Environmental Engineering Reference
In-Depth Information
than 7-10%. Therefore, it was concluded that observed cracks could not
signifi cantly propagate in the bulk of the plate, where the deformation is
low, even after 60 years. 19
2.3.3 Pressurizers
Since 1997, cracking incidences of pressurizer heaters were encountered
in French PWRs. 20 , 21 The heater sheath (outer diameter of 22 mm, 2 mm
thick) is made of 316 L stainless steel. The lower part of the heater is
attached to electrical connectors, out of the pressurizer, while the upper
part, introduced into the pressurizer, consists of a coaxial heating element
coiled round a copper mandrel. After the fi nal assembly of the heater, the
sheath is cold swaged to reduce the gaps between the heating element and
the sheath and therefore, to improve the thermal exchanges. Under operat-
ing conditions, the heaters are exposed to hydrogenated and non-polluted
primary environment at 345°C. Nevertheless, the temperature at the outer
surface of the sheath, in nominal condition, could reach 360°C. Nine cases
of SCC were found after 12 destructive examinations out of the 1200 fail-
ures observed on heaters. When SCC leads to primary leakage (boron
traces on connectors), the heater has to be replaced no later than the next
outage. Elemental analyses (energy-dispersive X-ray spectroscopy) did not
reveal any trace of pollutant at the surface of the retired heaters. Therefore,
it was concluded that SCC occurred in the nominal hydrogenated primary
environment. No chromium depletion was present at the grain boundaries
of the stainless steel. Such depletion can result in the precipitation of chro-
mium carbides at grain boundaries and promotes IGSCC in an oxidizing
environment.
A surface annealing heat treatment was developed (induction heating)
to counteract the initiation of SCC on the original cold-worked outer layer
exposed to the primary water. The goal of the heat treatment is to anneal
the surface of the material, decreasing strain-hardening and residual stresses
without any damage of the electrical properties of the heater element. As a
result, the Vickers micro hardness decreased from 320 HV 1 (higher than the
threshold necessary to initiate SCC) 22 down to 200 HV 1 (below the threshold
necessary to initiate SCC) at the surface of heat treated heaters, and resid-
ual stresses were removed as shown by corrosion tests in MgCl 2 medium.
￿ ￿ ￿ ￿ ￿ ￿
2.4 Conclusion
Despite the original stringent selection of the materials used to manufac-
ture the components, uniform and localized corrosion occurs in PWR envi-
ronments. Remedies can be of different types: adjusting the water chemistry,
reducing superfi cial strains and stresses, replacing materials or changing
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