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Added description of absorption reactions in documentation.

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Paul Romano 2012-07-26 12:44:27 -04:00
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49
docs/source/methods/physics.rst
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@ -767,6 +767,17 @@ of the particle is changed also using the procedure in
Although inelastic scattering leaves the target nucleus in an excited state, no
secondary photons from nuclear de-excitation are tracked in OpenMC.
------------------------
:math:`(n,xn)` Reactions
------------------------
These types of reactions are just treated as inelastic scattering and as such
are subject to the same procedure as described in
:ref:`inelastic-scatter`. Rather than tracking multiple secondary neutrons, the
weight of the outgoing neutron is multiplied by the number of secondary
neutrons, e.g. for (n,2n), only one outgoing neutron is tracked but its weight
is doubled.
-------
Fission
-------
@ -842,20 +853,34 @@ position of the collision site are stored in an array called the fission
bank. In a subsequent generation, these fission bank sites are used as starting
source sites.
------------------------
:math:`(n,xn)` Reactions
------------------------
----------------------------------------------------
:math:`(n,\gamma)` and Other Disappearance Reactions
----------------------------------------------------
These types of reactions are just treated as inelastic scattering and as such
are subject to the same procedure as described in
:ref:`inelastic-scatter`. Rather than tracking multiple secondary neutrons, the
weight of the outgoing neutron is multiplied by the number of secondary
neutrons, e.g. for (n,2n), only one outgoing neutron is tracked but its weight
is doubled.
All absorption reactions other than fission do not produce any secondary
neutrons. As a result, these are the easiest type of reactions to handle. When a
collision occurs, the first step is to sample a nuclide within a material. Once
the nuclide has been sampled, then a specific reaction for that nuclide is
sampled. Since the total absorption cross section is pre-calculated at the
beginning of a simulation, the first step in sampling a reaction is to determine
whether a "disappearance" reaction occurs where no secondary neutrons are
produced. This is done by sampling a random number :math:`\xi` on the interval
:math:`[0,1)` and checking whether
-------------------------------------------------
:math:`(n,\gamma)` and Other Absorption Reactions
-------------------------------------------------
.. math::
:label: absorption-condition
\xi \sigma_t (E) < \sigma_a (E) - \sigma_f (E)
where :math:`\sigma_t` is the total cross section, :math:`\sigma_a` is the
absorption cross section (this includes fission), and :math:`\sigma_f` is the
total fission cross section. If this condition is met, then the neutron is
killed and we proceed to simulate the next neutron from the source bank.
No secondary particles from disappearance reactions such as photons or
alpha-particles are produced or tracked. To truly capture the affects of gamma
heating in a problem, it would be necessary to explicitly track photons
originating from :math:`(n,\gamma)` and other reactions.
.. _freegas: