diff --git a/docs/source/usersguide/geometry.rst b/docs/source/usersguide/geometry.rst
index 70a665c53..80e60bd34 100644
--- a/docs/source/usersguide/geometry.rst
+++ b/docs/source/usersguide/geometry.rst
@@ -88,7 +88,7 @@ parameters for a sphere are the :math:`x,y,z` coordinates of the center of the
 sphere and the radius of the sphere. All of these parameters can be set either
 as optional keyword arguments to the class constructor or via attributes::
 
-  sphere = openmc.Sphere(R=10.0)
+  sphere = openmc.Sphere(r=10.0)
 
   # This is equivalent
   sphere = openmc.Sphere()
@@ -98,7 +98,7 @@ Once a surface has been created, half-spaces can be obtained by applying the
 unary ``-`` or ``+`` operators, corresponding to the negative and positive
 half-spaces, respectively. For example::
 
-   >>> sphere = openmc.Sphere(R=10.0)
+   >>> sphere = openmc.Sphere(r=10.0)
    >>> inside_sphere = -sphere
    >>> outside_sphere = +sphere
    >>> type(inside_sphere)
@@ -140,10 +140,10 @@ may want to specify different behavior for particles passing through a
 surface. To specify a vacuum boundary condition, simply change the
 :attr:`Surface.boundary_type` attribute to 'vacuum'::
 
-   outer_surface = openmc.Sphere(R=100.0, boundary_type='vacuum')
+   outer_surface = openmc.Sphere(r=100.0, boundary_type='vacuum')
 
    # This is equivalent
-   outer_surface = openmc.Sphere(R=100.0)
+   outer_surface = openmc.Sphere(r=100.0)
    outer_surface.boundary_type = 'vacuum'
 
 Reflective and periodic boundary conditions can be set with the strings
@@ -154,8 +154,8 @@ can be determined automatically. For non-axis-aligned planes, it is necessary to
 specify pairs explicitly using the :attr:`Surface.periodic_surface` attribute as
 in the following example::
 
-  p1 = openmc.Plane(A=0.3, B=5.0, D=1.0, boundary_type='periodic')
-  p2 = openmc.Plane(A=0.3, B=5.0, D=-1.0, boundary_type='periodic')
+  p1 = openmc.Plane(a=0.3, b=5.0, d=1.0, boundary_type='periodic')
+  p2 = openmc.Plane(a=0.3, b=5.0, d=-1.0, boundary_type='periodic')
   p1.periodic_surface = p2
 
 Rotationally-periodic boundary conditions can be specified for a pair of
diff --git a/docs/source/usersguide/volume.rst b/docs/source/usersguide/volume.rst
index 482f66fd0..e4bdbf715 100644
--- a/docs/source/usersguide/volume.rst
+++ b/docs/source/usersguide/volume.rst
@@ -30,7 +30,7 @@ arguments are not necessary. For example,
 
 ::
 
-   sphere = openmc.Sphere(R=10.0)
+   sphere = openmc.Sphere(r=10.0)
    cell = openm.Cell(region=-sphere)
    vol_calc = openmc.VolumeCalculation([cell], 1000000)
 
diff --git a/examples/python/basic/build-xml.py b/examples/python/basic/build-xml.py
index 0dc83ed44..5caed2810 100644
--- a/examples/python/basic/build-xml.py
+++ b/examples/python/basic/build-xml.py
@@ -36,9 +36,9 @@ materials_file.export_to_xml()
 ###############################################################################
 
 # Instantiate ZCylinder surfaces
-surf1 = openmc.ZCylinder(surface_id=1, x0=0, y0=0, R=7, name='surf 1')
-surf2 = openmc.ZCylinder(surface_id=2, x0=0, y0=0, R=9, name='surf 2')
-surf3 = openmc.ZCylinder(surface_id=3, x0=0, y0=0, R=11, name='surf 3')
+surf1 = openmc.ZCylinder(surface_id=1, x0=0, y0=0, r=7, name='surf 1')
+surf2 = openmc.ZCylinder(surface_id=2, x0=0, y0=0, r=9, name='surf 2')
+surf3 = openmc.ZCylinder(surface_id=3, x0=0, y0=0, r=11, name='surf 3')
 surf3.boundary_type = 'vacuum'
 
 # Instantiate Cells
diff --git a/examples/python/lattice/hexagonal/build-xml.py b/examples/python/lattice/hexagonal/build-xml.py
index 24087be9e..c0aa815b4 100644
--- a/examples/python/lattice/hexagonal/build-xml.py
+++ b/examples/python/lattice/hexagonal/build-xml.py
@@ -43,7 +43,7 @@ left = openmc.XPlane(surface_id=1, x0=-3, name='left')
 right = openmc.XPlane(surface_id=2, x0=3, name='right')
 bottom = openmc.YPlane(surface_id=3, y0=-4, name='bottom')
 top = openmc.YPlane(surface_id=4, y0=4, name='top')
-fuel_surf = openmc.ZCylinder(surface_id=5, x0=0, y0=0, R=0.4)
+fuel_surf = openmc.ZCylinder(surface_id=5, x0=0, y0=0, r=0.4)
 
 left.boundary_type = 'vacuum'
 right.boundary_type = 'vacuum'
diff --git a/examples/python/lattice/nested/build-xml.py b/examples/python/lattice/nested/build-xml.py
index ef74173f5..37483a939 100644
--- a/examples/python/lattice/nested/build-xml.py
+++ b/examples/python/lattice/nested/build-xml.py
@@ -39,9 +39,9 @@ left = openmc.XPlane(surface_id=1, x0=-2, name='left')
 right = openmc.XPlane(surface_id=2, x0=2, name='right')
 bottom = openmc.YPlane(surface_id=3, y0=-2, name='bottom')
 top = openmc.YPlane(surface_id=4, y0=2, name='top')
-fuel1 = openmc.ZCylinder(surface_id=5, x0=0, y0=0, R=0.4)
-fuel2 = openmc.ZCylinder(surface_id=6, x0=0, y0=0, R=0.3)
-fuel3 = openmc.ZCylinder(surface_id=7, x0=0, y0=0, R=0.2)
+fuel1 = openmc.ZCylinder(surface_id=5, x0=0, y0=0, r=0.4)
+fuel2 = openmc.ZCylinder(surface_id=6, x0=0, y0=0, r=0.3)
+fuel3 = openmc.ZCylinder(surface_id=7, x0=0, y0=0, r=0.2)
 
 left.boundary_type = 'vacuum'
 right.boundary_type = 'vacuum'
diff --git a/examples/python/lattice/simple/build-xml.py b/examples/python/lattice/simple/build-xml.py
index 6222cd366..1cdfa24f3 100644
--- a/examples/python/lattice/simple/build-xml.py
+++ b/examples/python/lattice/simple/build-xml.py
@@ -39,9 +39,9 @@ left = openmc.XPlane(surface_id=1, x0=-2, name='left')
 right = openmc.XPlane(surface_id=2, x0=2, name='right')
 bottom = openmc.YPlane(surface_id=3, y0=-2, name='bottom')
 top = openmc.YPlane(surface_id=4, y0=2, name='top')
-fuel1 = openmc.ZCylinder(surface_id=5, x0=0, y0=0, R=0.4)
-fuel2 = openmc.ZCylinder(surface_id=6, x0=0, y0=0, R=0.3)
-fuel3 = openmc.ZCylinder(surface_id=7, x0=0, y0=0, R=0.2)
+fuel1 = openmc.ZCylinder(surface_id=5, x0=0, y0=0, r=0.4)
+fuel2 = openmc.ZCylinder(surface_id=6, x0=0, y0=0, r=0.3)
+fuel3 = openmc.ZCylinder(surface_id=7, x0=0, y0=0, r=0.2)
 
 left.boundary_type = 'vacuum'
 right.boundary_type = 'vacuum'
diff --git a/examples/python/pincell/build-xml.py b/examples/python/pincell/build-xml.py
index a9e6a5a74..c728d5f36 100644
--- a/examples/python/pincell/build-xml.py
+++ b/examples/python/pincell/build-xml.py
@@ -49,9 +49,9 @@ materials_file.export_to_xml()
 ###############################################################################
 
