Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 339

transfer stages with complex heavy engineering are involved before the fi nal
stage generation of electric energy is achieved. At each stage of energy trans-
fer, the machinery involved undergoes ageing and the material properties
undergo degradation with continuous use. The severity of degradation may
vary from simple and minor to serious and complex. For the core components
of a nuclear reactor in a nuclear power plant (NPP), there is an additional
infl uence of the severe radiation environment that accelerates the ageing.
The types of nuclear reactors vary in their design features according to the
type of fuel and coolant used. The choice of materials for their construction
differs according to the reactor design as well as to previous experience in
operating nuclear reactors. The components in the reactor core must toler-
ate exposure to the coolant media (high-temperature water, liquid metals,
gas or liquid salts), stresses and vibrations as well as an intense fi eld of high
energy neutrons. Ageing of materials under this extreme environment can
lead to reduced performance and, in the worst cases, sudden failure of the
components.
A common consideration given in a power plant design at any installation
(nuclear/thermal) is the safety requirement. The concern for safety increases
as the material properties get degraded from their initial values with pro-
longed exposure to service conditions. Thus, intermittent surveillance cam-
paigns are mandatory in an operating installation for the evaluation of the
health of the components - even if the initial design adhered to strict safety
norms. For this, we must be able to identify the critical components that
can possibly undergo ageing degradation and decide the frequency of the
inspection campaigns. The outcome of such campaigns can forewarn of any
impending failure and suggest replacement of components such that the
designed life of the plant can be reached - and if reached, the campaign can
advise if the life of the component can be extended beyond the design life.
In the worst case, the campaign outcome may suggest shutdown of the plant
if safe continued operation of the component cannot be ensured. The cost of
such campaigns and subsequent component replacements should be recov-
erable by putting the plant back into operation. The following statement
with regard to nuclear installations is pertinent: 1

￿ ￿ ￿ ￿ ￿ ￿

With the present 60-year licenses beginning to expire between the years of
2029 and 2039 for the fi rst group of NPP that came online between 1969 and
1979 utilities are likely to initiate planning of base-load replacement power by
2014 or earlier. If the option to extend current plant lifetimes is not available,
strategic planning and investment required to maintain the current LWR fl eet
may not happen in a sustainable manner. The research window for support-
ing the utility's decisions to invest in lifetime extension and to support NRC
decisions to extend the license must start now and is likely to extend through
the following 20-year period (i.e. 2010 to 2029), with higher intensity for the
fi rst 10 years. The LWR's R&D Program represents the beginning of timely