Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 365

while it increases with fl uence thereby resulting in small yield points at high
fl uences (Fig. 1.14a). It is believed that at high fl uences saturation of radia-
tion hardening occurs resulting in deviations from the square-root depen-
dence of the hardening on the fl uence.
Since the Luders strain in steels varies linearly with yield stress, it increased
as cube-root of fl uence (Fig. 1.15) where we note that the datum point at the
highest neutron dose of 1.4 10 19 n/cm 2 is an extrapolation from high tem-

peratures to ambient. Thus, at room temperature the highly irradiated mate-
rial exhibited severe localized deformation and failed during Luders band
propagation itself before reaching the strain-hardening regime. The increase
in Luders strain and the decrease in source hardening, subsequent to irra-
diation, imply that the work hardening should decrease as neutron dose
increases. Indeed, the work-hardening exponent decreased from ~0.34 for
the unirradiated mild steel to ~0.19 at a neutron fl uence of 2

×

The fact that the source hardening in BCC metals such as steels decreases
on exposure to neutron irradiation implies that the concentration of inter-
stitial C and N in solution decreases with increased neutron fl uence. Murty 24
examined the effect of incremental neutron dose on static strain ageing
kinetics and demonstrated that the ageing kinetics are slowed and that fl u-
ences greater than 10 18 n/cm 2 rendered the steel non-ageing. In a correla-
tion between the effects of neutron irradiation and dry hydrogen treatment,
Murty and Charit 25 demonstrated that the concentration of nitrogen in solu-
tion decreases with neutron fl uence, reaching a value very close to zero at
10 18 n/cm 2 (Fig. 1.17). These results imply that interstitial impurities com-
bine with radiation-induced point defects such as vacancies and interstitials,
either with individual defects or loops, to form complexes. These complexes
are probably responsible for part of the increase in friction hardening and
the corresponding decrease in solution hardening. McLennan and Hall 26
found from internal friction experiments that the concentration of C in
solution decreased by a factor of four in steels after irradiation to about
10 19 n/cm 2 .
This is also the reason for the decrease in the intensity of dynamic strain
ageing (DSA) in annealed mild steel, as depicted in Fig. 1.18a-1.18e, where
the load drops in the stress-strain curves decreased with increase in radi-
ation fl uence, fi nally rendering the steel non-ageing after irradiation at
10 19 n/cm 2 . 27 It must be noted here that though radiation exposure results in
reduced concentration of interstitial C and N in solution leading to reduced
blue brittleness , radiation hardening and embrittlement can still occur. Thus
the competing and synergistic effects of DSA and neutron irradiation could
lead to increased ductility along with increased strength at appropriate
temperature and strain-rates. Comparison of stress-strain for unirradiated
material (~100°C) with those irradiated to different doses clearly reveals
( Fig. 1.19 ) 28 the typical embrittlement due to DSA in the unirradiated

× 10 18 n/cm 2 . 23

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