Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 210

the gas from the open volume within a fuel rod, but also provides data for
the calculation of the net internal void volume and the total quantity of gas
in the open volume. The gas sample is analyzed for composition by tech-
niques such as gas chromatography to identify the concentration of Xe, Kr,
CO-CO 2 , CH 4 and other constituents. The results are adjusted for decay and
combined with calculations or measurements of exposure to establish the
fraction of the generated fi ssion gas that was released to the free volume
within the fuel rod.
Metallographic mounts of the cladding and fuel may be prepared, pol-
ished and photographed. The mounts may be examined for clad oxide layer
(external and internal), fuel/clad interactions, fuel restructuring, rim effect
and agglomerate behaviour. The mounts may also be etched to reveal grain
boundaries for an analysis of the grain size.
Hydrogen/hydride analysis - A hot vacuum extraction process may be
used to determine clad hydrogen content such as that employed by the
LECO method. (LECO is a corporation which provides instrumentation
for elemental determination in organic and inorganic materials.) When such
analyses are done, it is important to separate the total hydrogen evolved
from the sample into concentrations from the surface layer(s) and from the
metal itself (which is of interest). Specifi cally for samples with thick oxides,
water molecules adsorbed at the cracked and porous zirconium oxide may
contribute to the recorded hydrogen content and give a metal hydrogen
content that is too large. It is therefore crucial that the adsorbed water mol-
ecules are removed before the hydrogen content in the sample is measured.
This can be done by grinding of the oxide. but in this case care is needed
so as not to remove massive hydrides at the outer metal surface of a fuel
rod. For components without a surface heat fl ux there is no concentration
of hydrides at the outer metal surface and there is lower risk of removing
hydrides from the sample by grinding.
In addition to gas-extraction methods, detailed destructive post-irradia-
tion examination (PIE) can be used to characterize the local oxide thick-
ness and hydrogen concentration. Backscattered electron imaging (BEI) of
polished cladding in the SEM may be used together with image analysis
to determine the local hydrogen concentration and radial hydrogen profi le
through the cladding wall. This is a relatively new application of BEI which
allows the relative positions of hydrides and oxides to be located more pre-
cisely than the gas-extraction methods.
Alternatively, the hydride distribution and orientation in the cladding
material is determined by optical examination after etching the sample to
reveal the hydride locations.
Crud analysis - The morphology of the crud may be examined by electron
probe microanalysis (EPMA).

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