Environmental Engineering Reference
In-Depth Information
observed that the swelling is maximal at an intermediate dislocation density. At
low dislocation densities there are few preferential sinks for interstitials, whereas
high dislocation densities provide so many sinks for vacancies that the vacancy
supersaturation remains too low for void nucleation and growth. At higher tem-
peratures, the recovery of cold work is responsible for the loss of swelling inhibi-
tion.
Since grain boundaries act as sinks for point defects, vacancy concentration
remains low at grain interiors if the grain size is sufficiently small and void nucle-
ation is hindered. It has been demonstrated that the swelling 0.45-
m grain size
stabilized by the addition of a fine precipitate of Al
2
O
3
was an order of magnitude
less than at 3.77
µ
m grain size [14].
Major and minor alloying additions to the base metal have been found to
counteract void swelling and this method has been accepted as an effective means
to increase the swelling resistance of practical alloys. In Fe-Ni-Cr alloys, the void
swelling is inversely proportional to their nickel content (Fig. 9.4). The swelling
increases rapidly with increasing chromium. The lowest swelling base composi-
tions are at low Cr and in the vicinity of 50 wt % Ni. A nimonic alloy, PE-16,
has been found to be more resistance to swelling than the austenitic stainless
steels. The swelling resistance of this alloys is attributed to very fine coherent
precipitates of Ni
3
(Al, Ti), which prevent dislocation climb and also prevent dis-
locations from continuing to operate as preferential sinks. Minor additions of Si,
Ti, Zn, and Nb have been found to appreciably reduce the peak swelling in stain-
less steels. Ferritic steels are more resistant to swelling than austenitic steels, but
the peak swelling temperature is considerably lower than for austenitic steels
with the same chromium content.
µ
9.5 RADIATION-ENHANCED CREEP
Creep is the deformation of a metal or alloy under sustained load that is normally
noticeable at temperatures above about
T
m
/2 where
T
m
is the melting temperature
of the metal or alloy in degrees Kelvin. Typical operating conditions in nuclear
reactors produce very low thermally induced creep rates, but the effect of the
damaging flux of particles is to greatly increase the creep rate of the material. This
phenomenon is called
radiation-enhanced creep
,
irradiation creep
, or simply
radiation creep
. Radiation-enhanced creep may be defined as the component of
the measured in-pile deformation resulting exclusively from the interaction of
various creep mechanisms that become operative when the material is under irra-
diation.
9.5.1 Characteristics
The radiation-enhanced creep is known to occur at relatively low temperatures
(
T
m
/3) where the thermal creep is generally negligibly small. Like thermal
