Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 370

100

50

Serrated
flow

0

300 400

Temperature, K

500 600

10 18 n/cm 2 ) Unirradiated Irradiated (2

1.20 Effect of DSA and neutron on the temperature variation of energy
to fracture (J). 29

and DSA could lead to benefi cial effects on strength and ductility in certain
temperature and strain-rate regimes. 29 While these descriptions are limited
to mild steels, such DSA and neutron radiation effects are also observed
in steels used for nuclear reactor pressure boundary applications; we will
discuss these in the next section under the radiation effects on radiation
embrittlement of nuclear RPVs and support structures.
Kass and Murty 30 have successfully used the Hall-Petch relation and fric-
tion/source hardening concepts to explain the infl uence of fast and thermal
neutrons, in the total neutron spectrum, on the grain size effects in pure iron
and low alloy steels. They evaluated the effects of total and fast neutron
spectra by irradiating samples with and without Cd-wrapping thereby elim-
inating low energy (<~0.5eV) neutrons in the Cd-wrapped samples. The bar
chart in Fig. 1.21 shows the effect of fast and total neutron radiation on the
yield stress for Armco-iron and steels (1020, A516 and A588). 30 All steels
exhibited increased hardening due to the total neutron radiation exposure
compared to only fast neutrons whereas in pure iron we note that as grain
size decreases from 190 to 50

￿ ￿ ￿ ￿ ￿ ￿

m, exposure to the total neutron spectrum
resulted in lower radiation hardening than only fast fl ux. This is explained
on the basis of the Hall-Petch plot where the y-axis intercept represent-
ing friction hardening increases with neutron radiation exposure while the
slope decreases following fast neutrons. Exposure to the total neutron spec-
trum with additional low energy neutrons would result in a further slight
increase in friction hardening (or y-axis intercept) accompanied by a slight
further decrease in the slope (i.e. decrease in the source hardening). This
implies that the two lines (Fig. 1.22) representing the effects of fast and
total (fast + low energy) fl uences will cross over at a critical grain size below

μ