 # Instantiate ZCylinder surfaces
-fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, R=0.39218, name='Fuel OR')
-clad_ir = openmc.ZCylinder(surface_id=2, x0=0, y0=0, R=0.40005, name='Clad IR')
-clad_or = openmc.ZCylinder(surface_id=3, x0=0, y0=0, R=0.45720, name='Clad OR')
+fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, r=0.39218, name='Fuel OR')
+clad_ir = openmc.ZCylinder(surface_id=2, x0=0, y0=0, r=0.40005, name='Clad IR')
+clad_or = openmc.ZCylinder(surface_id=3, x0=0, y0=0, r=0.45720, name='Clad OR')
 left = openmc.XPlane(surface_id=4, x0=-0.62992, name='left')
 right = openmc.XPlane(surface_id=5, x0=0.62992, name='right')
 bottom = openmc.YPlane(surface_id=6, y0=-0.62992, name='bottom')
diff --git a/examples/python/pincell_depletion/run_depletion.py b/examples/python/pincell_depletion/run_depletion.py
index a2a5e0e7f..ba34a3706 100644
--- a/examples/python/pincell_depletion/run_depletion.py
+++ b/examples/python/pincell_depletion/run_depletion.py
@@ -54,9 +54,9 @@ borated_water.add_s_alpha_beta('c_H_in_H2O')
 ###############################################################################
 
 # Instantiate ZCylinder surfaces
-fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, R=0.39218, name='Fuel OR')
-clad_ir = openmc.ZCylinder(surface_id=2, x0=0, y0=0, R=0.40005, name='Clad IR')
-clad_or = openmc.ZCylinder(surface_id=3, x0=0, y0=0, R=0.45720, name='Clad OR')
+fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, r=0.39218, name='Fuel OR')
+clad_ir = openmc.ZCylinder(surface_id=2, x0=0, y0=0, r=0.40005, name='Clad IR')
+clad_or = openmc.ZCylinder(surface_id=3, x0=0, y0=0, r=0.45720, name='Clad OR')
 left = openmc.XPlane(surface_id=4, x0=-0.62992, name='left')
 right = openmc.XPlane(surface_id=5, x0=0.62992, name='right')
 bottom = openmc.YPlane(surface_id=6, y0=-0.62992, name='bottom')
diff --git a/examples/python/pincell_multigroup/build-xml.py b/examples/python/pincell_multigroup/build-xml.py
index 337044508..995cd87db 100644
--- a/examples/python/pincell_multigroup/build-xml.py
+++ b/examples/python/pincell_multigroup/build-xml.py
@@ -98,7 +98,7 @@ materials_file.export_to_xml()
 ###############################################################################
 
 # Instantiate ZCylinder surfaces
-fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, R=0.54, name='Fuel OR')
+fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, r=0.54, name='Fuel OR')
 left = openmc.XPlane(surface_id=4, x0=-0.63, name='left')
 right = openmc.XPlane(surface_id=5, x0=0.63, name='right')
 bottom = openmc.YPlane(surface_id=6, y0=-0.63, name='bottom')
diff --git a/openmc/examples.py b/openmc/examples.py
index a5138377e..4e8c8cf92 100644
--- a/openmc/examples.py
+++ b/openmc/examples.py
@@ -51,8 +51,8 @@ def pwr_pin_cell():
 
     # Instantiate ZCylinder surfaces
     pitch = 1.26
-    fuel_or = openmc.ZCylinder(x0=0, y0=0, R=0.39218, name='Fuel OR')
-    clad_or = openmc.ZCylinder(x0=0, y0=0, R=0.45720, name='Clad OR')
+    fuel_or = openmc.ZCylinder(x0=0, y0=0, r=0.39218, name='Fuel OR')
+    clad_or = openmc.ZCylinder(x0=0, y0=0, r=0.45720, name='Clad OR')
     left = openmc.XPlane(x0=-pitch/2, name='left', boundary_type='reflective')
     right = openmc.XPlane(x0=pitch/2, name='right', boundary_type='reflective')
     bottom = openmc.YPlane(y0=-pitch/2, name='bottom',
@@ -256,14 +256,14 @@ def pwr_core():
                        bot_nozzle, top_nozzle, top_fa, bot_fa)
 
     # Define surfaces.
-    s1 = openmc.ZCylinder(R=0.41, surface_id=1)
-    s2 = openmc.ZCylinder(R=0.475, surface_id=2)
-    s3 = openmc.ZCylinder(R=0.56, surface_id=3)
-    s4 = openmc.ZCylinder(R=0.62, surface_id=4)
-    s5 = openmc.ZCylinder(R=187.6, surface_id=5)
-    s6 = openmc.ZCylinder(R=209.0, surface_id=6)
-    s7 = openmc.ZCylinder(R=229.0, surface_id=7)
-    s8 = openmc.ZCylinder(R=249.0, surface_id=8, boundary_type='vacuum')
+    s1 = openmc.ZCylinder(r=0.41, surface_id=1)
+    s2 = openmc.ZCylinder(r=0.475, surface_id=2)
+    s3 = openmc.ZCylinder(r=0.56, surface_id=3)
+    s4 = openmc.ZCylinder(r=0.62, surface_id=4)
+    s5 = openmc.ZCylinder(r=187.6, surface_id=5)
+    s6 = openmc.ZCylinder(r=209.0, surface_id=6)
+    s7 = openmc.ZCylinder(r=229.0, surface_id=7)
+    s8 = openmc.ZCylinder(r=249.0, surface_id=8, boundary_type='vacuum')
 
     s31 = openmc.ZPlane(z0=-229.0, surface_id=31, boundary_type='vacuum')
     s32 = openmc.ZPlane(z0=-199.0, surface_id=32)
@@ -473,8 +473,8 @@ def pwr_assembly():
     model.materials = (fuel, clad, hot_water)
 
     # Instantiate ZCylinder surfaces
-    fuel_or = openmc.ZCylinder(x0=0, y0=0, R=0.39218, name='Fuel OR')
-    clad_or = openmc.ZCylinder(x0=0, y0=0, R=0.45720, name='Clad OR')
+    fuel_or = openmc.ZCylinder(x0=0, y0=0, r=0.39218, name='Fuel OR')
+    clad_or = openmc.ZCylinder(x0=0, y0=0, r=0.45720, name='Clad OR')
 
     # Create boundary planes to surround the geometry
     pitch = 21.42
diff --git a/openmc/model/funcs.py b/openmc/model/funcs.py
index 44e7f95f1..0023a98d9 100644
--- a/openmc/model/funcs.py
+++ b/openmc/model/funcs.py
@@ -237,16 +237,16 @@ def get_hexagonal_prism(edge_length=1., orientation='y', origin=(0., 0.),
         c = sqrt(3.)/3.
 
         # y = -x/sqrt(3) + a
-        upper_right = Plane(A=c, B=1., D=l+x*c+y, boundary_type=boundary_type)
+        upper_right = Plane(a=c, b=1., d=l+x*c+y, boundary_type=boundary_type)
 
         # y = x/sqrt(3) + a
-        upper_left = Plane(A=-c, B=1., D=l-x*c+y, boundary_type=boundary_type)
+        upper_left = Plane(a=-c, b=1., d=l-x*c+y, boundary_type=boundary_type)
 
         # y = x/sqrt(3) - a
-        lower_right = Plane(A=-c, B=1., D=-l-x*c+y, boundary_type=boundary_type)
+        lower_right = Plane(a=-c, b=1., d=-l-x*c+y, boundary_type=boundary_type)
 
         # y = -x/sqrt(3) - a
-        lower_left = Plane(A=c, B=1., D=-l+x*c+y, boundary_type=boundary_type)
+        lower_left = Plane(a=c, b=1., d=-l+x*c+y, boundary_type=boundary_type)
 
         prism = -right & +left & -upper_right & -upper_left & \
                 +lower_right & +lower_left
@@ -262,17 +262,17 @@ def get_hexagonal_prism(edge_length=1., orientation='y', origin=(0., 0.),
         c = sqrt(3.)
 
         # y = -sqrt(3)*(x - a)
-        upper_right = Plane(A=c, B=1., D=c*l+x*c+y, boundary_type=boundary_type)
+        upper_right = Plane(a=c, b=1., d=c*l+x*c+y, boundary_type=boundary_type)
 
         # y = sqrt(3)*(x + a)
-        lower_right = Plane(A=-c, B=1., D=-c*l-x*c+y,
+        lower_right = Plane(a=-c, b=1., d=-c*l-x*c+y,
                             boundary_type=boundary_type)
 
         # y = -sqrt(3)*(x + a)
-        lower_left = Plane(A=c, B=1., D=-c*l+x*c+y, boundary_type=boundary_type)
+        lower_left = Plane(a=c, b=1., d=-c*l+x*c+y, boundary_type=boundary_type)
 
         # y = sqrt(3)*(x + a)
-        upper_left = Plane(A=-c, B=1., D=c*l-x*c+y, boundary_type=boundary_type)
+        upper_left = Plane(a=-c, b=1., d=c*l-x*c+y, boundary_type=boundary_type)
 
         prism = -top & +bottom & -upper_right & +lower_right & \
                             +lower_left & -upper_left
@@ -292,8 +292,8 @@ def get_hexagonal_prism(edge_length=1., orientation='y', origin=(0., 0.),
         t = l - corner_radius/c
 
         # Cylinder with corner radius and boundary type pre-applied
-        cyl1 = partial(ZCylinder, R=corner_radius, boundary_type=boundary_type)
-        cyl2 = partial(ZCylinder, R=corner_radius/(2*c),
+        cyl1 = partial(ZCylinder, r=corner_radius, boundary_type=boundary_type)
+        cyl2 = partial(ZCylinder, r=corner_radius/(2*c),
                        boundary_type=boundary_type)
 
         if orientation == 'x':
diff --git a/openmc/model/triso.py b/openmc/model/triso.py
index e04d7f589..2afc88d64 100644
--- a/openmc/model/triso.py
+++ b/openmc/model/triso.py
@@ -49,7 +49,7 @@ class TRISO(openmc.Cell):
     """
 
     def __init__(self, outer_radius, fill, center=(0., 0., 0.)):
-        self._surface = openmc.Sphere(R=outer_radius)
+        self._surface = openmc.Sphere(r=outer_radius)
         super().__init__(fill=fill, region=-self._surface)
         self.center = np.asarray(center)
 
diff --git a/openmc/region.py b/openmc/region.py
index e8e42776c..d12ae7112 100644
--- a/openmc/region.py
+++ b/openmc/region.py
@@ -261,7 +261,7 @@ class Intersection(Region, MutableSequence):
     following example:
 
     >>> equator = openmc.ZPlane(z0=0.0)
-    >>> earth = openmc.Sphere(R=637.1e6)
+    >>> earth = openmc.Sphere(r=637.1e6)
     >>> northern_hemisphere = -earth & +equator
     >>> southern_hemisphere = -earth & -equator
     >>> type(northern_hemisphere)
@@ -396,7 +396,7 @@ class Union(Region, MutableSequence):
     example:
 
     >>> s1 = openmc.ZPlane(z0=0.0)
-    >>> s2 = openmc.Sphere(R=637.1e6)
+    >>> s2 = openmc.Sphere(r=637.1e6)
     >>> type(-s2 | +s1)
     <class 'openmc.region.Union'>
 
diff --git a/openmc/surface.py b/openmc/surface.py
index 306fb6d73..4c44c68b6 100644
--- a/openmc/surface.py
+++ b/openmc/surface.py
@@ -4,6 +4,7 @@ from copy import deepcopy
 from functools import partial
 from numbers import Real, Integral
 from xml.etree import ElementTree as ET
+from warnings import warn
 
 import numpy as np
 
@@ -14,6 +15,11 @@ from openmc.mixin import IDManagerMixin
 
 _BOUNDARY_TYPES = ['transmission', 'vacuum', 'reflective', 'periodic']
 
+_WARNING_UPPER = """\
+"{}(...) accepts an argument named '{}', not '{}'. Future versions of OpenMC \
+will not accept the capitalized version.\
+"""
+
 
 class Surface(IDManagerMixin, metaclass=ABCMeta):
     """An implicit surface with an associated boundary condition.
@@ -287,29 +293,29 @@ class Surface(IDManagerMixin, metaclass=ABCMeta):
             surface = Plane(surface_id, bc, A, B, C, D, name)
 
         elif surf_type == 'x-cylinder':
-            y0, z0, R = coeffs
-            surface = XCylinder(surface_id, bc, y0, z0, R, name)
+            y0, z0, r = coeffs
+            surface = XCylinder(surface_id, bc, y0, z0, r, name)
 
         elif surf_type == 'y-cylinder':
-            x0, z0, R = coeffs
-            surface = YCylinder(surface_id, bc, x0, z0, R, name)
+            x0, z0, r = coeffs
+            surface = YCylinder(surface_id, bc, x0, z0, r, name)
 
         elif surf_type == 'z-cylinder':
-            x0, y0, R = coeffs
-            surface = ZCylinder(surface_id, bc, x0, y0, R, name)
+            x0, y0, r = coeffs
+            surface = ZCylinder(surface_id, bc, x0, y0, r, name)
 
         elif surf_type == 'sphere':
-            x0, y0, z0, R = coeffs
-            surface = Sphere(surface_id, bc, x0, y0, z0, R, name)
+            x0, y0, z0, r = coeffs
+            surface = Sphere(surface_id, bc, x0, y0, z0, r, name)
 
         elif surf_type in ['x-cone', 'y-cone', 'z-cone']:
-            x0, y0, z0, R2 = coeffs
+            x0, y0, z0, r2 = coeffs
             if surf_type == 'x-cone':
-                surface = XCone(surface_id, bc, x0, y0, z0, R2, name)
+                surface = XCone(surface_id, bc, x0, y0, z0, r2, name)
             elif surf_type == 'y-cone':
-                surface = YCone(surface_id, bc, x0, y0, z0, R2, name)
+                surface = YCone(surface_id, bc, x0, y0, z0, r2, name)
             elif surf_type == 'z-cone':
-                surface = ZCone(surface_id, bc, x0, y0, z0, R2, name)
+                surface = ZCone(surface_id, bc, x0, y0, z0, r2, name)
 
         elif surf_type == 'quadric':
             a, b, c, d, e, f, g, h, j, k = coeffs
@@ -331,13 +337,13 @@ class Plane(Surface):
         Boundary condition that defines the behavior for particles hitting the
         surface. Defaults to transmissive boundary condition where particles
         freely pass through the surface.
-    A : float, optional
+    a : float, optional
         The 'A' parameter for the plane. Defaults to 1.
-    B : float, optional
+    b : float, optional
         The 'B' parameter for the plane. Defaults to 0.
-    C : float, optional
+    c : float, optional
         The 'C' parameter for the plane. Defaults to 0.
-    D : float, optional
+    d : float, optional
         The 'D' parameter for the plane. Defaults to 0.
     name : str, optional
         Name of the plane. If not specified, the name will be the empty string.
@@ -370,56 +376,61 @@ class Plane(Surface):
     """
 
     _type = 'plane'
-    _coeff_keys = ('A', 'B', 'C', 'D')
+    _coeff_keys = ('a', 'b', 'c', 'd')
 
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 A=1., B=0., C=0., D=0., name=''):
+                 a=1., b=0., c=0., d=0., name='', **kwargs):
         super().__init__(surface_id, boundary_type, name=name)
         self._periodic_surface = None
-        self.a = A
-        self.b = B
-        self.c = C
-        self.d = D
+        self.a = a
+        self.b = b
+        self.c = c
+        self.d = d
+        for k, v in kwargs.items():
+            if k in 'ABCD':
+                warn(_WARNING_UPPER.format(type(self).__name__, k.lower(), k),
+                    FutureWarning)
+                setattr(self, k.lower(), v)
 
     @property
     def a(self):
-        return self.coefficients['A']
+        return self.coefficients['a']
 
     @property
     def b(self):
-        return self.coefficients['B']
+        return self.coefficients['b']
 
     @property
     def c(self):
-        return self.coefficients['C']
+        return self.coefficients['c']
 
     @property
     def d(self):
-        return self.coefficients['D']
+        return self.coefficients['d']
 
     @property
     def periodic_surface(self):
         return self._periodic_surface
 
     @a.setter
-    def a(self, A):
-        check_type('A coefficient', A, Real)
-        self._coefficients['A'] = A
+    def a(self, a):
+        check_type('A coefficient', a, Real)
+        self._coefficients['a'] = a
 
     @b.setter
-    def b(self, B):
-        check_type('B coefficient', B, Real)
-        self._coefficients['B'] = B
+    def b(self, b):
+        check_type('B coefficient', b, Real)
+        self._coefficients['b'] = b
 
     @c.setter
-    def c(self, C):
-        check_type('C coefficient', C, Real)
-        self._coefficients['C'] = C
+    def c(self, c):
+        check_type('C coefficient', c, Real)
+        self._coefficients['c'] = c
 
     @d.setter
-    def d(self, D):
-        check_type('D coefficient', D, Real)
-        self._coefficients['D'] = D
+    def d(self, d):
+        check_type('D coefficient', d, Real)
+        self._coefficients['d'] = d
 
     @periodic_surface.setter
     def periodic_surface(self, periodic_surface):
@@ -465,7 +476,7 @@ class Plane(Surface):
         if d == self.d:
             return self
         else:
-            return type(self)(A=self.a, B=self.b, C=self.c, D=d)
+            return type(self)(a=self.a, b=self.b, c=self.c, d=d)
 
     def to_xml_element(self):
         """Return XML representation of the surface
@@ -511,10 +522,10 @@ class Plane(Surface):
         n = np.cross(p2 - p1, p3 - p1)
 
