Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 396

dose level of 120 dpa indicate that SA 304L creeps faster than CW 316. 104
These results suggest that the correct grade of steel and optimum metal-
lurgical conditions can reduce the creep rate and prolong the life of these
components.

1.5 Conclusions

Structural components in NPPs undergo ageing with continuous opera-
tion and eventually reach the end of life. The rate of degradation depends
on their inherent ability to withstand the stress, temperature and service
environment. To get the best potential from a material the acumen of the
designer, the alacrity of the operator and dexterity of the surveillance per-
sonnel should play a non-compromising role. The engineering structures in
a NPP can be broadly classifi ed into two categories: (i) the components of
steam generators, turbines, etc., which experience thermal and mechanical
environment and (ii) in-pile components such as fuel clad, reactor pressure
vessel, etc., which are subjected to an added condition of intense neutron
irradiation. Materials in both categories also have to face high-temperature
fl owing water, the energy transfer medium, which corrodes/erodes the mate-
rial. The feedback data on the performance of materials in these environ-
ments help material scientists to modify the materials and to manoeuvre
their properties to perform better. This closed cycle needs to be kept active
to meet the required technological advancements.
The properties of materials used in LWR power plants are evaluated for
the service they have to render: a fl uctuating load requires material with
good fatigue strength, constant pressure at elevated temperature demands
good creep strength and stress relaxation, good toughness is needed even
after prolonged neutron irradiation, low tendency to absorb hydrogen so as
to minimize hydrogen related problems, etc. It is diffi cult to have one mate-
rial endowed with all these properties and hence more than a dozen materi-
als are used inside a reactor - these need to be joined in some way and this
adds to corrosion-related problems.
The elastic and plastic deformations of a material, whilst obeying a
generic relationship, will show a marginal difference in their properties
because of its metallurgical condition. The constants used in these rela-
tionships are material- or microstructure-specifi c. Despite such variability
it is still possible to isolate a material with the required microstructure to
serve under specifi ed environmental conditions, and above all, for a known
life expectancy. An indication of the crack length in a material helps to
keep a check on its degradation if its fracture toughness property is known.
Charpy impact tests provide an easier alternative to LEFM tests and are
used to grade the deterioration of the material. In situations where the
initial toughness of a material is unknown for comparison, the master

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