The divertor presented a much more substantial challenge to the materials because the heat fluxes are about an order of magnitude larger. Fortunately, the divertor is a relatively small component as compared with the first wall. Higher coolant velocities, internal fins, and other heat transfer enhancements that would require unacceptable pumping power in the first wall were employed here to raise the heat transfer coefficients. Their values are listed in Table 2. Further, much shorter modules might be used so that free-bending conditions could be approached.
However, the vanadium alloy divertor will not require a special tube
design. As Figure 5a
indicates, wall thicknesses up to
1.4 mm are acceptable under no-bending conditions.
Unfortunately, even with a five-fold increase in heat transfer
coefficient (3000 to 15000 W/mK) and completely unconstraining
supports, the SiC composite design fails to tolerate the divertor's
heat flux at any wall thickness. See Figure 5b.
For free-bending boundary conditions, the curve reaches
a maximum at 2.75 MW/m
.
An improvement of 19%in the permissible heat flux is needed
(in addition to some kind of erosion protection).
The largest thermal stresses in the divertor are a
strong function of both the heat transfer coefficient
to the coolant and the thermal conductivity. As these
parameters are increased, the maximum stresses are reduced.
Figure 6 illustrates the increases in these parameters needed
to make a viable divertor (assuming a 1 mm thick tube wall).
The analysis shows that further improvements in the heat transfer
coefficient will not improve the design unless the thermal
conductivity can also be increased. In fact, the thermal
conductivity must be at least 25 W/mK for a free-bending
design to become acceptable. As Figure 6 shows, a design constrained
from bending would require drastic improvements in these parameters;
for a successful SiC composite divertor, short tube lengths and
compliant headers are imperative.
