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If parameter studies are performed and the keff results are plotted, the curves should be smooth
and continuous. Discontinuities, outside of statistics, are generally, but not always, a signal of a
mistake in the model. Such discontinuities should be understood and explained before taking
them as indicators of errors.
The reviewer should be aware that apparently minor changes in calculational models could have
significant effects on the reactivity of the system. It is common for an analyst to gloss over
minor changes or variations when reporting results and still report large differences in keff. The
reviewer should seek to understand the underlying model changes that caused the reactivity
changes. Typical model variations to watch for in Monte-Carlo calculations include:
Boundary conditions (specular, albedo, vacuum, etc.);
Reflector type (types of concrete, water, polyethylene, etc.);
Reflector thickness;
Presence or absence of moderators;
Moderation type;
Minor spacing variations;
Number of neutron histories;
Neutron start distribution;
Reversed geometry;
Density variations; and
Mistakes in material compositions and locations.
These kinds of potential variations are particularly important to watch for when the CSE
references previous work. The CSE should clearly document why calculational results from
previous CSEs apply to the system currently being analyzed and why the variations, if any, exist
in the reactivity results for very similar models. The previous CSE should be summarized so that
the reviewer knows what was determined in the previous study and how it applies. There is no
need to produce a complete copy of the prior analysis results.
The reviewer should perform spot checks of the "worst case abnormal" models to see if they
match those described in Section 5.0. A common practice in Monte Carlo is to slightly change
the models for the "worst case" situations (i.e., to reduce or eliminate conservative assumptions)
to "tweak" the reactivity result under some arbitrary cutoff. The reviewer should be aware of this
tactic and assure himself that it is inconsequential if it occurs.
Key Review Issues
Calculational results for normal and abnormal scenarios are present and summarized.
Variations in keff from calculation to calculation are explained adequately in terms of model
changes corresponding to process variations.
A "sanity check" using handbooks, previous analyses and/or hand calculations is consistent
with reported results.
Results applied from previous CSEs are adequately justified.
No unexplained discontinuities are present in parameter studies.
Look for geometry plot to check the model configuration.

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