Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 340

collaborative research needed to retain the existing nuclear power infrastruc-
ture of the United States.

Our understanding of the behaviour of the service material in that envi-
ronment is based on years of operating experience of a reactor. Sustained
research and development is required to develop newer materials. Further,
it is from the examination of the ageing/aged materials we learn the role of
new environmental parameters that were unthought-of during the design
stage, and allow us to modify our safety codes in future designs. Specifi c
ageing and degradation mechanism depends on the component in question
and the various conditions such as temperature, load and environment to
which the materials are exposed. A typical NPP can be considered to con-
sist of seven different components: (i) fuel, (ii) structural components, (iii)
moderator/refl ector, (iv) control, (v) coolant, (vi) shields and (vii) safety
systems. Each of these components has specifi c requirements and selection
criteria based on which suitable and economic materials are chosen. 2
Fission based nuclear reactors can be classifi ed as thermal and fast, based
on the energy of the neutrons and the thermal reactors can further be cat-
egorized as boiling water reactor (BWR) and pressurized water reactor
(PWR). The latter type can further be classifi ed as light water cooled and
heavy water cooled. We will confi ne ourselves here to the LWRs that use the
steam-cycle conversion system wherein the steam produced by nuclear fi s-
sion drives a conventional turbine generator to produce electricity. A steam
generator is used in PWRs to produce steam while the direct cycle BWRs
generate steam in the reactor core thereby not requiring a separate steam
generator; Fig. 1.1a and 1.1b are schematics of PWR and BWR, respectively,
with important structural components indicated. 3
In a typical PWR which uses ceramic fuel, the fuel is separated from the
coolant by a physical barrier that prevents their direct contact. The barrier,
called the clad, has adequate thermal conductivity to transfer the fi ssion-
heat to the coolant and has a low thermal neutron absorption characteris-
tic to allow fi ssion neutrons to sustain a chain reaction. The cladded fuel is
immersed in a pressurized pool of coolant fl owing at an average temper-
ature of ~300°C under a pressure of around 16 MPa thus preventing the
water from boiling. Two separate water systems, the primary and secondary,
are contained in the steam generator which is a heat exchanger consist-
ing of a large number (~3000) of nickel-based super alloy tubes in a large
steel shell. Depending on the vendor, PWRs may have two, three or four
loops with respective coolant circuits, each with its own steam generator.
In BWRs, on the other hand, water is circulated through the reactor core
producing saturated steam that runs the turbine generator. Nuclear reaction
in BWRs is controlled using steel-clad boron carbide control rods that are
inserted from the bottom of the core while control rod cluster assemblies

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