Page 234 - Fundamentals of Magnetic Thermonuclear Reactor Design

First Wall Components Chapter | 7 215

The said integrated criterion entails a number of meaningful requirements
for the FW materials and structures. These include ‘inertness’ with respect to
plasma and the small inventory/capacitance with respect to fuel. Each of the
said properties refers to a wide range of physical and chemical phenomena, and
therefore, maybe a brief comment would be worthwhile.
The FW ‘inertness’ refers to the FW’s physical and technological ability to
introduce as little as possible impurity species into the plasma. Impurities in
fusion plasmas have detrimental effects, the most important being the increase
in energy losses and the reduction of produced fusion power (Section 6.2). To
ensure a desired inertness to FW, it is necessary to establish the physical, techno-
logical and temperature conditions allowing a decrease in ion- and heat-induced
erosion and use wall coating/armour containing low-Z elements, such as Li, Be,
B and C. In this respect, the use of tungsten (W) for ITER divertor targets is ques-
tionable. While being sputtering resistant, W has a high atomic number, which
makes the ultimate effect of its use ambiguous. The parameter used to evaluate
the inertness is ρ ii 2 i ρiZi2
Z (where ρ is the coefficient of sputtering of an armour with
a Z atomic number by plasma ions/atoms), which has to be minimised.
i
The FW fuel ‘capacitance’ problem is a difficult one and has not been solved
satisfactorily yet. The tricky aspect is the H isotope accumulation in the FW near-
surface layer as a result of sorption and implantation of incident ions. This prob-
lem is particularly acute with respect to the radiotoxic and very expensive tritium.
In short-cycle experimental devices, the sorption and diffusion processes
may directly affect plasma’s behaviour. The sorption of fuel onto the ‘clean’
wall depletes the plasma during a discharge, while desorption and an abrupt H
ejection from a gas-saturated wall may lead to a loss of stability.
In long-cycle reactors, two processes take place at a time: (1) the fuel mix
external ejection and (2) uncontrolled sorption onto a continuously regenerated
getter film, deposited on the wall through limiter sputtering. These two pro-
cesses determine the fuel mix current balance.
Tritium accumulation in wall materials, meaning a withdrawal from the fuel
cycle of more than 1 kg of tritium—a pretty large amount by fusion technol-
ogy standards—is an issue of critical importance to the MFR. This factor is
detrimental to the reactor’s technical and economic metrics and increases the
operational risk. It is this risk that constrains the use of the graphite armour,
despite the material’s low atomic number, heat resistivity and technological ef-
fectiveness. The reason is the very high sorption activity of carbon films freshly
deposited on the walls during reactor operation.
One possible solution to this problem is the operation mode optimisa-
tion and transition to new materials (Chapter 13). Although the technological
achievements of the past few decades are indisputable, the first-wall problem
is still there. The weaknesses that remain are the limiters and the targets’ short
durability and the need for their replacement, tritium accumulation in the near-
surface and the re-deposited layers, and the high cost of the FW components.
Therefore, the evolution of technological and physical solutions is an inevitable