*********************
Boundaries conditions
*********************

The boundaries condition for any external face of the geometry are defined using the ``BOUN`` keyword:

.. code-block::
  :caption: Block syntax

  REFL c1 c2 … 

=================     ===========
Input parameter       Description
=================     ===========
cx                    Reflection coefficient. Default is 0.
=================     ===========


The number of coefficient depends on the external volume shape and use the numbering convention of the volume faces (:ref:`Available finite shapes`).

The reflection probability value can take any real value.

The use the geometrical symmetries is generaly recommended whenever possible in order to:

-	simplify the geometrical description and reduce the number of volumes;
-	speed up the simulation time;
-	get better statistics of flux, reaction rates, etc.


.. rubric:: Example n°1

Applying a total reflection coefficient to two adjacent faces of a box is equivalent to describing a geometry with two perpendicular planes of symmetry.
Consequently, the box described below represents one-quarter of the device.

.. drawio-image:: resources/BOUN_0.drawio.svg

.. code-block::

  BOUN
    FACE F_1:V_v1:M_0 1.0
    FACE F_5:V_v1:M_0 1.0
  ENDB

.. rubric:: Example n°2

Applying a total reflection coefficient to two opposite faces of a box (or cylinder) is equivalent to describing an infinite number of boxes (or cylinders) replicated in the direction perpendicular to the reflective sides.

.. drawio-image:: resources/BOUN_1.drawio.svg


.. code-block::

  BOUN
    FACE F_1:V_v1:M_0 1.0
    FACE F_2:V_v1:M_0 1.0
  ENDB


The following figure show that repeating a geometry using a total reflection coefficient (A) is not always equivalent to an infinite lattice (B).

.. drawio-image:: resources/BOUN_2.drawio.svg


Here is a working example using boundaries reflections.

.. collapse:: Example of using boundaries conditions

  .. literalinclude :: examples/04_BOUN.m6
     :language: m6geo

  .. image:: resources/BOUN_example.png
      :width: 300px
      :align: center

|