Environmental Engineering Reference
In-Depth Information
The wet, in-core or in-mast sipping methods are used almost exclusively
in power reactors because of their speed and the need for short refuelling
outages. Vacuum or similar wet canister sipping is now used to confi rm or
resolve the results of the in-reactor methods.
Ultrasonic testing
Ultrasonic testing (UT) is being used to detect leaking BWR and PWR fuel
rods. The testing method makes use of differences in scattering by water
and gas of ultrasonic (pressure) waves as they are refl ected between the
inner and outer surfaces of fuel cladding. The UT process makes use of
two probes, which move laterally across a FA. The probes are arranged in a
“ pitch-catch ” confi guration in which one probe transmits a signal, which is
picked up by the cladding, refl ected between the inner and outer surfaces
and ultimately received by a second probe. The scattering by water at the
inner surface of a failed rod is greater than the scattering by gas at the inner
surface of a sound rod. Such differences are used to identify the specifi c rod
or rods that are leaking.
Eddy current testing
The zirconium oxide thickness can be measured by the eddy current (EC)
technique. However, before measuring the fuel rod oxide thickness it is
important to remove as much crud from the rods as possible. This may be
done by brushing the rod before the oxide is measured by the EC probe.
If crud is not removed, the oxide thickness value normally obtained is too
large. In many cases the crud deposited onto the fuel rods may be ferro-
magnetic and therefore the EC technique may fail to give reliable results
unless the EC equipment has been designed to compensate for ferromag-
netic crud. Eddy Current technique can also be used to detect fuel claddings
defects, for example non-penetrating cracks.
Visual examinations
Visual examination of the fuel assembly is used to:
Get a general impression of the condition of the fuel assembly.
Assess zirconium oxide type and whether oxide spallation has occurred
or not.
Identify the primary failure cause and characterize degradation of failed
fuel.
Measure component dimensions by means of translation stages and
position encoders.
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