         # The equation of the plane will by n·(<x,y,z> - p1) = 0. Determine
-        # coefficients A, B, C, and D based on that
-        A, B, C = n
-        D = np.dot(n, p1)
-        return cls(A=A, B=B, C=C, D=D, **kwargs)
+        # coefficients a, b, c, and d based on that
+        a, b, c = n
+        d = np.dot(n, p1)
+        return cls(a=a, b=b, c=c, d=d, **kwargs)
 
 
 class XPlane(Plane):
@@ -906,7 +917,7 @@ class Cylinder(Surface):
         Boundary condition that defines the behavior for particles hitting the
         surface. Defaults to transmissive boundary condition where particles
         freely pass through the surface.
-    R : float, optional
+    r : float, optional
         Radius of the cylinder. Defaults to 1.
     name : str, optional
         Name of the cylinder. If not specified, the name will be the empty
@@ -930,23 +941,23 @@ class Cylinder(Surface):
 
     """
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 R=1., name=''):
+                 r=1., name=''):
         super().__init__(surface_id, boundary_type, name=name)
-        self.r = R
+        self.r = r
 
     @property
     def r(self):
-        return self.coefficients['R']
+        return self.coefficients['r']
 
     @r.setter
-    def r(self, R):
-        check_type('R coefficient', R, Real)
-        self._coefficients['R'] = R
+    def r(self, r):
+        check_type('r coefficient', r, Real)
+        self._coefficients['r'] = r
 
 
 class XCylinder(Cylinder):
     """An infinite cylinder whose length is parallel to the x-axis of the form
-    :math:`(y - y_0)^2 + (z - z_0)^2 = R^2`.
+    :math:`(y - y_0)^2 + (z - z_0)^2 = r^2`.
 
     Parameters
     ----------
@@ -961,7 +972,7 @@ class XCylinder(Cylinder):
         y-coordinate of the center of the cylinder. Defaults to 0.
     z0 : float, optional
         z-coordinate of the center of the cylinder. Defaults to 0.
-    R : float, optional
+    r : float, optional
         Radius of the cylinder. Defaults to 0.
     name : str, optional
         Name of the cylinder. If not specified, the name will be the empty
@@ -988,11 +999,14 @@ class XCylinder(Cylinder):
     """
 
     _type = 'x-cylinder'
-    _coeff_keys = ('y0', 'z0', 'R')
+    _coeff_keys = ('y0', 'z0', 'r')
 
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 y0=0., z0=0., R=1., name=''):
-        super().__init__(surface_id, boundary_type, R, name=name)
+                 y0=0., z0=0., r=1., name='', *, R=None):
+        if R is not None:
+            warn(_WARNING_UPPER.format(type(self).__name__, 'r', 'R'), FutureWarning)
+            r = R
+        super().__init__(surface_id, boundary_type, r, name=name)
         self.y0 = y0
         self.z0 = z0
 
@@ -1058,7 +1072,7 @@ class XCylinder(Cylinder):
         Returns
         -------
         float
-            :math:`(y' - y_0)^2 + (z' - z_0)^2 - R^2`
+            :math:`(y' - y_0)^2 + (z' - z_0)^2 - r^2`
 
         """
         y = point[1] - self.y0
@@ -1085,12 +1099,12 @@ class XCylinder(Cylinder):
         else:
             y0 = self.y0 + vy
             z0 = self.z0 + vz
-            return type(self)(y0=y0, z0=z0, R=self.r)
+            return type(self)(y0=y0, z0=z0, r=self.r)
 
 
 class YCylinder(Cylinder):
     """An infinite cylinder whose length is parallel to the y-axis of the form
-    :math:`(x - x_0)^2 + (z - z_0)^2 = R^2`.
+    :math:`(x - x_0)^2 + (z - z_0)^2 = r^2`.
 
     Parameters
     ----------
@@ -1105,7 +1119,7 @@ class YCylinder(Cylinder):
         x-coordinate of the center of the cylinder. Defaults to 0.
     z0 : float, optional
         z-coordinate of the center of the cylinder. Defaults to 0.
-    R : float, optional
+    r : float, optional
         Radius of the cylinder. Defaults to 1.
     name : str, optional
         Name of the cylinder. If not specified, the name will be the empty
@@ -1132,11 +1146,14 @@ class YCylinder(Cylinder):
     """
 
     _type = 'y-cylinder'
-    _coeff_keys = ('x0', 'z0', 'R')
+    _coeff_keys = ('x0', 'z0', 'r')
 
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 x0=0., z0=0., R=1., name=''):
-        super().__init__(surface_id, boundary_type, R, name=name)
+                 x0=0., z0=0., r=1., name='', *, R=None):
+        if R is not None:
+            warn(_WARNING_UPPER.format(type(self).__name__, 'r', 'R'), FutureWarning)
+            r = R
+        super().__init__(surface_id, boundary_type, r, name=name)
         self.x0 = x0
         self.z0 = z0
 
@@ -1202,7 +1219,7 @@ class YCylinder(Cylinder):
         Returns
         -------
         float
-            :math:`(x' - x_0)^2 + (z' - z_0)^2 - R^2`
+            :math:`(x' - x_0)^2 + (z' - z_0)^2 - r^2`
 
         """
         x = point[0] - self.x0
@@ -1229,12 +1246,12 @@ class YCylinder(Cylinder):
         else:
             x0 = self.x0 + vx
             z0 = self.z0 + vz
-            return type(self)(x0=x0, z0=z0, R=self.r)
+            return type(self)(x0=x0, z0=z0, r=self.r)
 
 
 class ZCylinder(Cylinder):
     """An infinite cylinder whose length is parallel to the z-axis of the form
-    :math:`(x - x_0)^2 + (y - y_0)^2 = R^2`.
+    :math:`(x - x_0)^2 + (y - y_0)^2 = r^2`.
 
     Parameters
     ----------
@@ -1249,7 +1266,7 @@ class ZCylinder(Cylinder):
         x-coordinate of the center of the cylinder. Defaults to 0.
     y0 : float, optional
         y-coordinate of the center of the cylinder. Defaults to 0.
-    R : float, optional
+    r : float, optional
         Radius of the cylinder. Defaults to 1.
     name : str, optional
         Name of the cylinder. If not specified, the name will be the empty
@@ -1276,11 +1293,14 @@ class ZCylinder(Cylinder):
     """
 
     _type = 'z-cylinder'
-    _coeff_keys = ('x0', 'y0', 'R')
+    _coeff_keys = ('x0', 'y0', 'r')
 
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 x0=0., y0=0., R=1., name=''):
-        super().__init__(surface_id, boundary_type, R, name=name)
+                 x0=0., y0=0., r=1., name='', *, R=None):
+        if R is not None:
+            warn(_WARNING_UPPER.format(type(self).__name__, 'r', 'R'), FutureWarning)
+            r = R
+        super().__init__(surface_id, boundary_type, r, name=name)
         self.x0 = x0
         self.y0 = y0
 
@@ -1346,7 +1366,7 @@ class ZCylinder(Cylinder):
         Returns
         -------
         float
-            :math:`(x' - x_0)^2 + (y' - y_0)^2 - R^2`
+            :math:`(x' - x_0)^2 + (y' - y_0)^2 - r^2`
 
         """
         x = point[0] - self.x0
@@ -1373,11 +1393,11 @@ class ZCylinder(Cylinder):
         else:
             x0 = self.x0 + vx
             y0 = self.y0 + vy
-            return type(self)(x0=x0, y0=y0, R=self.r)
+            return type(self)(x0=x0, y0=y0, r=self.r)
 
 
 class Sphere(Surface):
-    """A sphere of the form :math:`(x - x_0)^2 + (y - y_0)^2 + (z - z_0)^2 = R^2`.
+    """A sphere of the form :math:`(x - x_0)^2 + (y - y_0)^2 + (z - z_0)^2 = r^2`.
 
