Page 243 - Fundamentals of Magnetic Thermonuclear Reactor Design

224 Fundamentals of Magnetic Thermonuclear Reactor Design

7.2.3.5 Armour Erosion Lifetime
The longevity of ITER’s most heat loaded in-chamber components is limited
by the armour erosion. Tiles grow thinner in the course of reactor operation and
finally become unable to protect the heat sink panel. When calculating the thin-
ning rate, the following parameters should be accounted for:

l thermal erosion due to surface overheating during transient events
l sputtering
l in-chamber transfer of eroded particles

The physical mechanisms of thermal erosion are quite diverse. The list in-
cludes, first, evaporation.
The total amount of evaporated matter depends on the surface temperature
and duration of the surface heated state, as well as on the pressure of saturated
vapours in the ‘working’ temperature range. The heat load baseline values in
Table 7.1 are not enough to estimate the heating kinetics of evaporating sur-
faces. In processes involving fast energy exchange, (e.g. a current disruption) a
plasma cloud of evaporated material appearing near the wall (with a pressure of
up to several MPa) helps shield the surface. The result is a many-fold (10× and
more!) reduction of the heat load on the wall and a lower evaporation rate. It
is difficult to analyse this phenomenon qualitatively, and experimental data are
needed to verify relevant computational simulations.
Second, heat acting on graphite and similar materials may give rise to mac-
roscopic erosion, when grains and even pieces are emitted from the material.
Mass lost due to macroerosion is estimated at 10–100 µm per each current
disruption event. As several hundred current disruptions are projected to occur
during ITER’s operation, the evaporated layer may become thinner by tens of
millimetres.
Third, the surface melting and the melted layer removal by electromagnetic
forces, the plasma ‘wind’ and the MHD-instabilities in a liquid-metal film may
contribute much to thermal erosion. The fourth physical mechanism of erosion,
ion sputtering, may be regarded as the most significant. For example, a graphite
armour layer lost due to sputtering during ITER operation is estimated to be up
to 10 m (!?) thick.
However, these estimates ignore one notable factor. Atoms leaving the wall
surface as a result of sputtering get ionised and are deposited back to the wall
(the redeposition process). The ion sputtering rate can be estimated realisti-
cally with sophisticated code packages for a 3D kinetic modelling of sputtered
particle transport, such as the REDEP [5]. The use of these better precision
tools results in 10× to 100× lower sputtering rate in regions subject to the
highest loads.
Thus, the maximum theoretical value of a sputtered off graphite wall layer
2
3
can be conservatively put at 10 –10 mm. With erosion being the prevailing
factor determining the FW’s lifetime, the tiles should be as thick as practically
feasible, the only constraint being the material’s temperature limit.