Merge remote-tracking branch 'upstream/develop' into hex_lattice

Conflicts:
	src/geometry.F90
	src/hdf5_summary.F90

docs/source/methods/cross_sections.rst

@@ -47,45 +47,28 @@ there would be for burnup calculations. Thus, there is a strong motive to
 implement a method of reducing the number of energy grid searches in order to
 speed up the calculation.
 
-Unionized Energy Grid
----------------------
+Logarithmic Mapping
+-------------------
 
-The most naïve method to reduce the number of energy grid searches is to
-construct a new energy grid that consists of the union of the energy points of
-each nuclide and use this energy grid for all nuclides. This method is
-computationally very efficient as it only requires one energy grid search at
-each collision as well as one interpolation between cross section values since
-the interpolation factor can be used for all nuclides. However, it requires
-redundant storage of cross section values at points which were added to each
-nuclide grid. This additional burden on memory storage can become quite
-prohibitive. To lessen that burden, the unionized energy grid can be thinned
-with cross sections reconstructed on the thinned energy grid. This method is
-currently used by default in the Serpent Monte Carlo code.
+To speed up energy grid searches, OpenMC uses logarithmic mapping technique
+[Brown]_ to limit the range of energies that must be searched for each
+nuclide. The entire energy range is divided up into equal-lethargy segments, and
+the bounding energies of each segment are mapped to bounding indices on each of
+the nuclide energy grids. By default, OpenMC uses 8000 equal-lethargy segments
+as recommended by Brown.
 
-Unionized Energy Grid with Nuclide Pointers
--------------------------------------------
+Other Methods
+-------------
 
-While having a unionized grid that is used for all nuclides allows for very fast
-lookup of cross sections, the burden on memory is in many circumstances
-unacceptable. The OpenMC Monte Carlo code utilizes a method that allows for a
-single energy grid search to be performed at every collision while avoiding the
-redundant storage of cross section values. Instead of using the unionized grid
-for every nuclide, the original energy grid of each nuclide is kept and a list
-of pointers (of the same length as the unionized energy grid) is constructed for
-each nuclide that gives the corresponding grid index on the nuclide grid for a
-given grid index on the unionized grid. One must still interpolate on cross
-section values for each nuclide since the interpolation factors will generally
-be different. The figure below illustrates this method. All values within the
-dashed box would need to be stored on a per-nuclide basis, and the union grid
-would need to be stored once. This method is also referred to as *double
-indexing* and is available as an option in Serpent (see paper by Leppanen_).
+A good survey of other energy grid techniques, including unionized energy grids,
+can be found in a paper by Leppanen_.
 
-.. figure:: ../_images/uniongrid.*
-    :width: 600px
-    :align: center
-    :figclass: align-center
+----------
+References
+----------
 
-    Mapping of union energy grid to nuclide energy grid through pointers.
+.. [Brown] Forrest B. Brown, "New Hash-based Energy Lookup Algorithm for Monte
+           Carlo codes," LA-UR-14-24530, Los Alamos National Laboratory (2014).
 
 .. _MCNP: http://mcnp.lanl.gov
 .. _Serpent: http://montecarlo.vtt.fi

docs/source/methods/geometry.rst

@@ -516,7 +516,7 @@ satisfy the following equations
 
     x^2 + y^2 + z^2 - 10^2 < 0 \\
     x - (-3) > 0 \\
-    x - 2 < 0
+    y - 2 < 0
 
 In order to determine if a point is inside the cell, we would substitute its
 coordinates into equation :eq:`cell-contains-example`. If the inequalities are

docs/source/methods/physics.rst

@@ -682,17 +682,20 @@ nuclear temperature, which is a function of the incoming energy of the
 neutron. The ACE format contains a list of nuclear temperatures versus incoming
 energies. The nuclear temperature is interpolated between neighboring incoming
 energies using a specified interpolation law. Once the temperature :math:`T` is
-determined, we then calculate a candidate outgoing energy based on rule C45 in
-the `Monte Carlo Sampler`_:
+determined, we then calculate a candidate outgoing energy based on the algorithm
+given in LA-UR-14-27694_:
 
 .. math::
     :label: evaporation-E
 
-    E' = -T \log (\xi_1 \xi_2)
+    E' = -T \log ((1 - g\xi_1)(1 - g\xi_2))
 
-where :math:`\xi_1, \xi_2` are random numbers sampled on the unit
-interval. The outgoing energy is only accepted according to a specified
-restriction energy as in equation :eq:`maxwell-restriction`.
+where :math:`g = 1 - e^{-w}`, :math:`w = (E - U)/T`, :math:`U` is the
+restriction energy, and :math:`\xi_1, \xi_2` are random numbers sampled on the
+unit interval. The outgoing energy is only accepted according to the restriction
+energy as in equation :eq:`maxwell-restriction`. This algorithm has a much
+higher rejection efficiency than the standard technique, i.e. rule C45 in the
+`Monte Carlo Sampler`_.
 
 ACE Law 11 - Energy-Dependent Watt Spectrum
 +++++++++++++++++++++++++++++++++++++++++++
@@ -1591,6 +1594,8 @@ References
 
 .. _Monte Carlo Sampler: https://laws.lanl.gov/vhosts/mcnp.lanl.gov/pdf_files/la-9721_3rdmcsampler.pdf
 
+.. _LA-UR-14-27694: http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-14-27694
+
 .. _MC21: http://www.osti.gov/bridge/servlets/purl/903083-HT5p1o/903083.pdf
 
 .. _Sutton and Brown: http://www.osti.gov/bridge/product.biblio.jsp?osti_id=307911