     Parameters
     ----------
@@ -1394,7 +1414,7 @@ class Sphere(Surface):
         y-coordinate of the center of the sphere. Defaults to 0.
     z0 : float, optional
         z-coordinate of the center of the sphere. Defaults to 0.
-    R : float, optional
+    r : float, optional
         Radius of the sphere. Defaults to 1.
     name : str, optional
         Name of the sphere. If not specified, the name will be the empty string.
@@ -1424,15 +1444,18 @@ class Sphere(Surface):
     """
 
     _type = 'sphere'
-    _coeff_keys = ('x0', 'y0', 'z0', 'R')
+    _coeff_keys = ('x0', 'y0', 'z0', 'r')
 
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 x0=0., y0=0., z0=0., R=1., name=''):
+                 x0=0., y0=0., z0=0., r=1., name='', *, R=None):
+        if R is not None:
+            warn(_WARNING_UPPER.format(type(self).__name__, 'r', 'R'), FutureWarning)
+            r = R
         super().__init__(surface_id, boundary_type, name=name)
         self.x0 = x0
         self.y0 = y0
         self.z0 = z0
-        self.r = R
+        self.r = r
 
     @property
     def x0(self):
@@ -1448,7 +1471,7 @@ class Sphere(Surface):
 
     @property
     def r(self):
-        return self.coefficients['R']
+        return self.coefficients['r']
 
     @x0.setter
     def x0(self, x0):
@@ -1466,9 +1489,9 @@ class Sphere(Surface):
         self._coefficients['z0'] = z0
 
     @r.setter
-    def r(self, R):
-        check_type('R coefficient', R, Real)
-        self._coefficients['R'] = R
+    def r(self, r):
+        check_type('r coefficient', r, Real)
+        self._coefficients['r'] = r
 
     def bounding_box(self, side):
         """Determine an axis-aligned bounding box.
@@ -1515,7 +1538,7 @@ class Sphere(Surface):
         Returns
         -------
         float
-            :math:`(x' - x_0)^2 + (y' - y_0)^2 + (z' - z_0)^2 - R^2`
+            :math:`(x' - x_0)^2 + (y' - y_0)^2 + (z' - z_0)^2 - r^2`
 
         """
         x = point[0] - self.x0
@@ -1544,7 +1567,7 @@ class Sphere(Surface):
             x0 = self.x0 + vx
             y0 = self.y0 + vy
             z0 = self.z0 + vz
-            return type(self)(x0=x0, y0=y0, z0=z0, R=self.r)
+            return type(self)(x0=x0, y0=y0, z0=z0, r=self.r)
 
 
 class Cone(Surface):
@@ -1565,7 +1588,7 @@ class Cone(Surface):
         y-coordinate of the apex. Defaults to 0.
     z0 : float
         z-coordinate of the apex. Defaults to 0.
-    R2 : float
+    r2 : float
         Parameter related to the aperature. Defaults to 1.
     name : str
         Name of the cone. If not specified, the name will be the empty string.
@@ -1594,15 +1617,18 @@ class Cone(Surface):
 
     """
 
-    _coeff_keys = ('x0', 'y0', 'z0', 'R2')
+    _coeff_keys = ('x0', 'y0', 'z0', 'r2')
 
     def __init__(self, surface_id=None, boundary_type='transmission',
-                 x0=0., y0=0., z0=0., R2=1., name=''):
+                 x0=0., y0=0., z0=0., r2=1., name='', *, R2=None):
+        if R2 is not None:
+            warn(_WARNING_UPPER.format(type(self).__name__, 'r2', 'R2'), FutureWarning)
+            r2 = R2
         super().__init__(surface_id, boundary_type, name=name)
         self.x0 = x0
         self.y0 = y0
         self.z0 = z0
-        self.r2 = R2
+        self.r2 = r2
 
     @property
     def x0(self):
@@ -1618,7 +1644,7 @@ class Cone(Surface):
 
     @property
     def r2(self):
-        return self.coefficients['R2']
+        return self.coefficients['r2']
 
     @x0.setter
     def x0(self, x0):
@@ -1636,9 +1662,9 @@ class Cone(Surface):
         self._coefficients['z0'] = z0
 
     @r2.setter
-    def r2(self, R2):
-        check_type('R^2 coefficient', R2, Real)
-        self._coefficients['R2'] = R2
+    def r2(self, r2):
+        check_type('r^2 coefficient', r2, Real)
+        self._coefficients['r2'] = r2
 
     def translate(self, vector):
         """Translate surface in given direction
@@ -1661,12 +1687,12 @@ class Cone(Surface):
             x0 = self.x0 + vx
             y0 = self.y0 + vy
             z0 = self.z0 + vz
-            return type(self)(x0=x0, y0=y0, z0=z0, R2=self.r2)
+            return type(self)(x0=x0, y0=y0, z0=z0, r2=self.r2)
 
 
 class XCone(Cone):
     """A cone parallel to the x-axis of the form :math:`(y - y_0)^2 + (z - z_0)^2 =
-    R^2 (x - x_0)^2`.
+    r^2 (x - x_0)^2`.
 
     Parameters
     ----------
@@ -1683,7 +1709,7 @@ class XCone(Cone):
         y-coordinate of the apex. Defaults to 0.
     z0 : float, optional
         z-coordinate of the apex. Defaults to 0.
-    R2 : float, optional
+    r2 : float, optional
         Parameter related to the aperature. Defaults to 1.
     name : str, optional
         Name of the cone. If not specified, the name will be the empty string.
@@ -1696,7 +1722,7 @@ class XCone(Cone):
         y-coordinate of the apex
     z0 : float
         z-coordinate of the apex
-    R2 : float
+    r2 : float
         Parameter related to the aperature
     boundary_type : {'transmission, 'vacuum', 'reflective'}
         Boundary condition that defines the behavior for particles hitting the
@@ -1726,7 +1752,7 @@ class XCone(Cone):
         Returns
         -------
         float
-            :math:`(y' - y_0)^2 + (z' - z_0)^2 - R^2(x' - x_0)^2`
+            :math:`(y' - y_0)^2 + (z' - z_0)^2 - r^2(x' - x_0)^2`
 
         """
         x = point[0] - self.x0
@@ -1737,7 +1763,7 @@ class XCone(Cone):
 
 class YCone(Cone):
     """A cone parallel to the y-axis of the form :math:`(x - x_0)^2 + (z - z_0)^2 =
-    R^2 (y - y_0)^2`.
+    r^2 (y - y_0)^2`.
 
     Parameters
     ----------
@@ -1754,7 +1780,7 @@ class YCone(Cone):
         y-coordinate of the apex. Defaults to 0.
     z0 : float, optional
         z-coordinate of the apex. Defaults to 0.
-    R2 : float, optional
+    r2 : float, optional
         Parameter related to the aperature. Defaults to 1.
     name : str, optional
         Name of the cone. If not specified, the name will be the empty string.
@@ -1767,7 +1793,7 @@ class YCone(Cone):
         y-coordinate of the apex
     z0 : float
         z-coordinate of the apex
-    R2 : float
+    r2 : float
         Parameter related to the aperature
     boundary_type : {'transmission, 'vacuum', 'reflective'}
         Boundary condition that defines the behavior for particles hitting the
@@ -1797,7 +1823,7 @@ class YCone(Cone):
         Returns
         -------
         float
-            :math:`(x' - x_0)^2 + (z' - z_0)^2 - R^2(y' - y_0)^2`
+            :math:`(x' - x_0)^2 + (z' - z_0)^2 - r^2(y' - y_0)^2`
 
         """
         x = point[0] - self.x0
@@ -1808,7 +1834,7 @@ class YCone(Cone):
 
 class ZCone(Cone):
     """A cone parallel to the x-axis of the form :math:`(x - x_0)^2 + (y - y_0)^2 =
-    R^2 (z - z_0)^2`.
+    r^2 (z - z_0)^2`.
 
     Parameters
     ----------
@@ -1825,7 +1851,7 @@ class ZCone(Cone):
         y-coordinate of the apex. Defaults to 0.
     z0 : float, optional
         z-coordinate of the apex. Defaults to 0.
-    R2 : float, optional
+    r2 : float, optional
         Parameter related to the aperature. Defaults to 1.
     name : str, optional
         Name of the cone. If not specified, the name will be the empty string.
@@ -1838,7 +1864,7 @@ class ZCone(Cone):
         y-coordinate of the apex
     z0 : float
         z-coordinate of the apex
-    R2 : float
+    r2 : float
         Parameter related to the aperature
     boundary_type : {'transmission, 'vacuum', 'reflective'}
         Boundary condition that defines the behavior for particles hitting the
@@ -1868,7 +1894,7 @@ class ZCone(Cone):
         Returns
         -------
         float
-            :math:`(x' - x_0)^2 + (y' - y_0)^2 - R^2(z' - z_0)^2`
+            :math:`(x' - x_0)^2 + (y' - y_0)^2 - r^2(z' - z_0)^2`
 
         """
         x = point[0] - self.x0
@@ -2080,7 +2106,7 @@ class Halfspace(Region):
     can be created from an existing Surface through the __neg__ and __pos__
     operators, as the following example demonstrates:
 
