Add TRISO particle and virtual lattice attributes to geometry docs

Updates the documentation for the geometry format to include new
attributes for TRISO particle and virtual lattice handling. These
attributes specify whether a cell contains a TRISO particle, whether
a virtual lattice is used, and the lattice's shape if applicable.

docs/source/io_formats/geometry.rst

@@ -219,6 +219,30 @@ Each ``<cell>`` element can have the following attributes or sub-elements:
 
     *Default*: None
 
+  :triso_particle:
+    If the cell is filled with a TRISO particle,  use this element to mark it.
+
+    .. note:: Only cells with spherical area can be marked.
+
+    *Default*: false
+
+  :virtual_lattice:
+    If the cell is filled with a matrix containing the TRISO particle,  use this
+    element to mark it. This can accelerate the search speed of neutrons in the 
+    region containing a large number of TRISO particles.
+
+    *Default*: false
+  
+  :shape:
+    If the virtual_lattice is True. This element specifies the shape of the
+    lattice.
+
+    .. note:: The shape of the lattice must be specified if the virtual_lattice
+              is True. Related methods can be referred to Liang, J., Li, R., Liu, Z., 
+              2024. Virtual lattice method for efficient Monte Carlo transport simulation
+              of dispersion nuclear fuels. Computer Physics Communications 295, 108985. 
+              https://doi.org/10.1016/j.cpc.2023.108985
+
 
 ---------------------
 ``<lattice>`` Element

openmc/model/triso.py

@@ -819,6 +819,11 @@ def create_triso_lattice(trisos, lower_left, pitch, shape, background, virtual=F
     background : openmc.Material
         A background material that is used anywhere within the lattice but
         outside a TRISO particle
+    virtual : bool
+        If True, create a virtual lattice where each cell is repeated
+        according to the pitch and shape. This is useful for creating a
+        lattice with a very large number of elements.
+        Default is False.
 
     Returns
     -------