Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

The material had been supplied by CEA to SCIP

II. The campaigns were run without iodine added

and completed using 3D-profilometry post-test

characterization and microscopy cross-sections of

select samples. The results obtained were used for

comparisons with the results from the modelling

team. In order to study ceramic geometry effects

on strain or time to failure further tests performed

with iodine have been suggested.

Subtask 2.2.1 FE modelling of the

mandrel test

Estimations of the local strains and stresses

induced during the strain-rate controlled mandrel

tests using ceramic inserts simulating different

crack patterns of the fuel pellet are performed

using Finite Element (FE) modelling. The FE modelling

is performed by CEA using a two-dimensional

simulation (in the radial-tangential plane of the

cladding) with the CAST3M FE code. The simulation

takes into account the friction between the

ceramic sheath and the cladding and a viscoplastic

behaviour of the cladding. The modelling results

will be validated using the experimental results.

During the year CEA has performed simulations

using two different notch patterns (4 or 16

notches).

Subtask 2.2.2 EBSD analysis (tentative) of

the mandrel tested samples

Subsequent to mandrel testing, four to five unirradiated

specimens will be characterized using the

Electron Backscatter Diffraction (EBSD) technique

in a Scanning Electron Micro-scope (SEM). Using

EBSD, it is possible to determine the degree of lattice

rotation in individual grains as a function of

macroscopic strain.The mandrel testing equipment

will be used in order for the specimens to obtain

different macroscopic strains (0–2.0%).

During the second year, evaluation of the EBSD

method was performed to investigate its possible

application for mandrel tested samples. The first

test has been performed on CW SR Zircaloy-4

material, which will also be used in the modelling

task. The EBSD technique can reveal misorientations

inside a grain which could be a consequence

of the obtained strain and the grain-versus-stress

orientation (i.e. dislocation activity and slip planes).

An average of the crystal rotations in the mapping

can be correlated to the applied strain.

The EBSD tests on CW SRA Zircaloy-4 cladding

material indicated that the microstructure was

not suitable for EBSD due to the deformed microstructure

from the fabrication of CW material. This

results in very poor contrast in the EBSD evaluation.

RXA Zircaloy-2 material was hence chosen as a

replacement material for this purpose. The Zry-2

material was tested successfully. The initial test

was made on an undeformed specimen and good

qual-ity of the EBSD measurements was confirmed

with a 90% accuracy.

Subtask 2.3 The effect of operational

parameters

The main objective of this subtask is to study the

effect of different ramp types on PCI (specifically

Iodine Induced Stress Corrosion Cracking, ISCC).

An outline for the first mandrel test in this subtask

was presented at the SCIP II May 2011 meeting.

The first test has been designed to simulate a ramp

tested rod that failed after a six step (1h hold time)

ramp test close to the test rod peak power position

(55 kW/m). The mandrel test will use one out

of three samples available from the father rod and

the test will consist of six 0.23% strain steps up to

a total strain of 1.4%, with a corresponding stepwise

increase in temperature at each step, simulating

the cladding inside temperature during respective

ramp step. Less than 60 Pa iodine pressure

will be used. Reference tests were performed to

evaluate the experimental capabilities of the mandrel

test equipment to perform multi-step ramps in

temperature and strain.

Subtask 2.4 The effect of burnup on the

PCI behaviour

The main objective of this subtask is to obtain a

better understanding of the effect of burn-up level

on PCI induced failures. It is the aim to identify

the changes in critical parameters with increasing

burn-up levels. The observed changes will then be

used as a base to identify the effect of burn-up on

the PCI behaviour. A second objective is to study

simultaneous PCI and DHC failures.

The investigation of the inner hardened layer has

started with the Ga sample after ramp test. The

objective is to study the formation of the recoil

hardened inner layer of the cladding or liner and

the formation of incipient cracks at the inner cladding/liner

surface. The Vickers micro-hardness

investigations of the Zry-2 cladding and Fe-alloyed

liner of rod Ga after ramp test show that a linear

decrease of the hardness of the liner from the surface

(fuel side) is observed. The depth of the inner

layer of the liner impacted is estimated to about 8

µm. No incipient cracks could be identified in the

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ENSI Erfahrungs- und Forschungsbericht 2012

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