-    >>> sphere = openmc.Sphere(surface_id=1, R=10.0)
+    >>> sphere = openmc.Sphere(surface_id=1, r=10.0)
     >>> inside_sphere = -sphere
     >>> outside_sphere = +sphere
     >>> type(inside_sphere)
diff --git a/tests/regression_tests/deplete/example_geometry.py b/tests/regression_tests/deplete/example_geometry.py
index e54c087f6..d4fdf585d 100644
--- a/tests/regression_tests/deplete/example_geometry.py
+++ b/tests/regression_tests/deplete/example_geometry.py
@@ -182,14 +182,14 @@ def segment_pin(n_rings, n_wedges, r_fuel, r_gap, r_clad):
     theta = np.linspace(0, 2*math.pi, n_wedges + 1)
 
     # Compute surfaces
-    fuel_rings = [openmc.ZCylinder(x0=0, y0=0, R=r_rings[i])
+    fuel_rings = [openmc.ZCylinder(x0=0, y0=0, r=r_rings[i])
                   for i in range(n_rings)]
 
-    fuel_wedges = [openmc.Plane(A=math.cos(theta[i]), B=math.sin(theta[i]))
+    fuel_wedges = [openmc.Plane(a=math.cos(theta[i]), b=math.sin(theta[i]))
                    for i in range(n_wedges)]
 
-    gap_ring = openmc.ZCylinder(x0=0, y0=0, R=r_gap)
-    clad_ring = openmc.ZCylinder(x0=0, y0=0, R=r_clad)
+    gap_ring = openmc.ZCylinder(x0=0, y0=0, r=r_gap)
+    clad_ring = openmc.ZCylinder(x0=0, y0=0, r=r_clad)
 
     # Create cells
     fuel_cells = []
diff --git a/tests/regression_tests/distribmat/test.py b/tests/regression_tests/distribmat/test.py
index cb598b9d3..33f64ba03 100644
--- a/tests/regression_tests/distribmat/test.py
+++ b/tests/regression_tests/distribmat/test.py
@@ -32,7 +32,7 @@ class DistribmatTestHarness(PyAPITestHarness):
         c1 = openmc.Cell(cell_id=1, fill=moderator)
         mod_univ = openmc.Universe(universe_id=1, cells=[c1])
 
-        r0 = openmc.ZCylinder(R=0.3)
+        r0 = openmc.ZCylinder(r=0.3)
         c11 = openmc.Cell(cell_id=11, region=-r0)
         c11.fill = [dense_fuel, None, light_fuel, dense_fuel]
         c12 = openmc.Cell(cell_id=12, region=+r0, fill=moderator)
diff --git a/tests/regression_tests/fixed_source/test.py b/tests/regression_tests/fixed_source/test.py
index 7d98de9fc..74908f87c 100644
--- a/tests/regression_tests/fixed_source/test.py
+++ b/tests/regression_tests/fixed_source/test.py
@@ -37,7 +37,7 @@ def test_fixed_source():
     mat.add_nuclide('U238', 0.0001)
     mat.set_density('g/cc', 7.5)
 
-    surf = openmc.Sphere(R=10.0, boundary_type='vacuum')
+    surf = openmc.Sphere(r=10.0, boundary_type='vacuum')
     cell = openmc.Cell(fill=mat, region=-surf)
 
     model = openmc.model.Model()
diff --git a/tests/regression_tests/multipole/test.py b/tests/regression_tests/multipole/test.py
index c826ff0d7..4c4353c84 100644
--- a/tests/regression_tests/multipole/test.py
+++ b/tests/regression_tests/multipole/test.py
@@ -27,7 +27,7 @@ def make_model():
     c1 = openmc.Cell(cell_id=1, fill=moderator)
     mod_univ = openmc.Universe(universe_id=1, cells=(c1,))
 
-    r0 = openmc.ZCylinder(R=0.3)
+    r0 = openmc.ZCylinder(r=0.3)
     c11 = openmc.Cell(cell_id=11, fill=dense_fuel, region=-r0)
     c11.temperature = [500, 700, 0, 800]
     c12 = openmc.Cell(cell_id=12, fill=moderator, region=+r0)
diff --git a/tests/regression_tests/periodic/test.py b/tests/regression_tests/periodic/test.py
index d746ebdd5..5df935b22 100644
--- a/tests/regression_tests/periodic/test.py
+++ b/tests/regression_tests/periodic/test.py
@@ -30,7 +30,7 @@ class PeriodicTest(PyAPITestHarness):
 
         z_min = openmc.ZPlane(5, z0=-5., boundary_type='reflective')
         z_max = openmc.ZPlane(6, z0=5., boundary_type='reflective')
-        z_cyl = openmc.ZCylinder(7, x0=-2.5, y0=2.5, R=2.0)
+        z_cyl = openmc.ZCylinder(7, x0=-2.5, y0=2.5, r=2.0)
 
         outside_cyl = openmc.Cell(1, fill=water, region=(
             +x_min & -x_max & +y_min & -y_max & +z_min & -z_max & +z_cyl))
diff --git a/tests/regression_tests/photon_production/test.py b/tests/regression_tests/photon_production/test.py
index 68992c867..df6375e13 100644
--- a/tests/regression_tests/photon_production/test.py
+++ b/tests/regression_tests/photon_production/test.py
@@ -14,7 +14,7 @@ class SourceTestHarness(PyAPITestHarness):
         materials = openmc.Materials([mat])
         materials.export_to_xml()
 
-        cyl = openmc.XCylinder(boundary_type='vacuum', R=1.0e-6)
+        cyl = openmc.XCylinder(boundary_type='vacuum', r=1.0e-6)
         x_plane_left = openmc.XPlane(boundary_type='vacuum', x0=-1.0)
         x_plane_center = openmc.XPlane(boundary_type='transmission', x0=1.0)
         x_plane_right = openmc.XPlane(boundary_type='vacuum', x0=1.0e9)
diff --git a/tests/regression_tests/photon_source/test.py b/tests/regression_tests/photon_source/test.py
index 61367d375..94b028003 100644
--- a/tests/regression_tests/photon_source/test.py
+++ b/tests/regression_tests/photon_source/test.py
@@ -15,19 +15,19 @@ class SourceTestHarness(PyAPITestHarness):
         materials = openmc.Materials([mat])
         materials.export_to_xml()
 
-        sphere = openmc.Sphere(R=1.0e9, boundary_type='reflective')
+        sphere = openmc.Sphere(r=1.0e9, boundary_type='reflective')
         inside_sphere = openmc.Cell()
         inside_sphere.region = -sphere
         inside_sphere.fill = mat
         geometry = openmc.Geometry([inside_sphere])
         geometry.export_to_xml()
- 
+
         source = openmc.Source()
         source.space = openmc.stats.Point((0, 0, 0))
         source.angle = openmc.stats.Isotropic()
         source.energy = openmc.stats.Discrete([10.0e6], [1.0])
         source.particle = 'photon'
- 
+
         settings = openmc.Settings()
         settings.particles = 10000
         settings.batches = 1
@@ -37,7 +37,7 @@ class SourceTestHarness(PyAPITestHarness):
         settings.run_mode = 'fixed source'
         settings.source = source
         settings.export_to_xml()
- 
+
         particle_filter = openmc.ParticleFilter('photon')
         tally = openmc.Tally()
         tally.filters = [particle_filter]
diff --git a/tests/regression_tests/source/test.py b/tests/regression_tests/source/test.py
index 00171a255..cd468c5b8 100644
--- a/tests/regression_tests/source/test.py
+++ b/tests/regression_tests/source/test.py
@@ -14,7 +14,7 @@ class SourceTestHarness(PyAPITestHarness):
         materials = openmc.Materials([mat1])
         materials.export_to_xml()
 
-        sphere = openmc.Sphere(surface_id=1, R=10.0, boundary_type='vacuum')
+        sphere = openmc.Sphere(surface_id=1, r=10.0, boundary_type='vacuum')
         inside_sphere = openmc.Cell(cell_id=1)
         inside_sphere.region = -sphere
         inside_sphere.fill = mat1
diff --git a/tests/regression_tests/surface_tally/test.py b/tests/regression_tests/surface_tally/test.py
index 24dd66314..96199ec2a 100644
--- a/tests/regression_tests/surface_tally/test.py
+++ b/tests/regression_tests/surface_tally/test.py
@@ -25,8 +25,8 @@ class SurfaceTallyTestHarness(PyAPITestHarness):
         materials_file.export_to_xml()
 
