Page 256 - Fundamentals of Magnetic Thermonuclear Reactor Design

First Wall Components Chapter | 7 237

TABLE 7.5 Tokamak Divertor Operating Conditions (Project Evaluations)
Reactor
Parameter ITER DEMO
Heat flow (MW) ∼100 >300
2
Heat flow density (MW/m )
On sloped target 20 ∼60
In divertor plasma, max ∼500 ∼2000
Pulse length(s) 400 >10 4
Number of pulses 310 4 ∼10 4
Plasma total burn time during operation cycle(s) ∼10 7 >10 8
Plasma relative burn time during operation cycle (%) 4 up to 40
3
2
Number of current disruptions 3 × 10 (10%) ≤10 (1%)
Expected number of target replacements ≥ 5 up to 2
Radiation-induced material degradation over reactor ∼1 ≥30
operation time (dpa)
Dust generation (kg) 100 >1000
Materials relative activation over reactor operation time 1 >30
3
Helium evacuation flow (Pa m /s) 0.7 >2.0

longer. Given these data, the theoretical erosion lifetime of DEMO ‘classical’
divertor targets will be at least 10× lower, by optimistic estimates. As the target-
replacement outages are bound to affect DEMO’s economic performance, the
unambiguous conclusion is that the conventional FW in the form of a static
water-cooled configuration with periodically replaceable elements will not do
the trick in the next-generation reactors.
A new conceptual approach is needed to achieve a few orders of magnitude in-
crease in the divertor target lifetime and improve the heat sinking capability. Also,
it is necessary to increase the coolant’s temperature, helium evacuation efficiency,
the components’ radiation resistance, and radically decrease dust generation.

7.4.2 Possible Engineering and Physical Solutions
Let us first consider the erosion lifetime issue. There may be three possible ways
to address it: (1) suppress erosion (rate) by physical and technological methods,
(2) build up the sacrificial material initial mass and (3) apply additional sacrifi-
cial layer during reactor operation (Table 7.6).
One well-established way to increase the erosion lifetime is to use an armour
material with the lowest possible sputtering coefficient, such as tungsten, and