In order to establish that a proposed system or process will be subcritical under the normal
conditions and under postulated process upset conditions, it is first necessary to establish
acceptable subcritical values for the operation and then show that the proposed operation will not
exceed those values. There are currently four acceptable methods for the establishment of these
values. They are:
By reference to national standards that present Subcritical Limits.
By reference to widely accepted handbooks of Subcritical Limits.
By reference to experiments with appropriate adjustments to ensure subcriticality when the
uncertainties of parameters reported in the experiment documentation are considered.
By validated calculational techniques.
The method used in a given evaluation must be supported in the text of this section.
When employing methods 1 or 2, simply provide the reference giving the Subcritical Limit. When
employing method 3, provide the references giving the critical parameters, and fully explain your
consideration of uncertainty in the reported critical parameters when determining limits.
When employing method 4, indicate the specific methods that were used in the assessment of
subcriticality. References should be provided to allow a reviewer the opportunity to further
research the methods used in the evaluation. It is not necessary to describe the theory behind any
calculational methods used. (This should be done by reference.) It is only necessary to indicate
what methods were used.
Examples of calculational methods are: the three-dimensional Monte Carlo code, KENO-V.a or
MCNP; the one-dimensional Sn discrete-ordinates transport theory code, ANISN; and hand
calculation methods such as limited surface density, density analog, or solid angle methods.
When applicable, identify the nuclear cross section data that were used and the methods used to
process these data.
Compliance with Section 4.3, "Validation of a Calculational Method," of ANS-8.1 should also be
demonstrated in this section. Reference can be made to more detailed validation reports;
however, the results of these validation reports should be summarized, the method used to
determine the bias (if any) should be described, and the overall conclusion(s) of validation efforts
and how the conclusions were applied to the evaluation results should be stated.
When computer neutronics calculations were used, the type of computing platform should be
stated along with relevant code configuration control information. This information may be
provided by reference.