         # Instantiate ZCylinder surfaces
-        fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, R=1, \
-             name='Fuel OR')
+        fuel_or = openmc.ZCylinder(surface_id=1, x0=0, y0=0, r=1,
+            name='Fuel OR')
         left = openmc.XPlane(surface_id=2, x0=-2, name='left')
         right = openmc.XPlane(surface_id=3, x0=2, name='right')
         bottom = openmc.YPlane(y0=-2, name='bottom')
diff --git a/tests/regression_tests/triso/test.py b/tests/regression_tests/triso/test.py
index 9ede6dbb0..3b858e6a7 100644
--- a/tests/regression_tests/triso/test.py
+++ b/tests/regression_tests/triso/test.py
@@ -44,7 +44,7 @@ class TRISOTestHarness(PyAPITestHarness):
         graphite.add_s_alpha_beta('c_Graphite')
 
         # Create TRISO particles
-        spheres = [openmc.Sphere(R=r*1e-4)
+        spheres = [openmc.Sphere(r=r*1e-4)
                    for r in [212.5, 312.5, 347.5, 382.5]]
         c1 = openmc.Cell(fill=fuel, region=-spheres[0])
         c2 = openmc.Cell(fill=porous_carbon, region=+spheres[0] & -spheres[1])
diff --git a/tests/regression_tests/volume_calc/test.py b/tests/regression_tests/volume_calc/test.py
index fda4b3321..da8399c1f 100644
--- a/tests/regression_tests/volume_calc/test.py
+++ b/tests/regression_tests/volume_calc/test.py
@@ -25,10 +25,10 @@ class VolumeTest(PyAPITestHarness):
         materials = openmc.Materials((water, fuel))
         materials.export_to_xml()
 
-        cyl = openmc.ZCylinder(1, R=1.0, boundary_type='vacuum')
-        top_sphere = openmc.Sphere(2, z0=5., R=1., boundary_type='vacuum')
+        cyl = openmc.ZCylinder(1, r=1.0, boundary_type='vacuum')
+        top_sphere = openmc.Sphere(2, z0=5., r=1., boundary_type='vacuum')
         top_plane = openmc.ZPlane(3, z0=5.)
-        bottom_sphere = openmc.Sphere(4, z0=-5., R=1., boundary_type='vacuum')
+        bottom_sphere = openmc.Sphere(4, z0=-5., r=1., boundary_type='vacuum')
         bottom_plane = openmc.ZPlane(5, z0=-5.)
 
         # Define geometry
diff --git a/tests/unit_tests/conftest.py b/tests/unit_tests/conftest.py
index 94fa6abc7..aa28be9c3 100644
--- a/tests/unit_tests/conftest.py
+++ b/tests/unit_tests/conftest.py
@@ -46,7 +46,7 @@ def cell_with_lattice():
     m_inside = [openmc.Material(), openmc.Material(), None, openmc.Material()]
     m_outside = openmc.Material()
 
-    cyl = openmc.ZCylinder(R=1.0)
+    cyl = openmc.ZCylinder(r=1.0)
     inside_cyl = openmc.Cell(fill=m_inside, region=-cyl)
     outside_cyl = openmc.Cell(fill=m_outside, region=+cyl)
     univ = openmc.Universe(cells=[inside_cyl, outside_cyl])
@@ -63,7 +63,7 @@ def cell_with_lattice():
 
 @pytest.fixture
 def mixed_lattice_model(uo2, water):
-    cyl = openmc.ZCylinder(R=0.4)
+    cyl = openmc.ZCylinder(r=0.4)
     c1 = openmc.Cell(fill=uo2, region=-cyl)
     c1.temperature = 600.0
     c2 = openmc.Cell(fill=water, region=+cyl)
diff --git a/tests/unit_tests/test_geometry.py b/tests/unit_tests/test_geometry.py
index b5d6076bb..4968c6fa3 100644
--- a/tests/unit_tests/test_geometry.py
+++ b/tests/unit_tests/test_geometry.py
@@ -10,8 +10,8 @@ def test_volume(run_in_tmpdir, uo2):
     # Create model with nested spheres
     model = openmc.model.Model()
     model.materials.append(uo2)
-    inner = openmc.Sphere(R=1.)
-    outer = openmc.Sphere(R=2., boundary_type='vacuum')
+    inner = openmc.Sphere(r=1.)
+    outer = openmc.Sphere(r=2., boundary_type='vacuum')
     c1 = openmc.Cell(fill=uo2, region=-inner)
     c2 = openmc.Cell(region=+inner & -outer)
     u = openmc.Universe(cells=[c1, c2])
@@ -44,8 +44,8 @@ def test_volume(run_in_tmpdir, uo2):
 
 
 def test_export_xml(run_in_tmpdir, uo2):
-    s1 = openmc.Sphere(R=1.)
-    s2 = openmc.Sphere(R=2., boundary_type='reflective')
+    s1 = openmc.Sphere(r=1.)
+    s2 = openmc.Sphere(r=2., boundary_type='reflective')
     c1 = openmc.Cell(fill=uo2, region=-s1)
     c2 = openmc.Cell(fill=uo2, region=+s1 & -s2)
     geom = openmc.Geometry([c1, c2])
diff --git a/tests/unit_tests/test_lattice.py b/tests/unit_tests/test_lattice.py
index 47dc1c029..fe86e2a78 100644
--- a/tests/unit_tests/test_lattice.py
+++ b/tests/unit_tests/test_lattice.py
@@ -7,7 +7,7 @@ import pytest
 
 @pytest.fixture(scope='module')
 def pincell1(uo2, water):
-    cyl = openmc.ZCylinder(R=0.35)
+    cyl = openmc.ZCylinder(r=0.35)
     fuel = openmc.Cell(fill=uo2, region=-cyl)
     moderator = openmc.Cell(fill=water, region=+cyl)
 
@@ -19,7 +19,7 @@ def pincell1(uo2, water):
 
 @pytest.fixture(scope='module')
 def pincell2(uo2, water):
-    cyl = openmc.ZCylinder(R=0.4)
+    cyl = openmc.ZCylinder(r=0.4)
     fuel = openmc.Cell(fill=uo2, region=-cyl)
     moderator = openmc.Cell(fill=water, region=+cyl)
 
diff --git a/tests/unit_tests/test_mesh_from_lattice.py b/tests/unit_tests/test_mesh_from_lattice.py
index 4d79fc579..feac87326 100644
--- a/tests/unit_tests/test_mesh_from_lattice.py
+++ b/tests/unit_tests/test_mesh_from_lattice.py
@@ -5,7 +5,7 @@ import pytest
 
 @pytest.fixture(scope='module')
 def pincell1(uo2, water):
-    cyl = openmc.ZCylinder(R=0.35)
+    cyl = openmc.ZCylinder(r=0.35)
     fuel = openmc.Cell(fill=uo2, region=-cyl)
     moderator = openmc.Cell(fill=water, region=+cyl)
 
@@ -17,7 +17,7 @@ def pincell1(uo2, water):
 
 @pytest.fixture(scope='module')
 def pincell2(uo2, water):
-    cyl = openmc.ZCylinder(R=0.4)
+    cyl = openmc.ZCylinder(r=0.4)
     fuel = openmc.Cell(fill=uo2, region=-cyl)
     moderator = openmc.Cell(fill=water, region=+cyl)
 
@@ -89,12 +89,12 @@ def rlat3(pincell1, pincell2, uo2, water, zr):
 def test_mesh2d(rlat2):
     shape = np.array(rlat2.shape)
     width = shape*rlat2.pitch
-    
+
     mesh1 = openmc.Mesh.from_rect_lattice(rlat2)
     assert np.array_equal(mesh1.dimension, (3, 3))
     assert np.array_equal(mesh1.lower_left, rlat2.lower_left)
     assert np.array_equal(mesh1.upper_right, rlat2.lower_left + width)
-    
+
     mesh2 = openmc.Mesh.from_rect_lattice(rlat2, division=3)
     assert np.array_equal(mesh2.dimension, (9, 9))
     assert np.array_equal(mesh2.lower_left, rlat2.lower_left)
diff --git a/tests/unit_tests/test_model_triso.py b/tests/unit_tests/test_model_triso.py
index 1d2e0905c..782373414 100644
--- a/tests/unit_tests/test_model_triso.py
+++ b/tests/unit_tests/test_model_triso.py
@@ -47,7 +47,7 @@ def centers_rectangular_prism():
 
 @pytest.fixture(scope='module')
 def centers_x_cylinder():
-    cylinder = openmc.XCylinder(R=1, y0=1, z0=2)
+    cylinder = openmc.XCylinder(r=1, y0=1, z0=2)
     min_x = openmc.XPlane(x0=0)
     max_x = openmc.XPlane(x0=1)
     region = +min_x & -max_x & -cylinder
@@ -57,7 +57,7 @@ def centers_x_cylinder():
 
