Page 261 - Fundamentals of Magnetic Thermonuclear Reactor Design

242 Fundamentals of Magnetic Thermonuclear Reactor Design

Similar approaches can be used to intensify heat removal from divertor tar-
gets (Table 7.7).
Reciprocation of a strongly peaked heat flow over a target surface can be
performed in two ways: (1) using movable targets and (2) repositioning the
separatrix through variations of the poloidal field. To achieve an appreciable
positive result, the amplitude of the separatrix and target displacement against
each other must be larger than the heat flow’s azimuthal ‘length’ (0.1–0.3 m),
with the least required displacement frequency close to 0.3 Hz.
The heat load can be dispersed by a hot target, but even at the highest tol-
2
erable temperature of ∼2500°C not more than 2.5 MW/m can be radiated, a
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number negligible compared with the ∼40 MW/m flow incident on the target.
For a flow coming at a small angle of incidence, the power reflection coef-
ficient may reach ∼50%. In divertor targets, the glancing angle of the incident
plasma flow is indeed very small (in the order of several degrees). However,
this reflection coefficient is impossible to achieve in practice. The reason is the
induced electric potential that develops near the surface and makes reflected
ions return to the target.
Therefore, the best method to effectively withstand the severe heat loads
in a stationary environment would be to evaporate the sacrificial layer of the
target. For example, the evaporation of a liquid lithium coating at 1200°C–
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1500°C enables the absorption of a heat flow of up to 50 MW/m . However,
this method has a fundamental weakness: plasma contamination by evaporat-
ing lithium.
Another method for intensifying heat transfer is the cooling mode optimisa-
tion. This can be achieved by improving the coolant’s thermal accumulation
capacity at the wall–coolant interface. Factors to be controlled include the
coolant itself, the coolant mass flow rate, the coolant in-channel warming up,
the channels’ inner microrelief, the flow turbulence degree, and the coolant’s
phase transitions.
Coolants employed in present-day power engineering have different heat
accumulation capacities. For example, water flowing at the highest permissible
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rate of 10–15 m/s can withstand stationary loads of up 40 MW/m . This is close
to loads expected in the MFR, but provides no margin. In addition, thermal
engineering systems using a water coolant have a relatively low heat-to-electric
energy conversion coefficient.
Many commercial cooling systems utilise gaseous helium which, with all its
operational merits, has a very low heat accumulation capacity. At a practically
maximum pressure of 20 MPa and a flow rate of up to 100 m/s, the highest heat
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load on a helium-based cooling system is within 10–15 MW/m .
Liquid metals as a specific class of coolants have important thermal engi-
neering advantages, but when employed in an MFR, they feature limitations
associated with magnetic fields that constrain the permissible pumping speed.
The liquid metals’ highest permissible speed of circulation in a closed-circuit
cooling system is close to 1 m/s, while the largest permissible heat load is