
If the weighting function is very well known, you can do good
transport calculations with very few groups, but with modern
computers, the trend has been to use more and more groups.
GROUPR contains a number of builtin options for group structures
(some of them of mostly historical interest), and the user can
enter any arbitrary structure that matches his or her needs.
Reactor physicists have always been fond the the variable called
"lethargy," with is defined as follows:
Lethargy increases as neutrons slow down. Note that in the
slowingdown region, where the flux varies like 1/E,
groups with constant lethargy width all contain the same
portion of the flux. In elastic scattering, neutrons loose
a fraction of their energy with each scattering; thus, the
lethargy increases by a fixed amount with each scattering.
Because lethargy is such a natural variable for neutron
slowingdown problems, many group structures are based on
groups with certain lethargy widths. For example,
 the LANL 30group structure uses "one lethargy" widths
in the resonance range,
 the ANL 27group structure uses even halflethargy steps, and
 the GAMI 68group structure uses even quarterlethargy steps.
Some group structures start with even lethargy groups and
then subdivide some groups that contain especially interesting
structure, such as the Fe56 resonance at 27 keV. In other
cases, the group structure starts with even lethargy groups
and then moves some bounds to try to center important
resonances (e.g., the WIMS 69group structure).
Another factor when choosing a group structure is its range.
Many group structures intended for reactor analysis stop at
10 MeV. They are clearly not suitable for fusion problems.
Other group structures only provide a few groups in the
thermal range, and they are most suitable for fastreactor
and fusion problems.
The neutron group structures currently available in GROUPR are as
follows:
 IGN=2, CSEWG 240group structure, a "supergroup" structure
containing bounds from many other structures. Not widely
used these days.
 IGN=3, LANL 30group structure, used for fusion problems.
Still in active use.
 IGN=4, ANL 27group structure, even halflethargy groups used
for fastreactor analysis after collapse from more detailed
structures.
 IGN=5, RRD 50group structure, a standard fastreactor set
that is now obsolete.
 IGN=6, GAMI 68group structure, even quarterlethargy set
used for the epithermal part of thermal reactor codes (most
recently, EPRICELL).
 IGN=7, GAMII 100group structure, an extension to higher
energies of GAMI. Note widely used these days.
 IGN=8, LASERTHERMOS 35group structure, a thermal structure
for the low energy part of powerreactor codes (most recently
EPRICELL).
 IGN=9, EPRICPM 69group structure, the WIMS structure, still
widely used all over the world for reactor calculations.
 IGN=10, LANL 187group structure, a single set of finelethargy
bounds suitable for thermal reactors, fast reactors, and fusion
reactors.
 IGN=11, LANL 70group structure, another standard fastreactor
set, now supplanted by the 80group structure.
 IGN=12, SANDII 620group structure, very fine group structure
used for flux unfolding applications. Mostly supplanted by the
640group structure.
 IGN=13, LANL 80group structure, optimized for fastreactor and
fusion systems. Still in use through MATXS files.
 IGN=14, EURLIB 100group structure, similar to GAMI except
for additional highenergy groups and groups near the iron
resonance. Used for a number of NEA exercises.
 IGN=15, SANDII 640group structure, used for dosimetry work
with flux unfolding codes. It extends the 620group structure
to higher energies.
 IGN=16, VITAMINE 174group structure, used for the widely
distributed finegroup library from Oak Ridge.
 IGN=17, VITAMINJ 175group structure, an extension of the
174group structure. This structure is currently widely
used in the fusion community, e.g., ITER design.
