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Key Review Issues
Ensure any referenced material in this section is available and valid.
4.0 Methodology
This section of the evaluation describes how the subcritical limits and controls for the process or
operation were obtained or derived. This section should contain a brief summary of the
method(s) relied upon to develop the subcritical limits on the process. The preferred method, as
stated in ANSI/ANS-8.1, is by reference to experiments or "accepted" published subcritical
limits. The alternative is to perform hand calculations or, more probably, computer calculations.
Detailed documentation of the verification and validation of computer codes need not be
included in this section. A reference to an existing validation document is sufficient. The
calculations themselves may reside in referenced documents so long as the documentation trail is
Review of this section is very straightforward if single parameter subcritical limits are used from
ANSI/ANS-8.1. These will invariably be conservative and only the value from the standard need
be quoted and a brief (i.e., a sentence or two) reason for why the limit is applicable.
If other handbook data is cited, the reviewer needs to be aware of some potential pitfalls. In
general, subcritical limits may be taken from ARH-600, LA-10860, TID-7016, etc. The data in
ARH-600 are from calculations that did not have extensive peer review but are nevertheless very
helpful in understanding sensitivity of controlled parameters. Although some caution is prudent,
the reference is nevertheless helpful. The data in LA-10860 and TID-7016 are mostly taken
from critical experiments, however, calculational results appear in spots. The intent is to
establish subcritical limits using critical data by applying an appropriate safety margin. A typical
rule of thumb is that the maximum subcritical value will not exceed 90% of the critical value
after a contingency occurs. Note that the minimum subcritical value does not correspond to a keff
of 0.95, or any other particular value of keff. In practice, values taken from these industry
accepted-handbooks, with appropriate allowances for uncertainty, are roughly equivalent, with
regards to establishing safety margins, to the single parameter limits contained in ANSI/ANS-
The trickiest analysis to review is one based on hand calculations. This is so because of the
many assumptions implicit in the method. Hand calculations are, in general, extrapolations from
known critical data to a specific application. The reviewer should be very familiar with the
hand-calculation method being used. The reviewer should be alert for mis-application and
incorrect mathematics. This section should include a reference to the publication or technical
data on which the hand calculation is based. If done properly within the area of applicability for
the method, hand calculations will yield conservative results. The reviewer should spot-check
the mathematics and review the reference against the model to assure compatibility.

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