Page 418 - Fundamentals of Magnetic Thermonuclear Reactor Design

396 Fundamentals of Magnetic Thermonuclear Reactor Design

l faster generation of nickel and zinc transmutants leading to thermal conduc-
tivity reduction,
l shift of the swelling temperature peak to lower temperatures,
l strength degradation as a result of dynamic destruction of fine strengthening
particles,
l faster radiation creep at 100–150°C.

13.5.2 Radiation Characteristics of Copper Alloys

Low-temperature radiation embrittlement of an alloy, first showing up at
−3
∼10 dpa, causes the alloy hardening by around 150 MPa, lower plasticity and
unstable plastic deformation. The yield strength increases with the dose grow-
ing ∼0.4 dpa, then stabilises until the dose is up to ∼2.5 dpa, the highest dose
applied experimentally. The uniform elongation strain also falls with radiation
loading to 0.5%–1.0% at ∼0.4 dpa and afterwards remains almost unchanged.
18
−3
The concentration of radiation-induced defects is close to 10 cm at 0.4 dpa
[11]. The total elongation is the only metric that changes monotonously with
dose: for the Cu–Cr–Zr IG and GlidCopAl 25 IG alloys, it decreases to 5%–7%
at a radiation temperature of 150°C and a dose of ∼2.5 dpa.
Other radiation-induced effects, including the swelling and helium embrit-
tlement show up at high doses of two to 10 dpa. A microstructural analysis of
irradiated specimens reveals small groups of high-density defect complexes like
SFT (stacking fault tetrahedra) and dislocation loops (interstitial type).
Swelling of copper alloys was studied during experiments on the SM-2
mixed-spectrum reactor at a radiation temperature of 300°C, corresponding to
the swelling peak, and on the BOR-60 fast experimental reactor at a radiation
temperature of 340°C. The following experimental results were obtained.
Pure copper exposed up to ∼10 dpa fast neutron irradiation features a swell-
ing close to 3%. The swelling of high-strength copper alloys at similar radia-
tion conditions is within 0.2%. However, the behaviour of the Cu–Cr–Zr IG
alloy is different from that of the GlidCopAl 25 IG alloy under the action of
a neutron flux with a mixed energy spectrum. The GlidCopAl 25 IG alloy is
prone to an appreciable void swelling at a dose range of 0.4–2.5 dpa, like that
typical of pure copper (∼1% per 1 dpa). For instance, at a dose of 0.6 dpa, the
alloy vacancy swelling is ∼0.4%. The intensive development of vacancy pores
is believed to be due to accumulation of transmutated helium. The latter is a
product of a nuclear reaction involving boron (∼200 appm), which is added to
alloys for deoxidation purposes.
A microstructure analysis of an irradiated Cu–Cr–Zr IG alloy with no boron
additions has shown only the presence of dislocation loops at the same radiation
conditions.

13.5.2.1 Transmutation Processes
The concentration of impurities in irradiated materials tends to increase due to
the accumulation of solid and gaseous transmutation products under the action