 @pytest.fixture(scope='module')
 def centers_y_cylinder():
-    cylinder = openmc.YCylinder(R=1, x0=1, z0=2)
+    cylinder = openmc.YCylinder(r=1, x0=1, z0=2)
     min_y = openmc.YPlane(y0=0)
     max_y = openmc.YPlane(y0=1)
     region = +min_y & -max_y & -cylinder
@@ -67,7 +67,7 @@ def centers_y_cylinder():
 
 @pytest.fixture(scope='module')
 def centers_z_cylinder():
-    cylinder = openmc.ZCylinder(R=1, x0=1, y0=2)
+    cylinder = openmc.ZCylinder(r=1, x0=1, y0=2)
     min_z = openmc.ZPlane(z0=0)
     max_z = openmc.ZPlane(z0=1)
     region = +min_z & -max_z & -cylinder
@@ -77,7 +77,7 @@ def centers_z_cylinder():
 
 @pytest.fixture(scope='module')
 def centers_sphere():
-    sphere = openmc.Sphere(R=1, x0=1, y0=2, z0=3)
+    sphere = openmc.Sphere(r=1, x0=1, y0=2, z0=3)
     region = -sphere
     return openmc.model.pack_spheres(radius=_RADIUS, region=region,
         pf=_PACKING_FRACTION, initial_pf=0.2)
@@ -85,8 +85,8 @@ def centers_sphere():
 
 @pytest.fixture(scope='module')
 def centers_spherical_shell():
-    sphere = openmc.Sphere(R=1, x0=1, y0=2, z0=3)
-    inner_sphere = openmc.Sphere(R=0.5, x0=1, y0=2, z0=3)
+    sphere = openmc.Sphere(r=1, x0=1, y0=2, z0=3)
+    inner_sphere = openmc.Sphere(r=0.5, x0=1, y0=2, z0=3)
     region = -sphere & +inner_sphere
     return openmc.model.pack_spheres(radius=_RADIUS, region=region,
         pf=_PACKING_FRACTION, initial_pf=0.2)
@@ -94,7 +94,7 @@ def centers_spherical_shell():
 
 @pytest.fixture(scope='module')
 def triso_universe():
-    sphere = openmc.Sphere(R=_RADIUS)
+    sphere = openmc.Sphere(r=_RADIUS)
     cell = openmc.Cell(region=-sphere)
     univ = openmc.Universe(cells=[cell])
     return univ
@@ -179,10 +179,10 @@ def test_packing_fraction(container, centers):
 def test_num_spheres():
     """Check that the function returns the correct number of spheres"""
     centers = openmc.model.pack_spheres(
-        radius=_RADIUS, region=-openmc.Sphere(R=1), num_spheres=50
+        radius=_RADIUS, region=-openmc.Sphere(r=1), num_spheres=50
     )
     assert len(centers) == 50
-    
+
 
 def test_triso_lattice(triso_universe, centers_rectangular_prism):
     trisos = [openmc.model.TRISO(_RADIUS, triso_universe, c)
@@ -203,7 +203,7 @@ def test_container_input(triso_universe):
     # Invalid container shape
     with pytest.raises(ValueError):
         centers = openmc.model.pack_spheres(
-            radius=_RADIUS, region=+openmc.Sphere(R=1), num_spheres=100
+            radius=_RADIUS, region=+openmc.Sphere(r=1), num_spheres=100
         )
 
 
@@ -211,17 +211,17 @@ def test_packing_fraction_input():
     # Provide neither packing fraction nor number of spheres
     with pytest.raises(ValueError):
         centers = openmc.model.pack_spheres(
-            radius=_RADIUS, region=-openmc.Sphere(R=1)
+            radius=_RADIUS, region=-openmc.Sphere(r=1)
         )
 
     # Specify a packing fraction that is too high for CRP
     with pytest.raises(ValueError):
         centers = openmc.model.pack_spheres(
-            radius=_RADIUS, region=-openmc.Sphere(R=1), pf=1
+            radius=_RADIUS, region=-openmc.Sphere(r=1), pf=1
         )
 
     # Specify a packing fraction that is too high for RSP
     with pytest.raises(ValueError):
         centers = openmc.model.pack_spheres(
-            radius=_RADIUS, region=-openmc.Sphere(R=1), pf=0.5, initial_pf=0.4
+            radius=_RADIUS, region=-openmc.Sphere(r=1), pf=0.5, initial_pf=0.4
         )
diff --git a/tests/unit_tests/test_plots.py b/tests/unit_tests/test_plots.py
index d48b609c1..840e7e635 100644
--- a/tests/unit_tests/test_plots.py
+++ b/tests/unit_tests/test_plots.py
@@ -45,7 +45,7 @@ def test_repr(myplot):
 
 def test_from_geometry():
     width = 25.
-    s = openmc.Sphere(R=width/2, boundary_type='vacuum')
+    s = openmc.Sphere(r=width/2, boundary_type='vacuum')
     c = openmc.Cell(region=-s)
     univ = openmc.Universe(cells=[c])
     geom = openmc.Geometry(univ)
diff --git a/tests/unit_tests/test_region.py b/tests/unit_tests/test_region.py
index b623cd606..d1074fd7d 100644
--- a/tests/unit_tests/test_region.py
+++ b/tests/unit_tests/test_region.py
@@ -75,7 +75,7 @@ def test_intersection(reset):
 
 
 def test_complement(reset):
-    zcyl = openmc.ZCylinder(surface_id=1, R=1.)
+    zcyl = openmc.ZCylinder(surface_id=1, r=1.)
     z0 = openmc.ZPlane(surface_id=2, z0=-5.)
     z1 = openmc.ZPlane(surface_id=3, z0=5.)
     outside = +zcyl | -z0 | +z1
diff --git a/tests/unit_tests/test_surface.py b/tests/unit_tests/test_surface.py
index 45fecb493..6ed657a4c 100644
--- a/tests/unit_tests/test_surface.py
+++ b/tests/unit_tests/test_surface.py
@@ -13,7 +13,7 @@ def assert_infinite_bb(s):
 
 
 def test_plane():
-    s = openmc.Plane(A=1, B=2, C=-1, D=3, name='my plane')
+    s = openmc.Plane(a=1, b=2, c=-1, d=3, name='my plane')
     assert s.a == 1
     assert s.b == 2
     assert s.c == -1
@@ -138,7 +138,7 @@ def test_zplane():
 
 def test_xcylinder():
     y, z, r = 3, 5, 2
-    s = openmc.XCylinder(y0=y, z0=z, R=r)
+    s = openmc.XCylinder(y0=y, z0=z, r=r)
     assert s.y0 == y
     assert s.z0 == z
     assert s.r == r
@@ -175,7 +175,7 @@ def test_periodic():
 
 def test_ycylinder():
     x, z, r = 3, 5, 2
-    s = openmc.YCylinder(x0=x, z0=z, R=r)
+    s = openmc.YCylinder(x0=x, z0=z, r=r)
     assert s.x0 == x
     assert s.z0 == z
     assert s.r == r
@@ -200,7 +200,7 @@ def test_ycylinder():
 
 def test_zcylinder():
     x, y, r = 3, 5, 2
-    s = openmc.ZCylinder(x0=x, y0=y, R=r)
+    s = openmc.ZCylinder(x0=x, y0=y, r=r)
     assert s.x0 == x
     assert s.y0 == y
     assert s.r == r
@@ -228,7 +228,7 @@ def test_zcylinder():
 
 def test_sphere():
     x, y, z, r = -3, 5, 6, 2
-    s = openmc.Sphere(x0=x, y0=y, z0=z, R=r)
+    s = openmc.Sphere(x0=x, y0=y, z0=z, r=r)
     assert s.x0 == x
     assert s.y0 == y
     assert s.z0 == z
@@ -258,7 +258,7 @@ def test_sphere():
 
 def cone_common(apex, r2, cls):
     x, y, z = apex
-    s = cls(x0=x, y0=y, z0=z, R2=r2)
+    s = cls(x0=x, y0=y, z0=z, r2=r2)
     assert s.x0 == x
     assert s.y0 == y
     assert s.z0 == z
diff --git a/tests/unit_tests/test_universe.py b/tests/unit_tests/test_universe.py
index 89904524b..674fc5c18 100644
--- a/tests/unit_tests/test_universe.py
+++ b/tests/unit_tests/test_universe.py
@@ -34,8 +34,8 @@ def test_basic():
 
 
 def test_bounding_box():
-    cyl1 = openmc.ZCylinder(R=1.0)
-    cyl2 = openmc.ZCylinder(R=2.0)
+    cyl1 = openmc.ZCylinder(r=1.0)
+    cyl2 = openmc.ZCylinder(r=2.0)
     c1 = openmc.Cell(region=-cyl1)
     c2 = openmc.Cell(region=+cyl1 & -cyl2)