Page 185 - Fundamentals of Magnetic Thermonuclear Reactor Design

166 Fundamentals of Magnetic Thermonuclear Reactor Design

peratures needed to cool down the coil windings may vary from 3.7 K to 4.3 K
depending on operational plasma scenarios.

A.5.1.3.1 Toroidal Field Magnet Model

A full-scale thermal–hydraulic model of one TF coil is composed of around
1000 basic models to reflect the coil configuration and hydraulic parameters in
a realistic way. The model describes the winding pack, the coil case, the helium
supply/return pipes, long cryogenic pipelines, various valves, quench lines, a
circulation pump, a cold compressor, two LHe baths, two SHe heat exchangers
immersed in the LHe baths and a vapor helium heat exchanger.
A TF coil winding consists of seven DP each having CIC-type conductor lay-
ers embedded in a steel plate. The winding pack is inside a steel case. Every CICC
in the DP is modelled separately, taking into account its length and transverse
mass and heat transfer of helium in the bundle area and in the central spiral.
The quasi-3D TF model allows accurate calculation of heat fluxes coming to
helium in the cooling channels of the winding pack from the steel structures (the
radial plates and the coil case). A 2D non-stationary heat conduction problem
is solved over 32 cross-sections, equally spaced in the poloidal direction. The
finite-difference approximation is applied using space and time discretisation. A
rectangular mesh is generated over every section. The temperature is calculated
at every node of the mesh. The number of nodes along the X and Y axes is the
same for all sections to ensure topological uniformity.
As an illustrative example, Fig. A.5.9 shows the meshing of a D-shaped TF
coil’s median cross-section. The mesh has a total of 1.7 million nodes for 32
sections.
Every cross-section is divided into 160 subdomains associated with differ-
ent structural materials, boundary conditions and heat deposited from various
sources including nuclear heat and eddy current losses. Head load density is
allocated individually for each subdomain and is time-variable. When a normal
tokamak operation is simulated, heat loads vary cyclically with time, with a rep-
etition period depending on a given plasma burn scenario. To model the plasma
current disruption or fast energy discharge, the normal heat loads are replaced
by heat loads typical for such scenarios. In addition to the conductive heat from
the coil structures and the case, SC strands contribute to heat loads due to hys-
teresis loss, cooperative heat transfer and neutron heating. This contribution is
time- and space-dependent and may vary among the pancakes. All heat loads
are identified in relevant input files.
CICC is modelled as double-channelled with two separate helium flows
joined by transverse mass and heat exchange. The model also implies helium
counterflows in adjacent pancakes.
The coil case and support structures have a separate cooling circuit. The
SHe is delivered into 74 cooling channels located poloidally in the coil case:
40 channels on the plasma-facing side and 34 channels on the outer sides.