Page 415 - Fundamentals of Magnetic Thermonuclear Reactor Design

Structural and Functional Materials Chapter | 13 393

In addition, ITER is not expected to fuse DT fuel until the years 2030s—and the
two decades may provide us with benefits such as a better understanding of the
tritium accumulation phenomenon and methods for controlling it.

13.4 PLASMA-FACING MATERIALS
13.4.1 Beryllium Alloys

Powder metallurgy techniques are used almost exclusively to provide beryl-
lium alloys for nuclear applications, including the DShG and TShG-56 alloys
produced in Russia and the US S65C grade. Apart from beryllium (90%–96%),
they contain BeO, aluminium and titanium. The operating conditions of beryl-
lium cladding tiles in tokamaks are very special. The temperature of their sur-
face layer bonded to a heat-sink substrate is close to 300°C. The key physical
mechanism governing the cladding tile evolution is the low-temperature radia-
tion embrittlement. The surface of plasma-facing tiles is heated to 600–700°C.
At such temperatures, beryllium alloys are mostly subject to swelling and
helium-induced embrittlement.
One and the same structural component undergoes radiative degradation due
to a number of mechanisms. This complicates the attainment of desired durability.
The plasticity of a non-irradiated beryllium is within 5% at temperatures
between 20 and 300°C. A neutron irradiation of up to ∼5 dpa at 300–400°C
results in an insignificant hardening and embrittlement of a specimen [9]. The
hardening increases with radiation loading due to the growing concentration of
dislocation loops. At ∼300°C, beryllium alloys show a classical LTRE, while
their plasticity remains close to 1% at irradiation of up to ∼5 dpa, and no abso-
lute brittle fracture occurs.
Beryllium is a leader among fusion materials by the rate of transmutant
helium accumulation. In the breeder reactor environment, it accumulates 1000×
more He than in austenitic Cr–Ni steel. In the MFR operating conditions, He
accumulation will be even higher: close to 2at.% per 1 dpa, meaning that at a
radiation dose equivalent to 5 dpa, beryllium alloys accumulate around 10% of
helium. This inevitably will lead to an intensive gas porosity at temperatures
between 450 and 600°C, accompanied by a radiation-induced development of
vacancy pores. A microstructural analysis of irradiated specimens proves the
presence of helium bubbles of sizes up to 200 nm. A gas-driven swelling of
beryllium-alloy specimens containing helium bubbles (at 5 dpa) is estimated at
around 4.5%. This effect should be accounted for in the FW design.
Another adverse effect of fast helium accumulation, commonly observed
in a temperature range of 550–800°C, is the helium-induced embrittlement. It
manifests itself in the form of micropores of up to 300 nm, developing at grain
boundaries and filled with helium. In-pore pressure may reach a few tens of
megapascals. In the mechanical strain field, the micropores grow, merge and
become intergranular cracks. This makes plasticity fall to zero and the material
break at stresses lower than the ultimate tensile strength.