.. _section-3.2:

|SAS| Channel Approach
-------------------------------

|SAS| is capable of using a multi-channel treatment. Each
channel represents a fuel pin, its associated coolant, and a fraction of
the subassembly duct wall, as indicated in :numref:`figure-3.2-1`. Usually, a
channel is used to represent an average pin in a fuel subassembly or a
group of similar subassemblies. A channel can also be used to represent
a blanket assembly or a control-rod channel, and the hottest pin in a
subassembly can be represented instead of the average pin. Different
channels can be used to account for radial and azimuthal power
variations within the core, as well as variations in coolant flow
orificing and fuel burn-up. In the multiple pin option, more than one
channel can be used to represent a subassembly.

.. _section-3.2.1:

Axial Mesh Structure
~~~~~~~~~~~~~~~~~~~~

A channel usually represents the whole length of the subassembly, from
coolant inlet to coolant outlet. A number of axial zones are used, as
indicated in :numref:`figure-3.2-2`. One zone represents the fuel-pin section,
including the core, axial blankets, and gas plenum. Other zones
represent reflector regions above and below the pin section. A maximum
of 7 zones can be used in a channel. In general, radial dimensions and
thermal properties are constant with a reflector zone. The pin section
zone is treated separately in considerably more detail than the
reflector zones. The gas plenum can be either above or below the core.

:numref:`figure-3.2-3` shows the axial mesh structure used for a channel. The
coolant and structure nodes run the whole length of the channel. The
coolant nodes are staggered with respect to the fuel, cladding,
reflector, and structure nodes. Using coolant temperatures defined at
the axial boundaries between cladding and structure nodes makes it
easier to calculate accurate coolant temperatures. If non-uniform axial
mesh sizes are used, a simple finite differencing of the coolant
temperature equation gives accurate coolant temperatures with a
staggered mesh, whereas if the coolant nodes were at the middle of the
cladding nodes, then obtaining accurate coolant temperatures would
require extra terms in the finite differencing of the coolant
temperature equation.

.. _figure-3.2-1:

..  figure:: media/Figure_3.2.1_SAS_Channel_Treatment.*
	:align: center
	:figclass: align-center


	|SAS| Channel Treatment

.. _figure-3.2-2:

..  figure:: media/Figure_3.2.2_Axial_Zones_SAS_Channels.png
	:align: center
	:figclass: align-center
	:height: 8.10764in

	Axial Zones in a |SAS| Channel

.. _figure-3.2-3:

..  figure:: media/image6.png
	:align: center
	:figclass: align-center
	:width: 6.22292in
	:height: 6.75417in

	Schematic of |SAS| Channel Discretization

.. _section-3.2.2:

Radial Mesh Structure
~~~~~~~~~~~~~~~~~~~~~

.. _section-3.2.2.1:

Core and Blanket Region
^^^^^^^^^^^^^^^^^^^^^^^

:numref:`figure-3.2-4` shows the radial mesh structure used for temperature
calculations in the core and blanket regions. This figure represents one
axial node. Between four and eleven radial nodes are used in the fuel,
three in the cladding, one in the coolant, and two in the structure. In
the fuel, the nodes can be set up on either an equal radial difference
basis or an equal mass basis. In either case, the first and last nodes
are half-size. For a given number of nodes, an equal radial difference
mesh will usually give more accurate center-line temperatures, but equal
mass nodes are sometimes used to get more nodes in the outer part of the
fuel, where temperature gradients are steeper. Steady-state fuel
restructuring can change the node sizes. Also, during the transient
calculation, the radii will move with the fuel as it expands or
contracts due to temperature changes. After the steady-state
initialization, the mass of fuel associated with a radial node is
constant, at least until fuel-pin disruption and coupling is made to
PLUTO2, PINACLE, or LEVITATE.

The inner fuel node is at :math:`r\  = \ 0` if there is no central void.
Otherwise, it is at the fuel inner surface. The outer fuel node is at
the fuel outer surface.

The "structure" represents each pin's share of the duct wall. A wrapper
wire can be lumped in with either the cladding or the structure.

.. _section-3.2.2.2:

Gas Plenum Region
^^^^^^^^^^^^^^^^^

The radial mesh structure used in the gas plenum region is shown in
:numref:`figure-3.2-5`. The plenum gas is represented by a single axial and radial
node. This gas is in contact with a number of axial cladding nodes. At
each axial node, there is one radial node in the cladding, one in the
coolant, and two in the structure.

.. _section-3.2.2.3:

Reflector Regions
^^^^^^^^^^^^^^^^^

The radial mesh structure in an axial node in a reflector region is
shown in :numref:`figure-3.2-6`. Two nodes are used in the reflector, one in the
coolant, and two in the structure.

.. _figure-3.2-4:

..  figure:: media/Figure_3.2.4_Radial_Temp_Nodes_Core.png
	:align: center
	:figclass: align-center


	Radial Temperature Nodes, Core and Axial Blanket Regions

.. _figure-3.2-5:

..  figure:: media/Figure_3.2.5_Radial_Temp_Nodes_GP.png
	:align: center
	:figclass: align-center
	:width: 5.87708in
	:height: 6.93056in

	Radial Temperature Nodes, Gas Plenum Region

.. _figure-3.2-6:

..  figure:: media/Figure_3.2.6_Radial_Temp_Nodes_Refl.png
	:align: center
	:figclass: align-center
	:width: 5.37708in
	:height: 4.06181in

	Radial Temperature Nodes, Reflector Region