
ivity analysis over the discretisation parameters\, the size of the seed a
s well as the definition of interfacial voxels having both solid and gas p
hases (called fuzzy voxels). Thus we propose to define several bounds and
estimates of the effective thermal conductivity depending on the definitio
n of the fuzzy voxels. Finally\, we compare the results of the DEM/FFT met
hod to experimental measurements available in the litterature[3\, 4] and s
how a good agreement between our simulations and the measurements.\n\n\n[1
] Frédéric Dubois\, Michel Jean\, Mathieu Renouf\, Rémy Mozul\, Alexand
re Martin\,\nand Marine Bagnéris. LMGC90. In 10e colloque national en cal
cul des structures\,\nGiens\, France\, May 2011.\n[2] J.C. Michel\, H. Mou
linec\, and P. Suquet. Eective properties of composite materials\nwith per
iodic microstructure : a computational approach. Computer Methods in\nAppl
ied Mechanics and Engineering\, 172(1) :109 - 143\, 1999.\n[3] JS Boegli a
nd RG Deissler. Measured effective thermal conductivity of uranium\noxide
powder in various gases and gas mixtures. 1955.\n[4] R.O.A. Hall and D.G.
Martin. The thermal conductivity of powder beds. a model\,\nsome measureme
nts on uo2 vibro-compacted microspheres\, and their correlation.\nJournal
of Nuclear Materials\, 101(1) :172 - 183\, 1981.\n\nhttps://indico.psi.ch/
event/6941/contributions/19112/
LOCATION:PSI Auditorium WHGA/001
URL:https://indico.psi.ch/event/6941/contributions/19112/
END:VEVENT
BEGIN:VEVENT
SUMMARY:3D-reconstruction via genetic algorithms: Application to metallic
fuel
DTSTART;VALUE=DATE-TIME:20191106T100000Z
DTEND;VALUE=DATE-TIME:20191106T102000Z
DTSTAMP;VALUE=DATE-TIME:20201125T203950Z
UID:indico-contribution-4352-19141@indico.psi.ch
DESCRIPTION:Speakers: Riccardo Genoni (Politecnico di Milano)\nInferring 3
D features of heterogeneous materials (e.g.\, solid materials with gaseous
porosity) from 2D sections is the subject of stereology. This inference p
rocess is critical for the determination of various and fundamental proper
ties of the 3D medium\, such as the pore size distribution and the percola
tion thresholds. Stereological approaches have been tailored to different
materials/properties\, but lack of generality and depend on very strong hy
potheses. In the attempt to overcome these limitations and allowing to app
roach a broader range of material/properties\, in this work we present a 3
D reconstruction technique leveraging a genetic algorithm. The 3D-reconstr
uction problem is treated as an optimization problem: a population of virt
ual 3D cubes – embodying a certain void fraction – is randomly generat
ed and selected by comparing cubes sections with a reference 2D image in t
erms of specific optimization metrics (e.g.\, the number of pores and the
pore average radius). Once the population of 3D cubes is optimized by the
genetic algorithm (i.e.\, it is possible to extract from it a population o
f 2D sections comparable with the reference image)\, it can be used to det
ermine correlations between 2D (measurable) quantities and 3D (to be infer
red) quantities. Moreover\, since the outcome of the genetic algorithm is
not a single optimal 3D cube\, but a whole population of cubes\, confidenc
e intervals can be derived for these correlations. The proposed approach i
s very broad and general\, applicable in many specific topics related to n
uclear fuel (e.g.\, high burnup structure\, grain growth\, porosity develo
pment\, and so on). In this work\, we showcase the application of the prop
osed 3D-reconstruction algorithm to a subset of fuel sections of metallic
fuel obtained as part of the post irradiation examination of the FUTURIX c
ampaign\, with focus on the development of porosity. The goal is to demons
trate the applicability of the proposed algorithm to the 3D-reconstruction
of metallic fuel\, in terms of total porosity\, porosity-2 and pore-size
distribution.\n\nhttps://indico.psi.ch/event/6941/contributions/19141/
LOCATION:PSI Auditorium WHGA/001
URL:https://indico.psi.ch/event/6941/contributions/19141/
END:VEVENT
END:VCALENDAR