Page 239 - Fundamentals of Magnetic Thermonuclear Reactor Design

220 Fundamentals of Magnetic Thermonuclear Reactor Design

for at later stages. The second step to be made at this stage is to better define the
FW layout, arrangement and acceptable size.
It is desirable to increase the FW surface to decrease the specific heat load.
However, this desire contradicts the pursuit of the best possible technical and eco-
nomic characteristics of the reactor as a whole. The space extending radially from
the plasma core near the equatorial plane is particularly insufficient, and it may be
feasible to place the energy receiving part above and/or under the plasma column.
Placing the targets further away from the X-point may help reduce the heat
loads and slow down the ion sputtering. The seemingly best solution to remove
the targets out of the magnet system is unviable due to the divertor’s poloidal
configuration. Probably, this could be achieved with a bundle divertor. However,
the use of the latter would give rise to new intricate problems.
Issues related to the replacement of the FW components should also be
considered in the FW’s design. For example, the bulky limiter panels in the
equatorial region can reasonably be placed outside the plasma column where
the widest vacuum ports are located.
The third step is to assess the consistency of the selected FW option with the
goal of achieving the highest possible economic performance and energy effective-
ness. A reactor’s economic performance is measured by its availability factor, de-
pending on the duration of outages needed for the replacement of in-chamber com-
ponents (ICCs) in accordance with the schedule. The experimental ITER machine
provides for such an opportunity. However, a demonstration reactor design does
not necessarily allow periodic and even one-off replacements of operating parts to
be made, and the divertor concept is likely to be substantially different from that for
the ITER. The availability factor also affects the susceptibility to radiation damage,
determining the range of candidate structural and functional materials.
The energy conversion efficiency depends mostly on the coolant. This is why
high-temperature coolants, including helium and liquid metals, are preferred.
At the end of the initial design stage we determine the following:
l ICCs’ relative positions and limit sizes
l maximum loads to which ICCs are exposed under quasi-stationary operating
conditions
l frequency and method of ICCs replacement
l most suitable coolant
l candidate materials and their neutron irradiation doses

7.2.3 Estimation of the Engineering and Physical Characteristics
of the First-Wall Components
The purpose of the second design stage is to define the FW’s compositional
structure. To this end, one has to
l estimate heat loads on different components,
l define coolant’s parameters,