In accordance with the definition of Evaluation Guideline (EG) provided by this standard,
this appendix specifies a numerical radiological dose value to be used in identifying safety-
class (SC) Systems, Structures, and Components (SSC). Calculation methods and
assumptions needed to provide general consistency in dose estimation are also described,
with relevant background and interpretation discussions included as appropriate.
The methodology provided by this standard focuses on characterizing facility safety with
or without well-documented design information. The EG construct as described in this
appendix is primarily intended for use with existing facilities. Discussions relevant to new
facilities are provided in the Implementation Guide for Section 4.1 (Nuclear and Explosive
Safety Design Criteria) of DOE Order 420.1, "Facility Safety."
The EG is 25 rem total effective dose equivalent (TEDE). The dose estimates to be
compared to it are those received by a hypothetical maximally-exposed offsite individual
(MOI) at the site boundary for an exposure duration of 2 hours. The nominal exposure
duration of 2 hours may be extended to 8 hours for those release scenarios that are
especially slow to develop. Dose calculations for comparison against the EG are based on
the concept of an unmitigated release to determine whether the potential level of hazard in
the specific facility warrants SC SSC designation (see Section A.3.1 for details).
The value of 25 rem TEDE is not to be used as a `hard' pass/fail level. Unmitigated
releases should be compared against the EG to determine whether they challenge the
EG, rather than exceed it. This is because consequence calculations are highly
assumption driven and uncertain.
It should be made clear that the EG is not to be treated as a design acceptance criterion, nor
as justification for nullifying the general design criteria relative to defense- in-depth safety
measures. The value of 25 rem TEDE is not considered an acceptable public exposure
either. It is, however, generally accepted as a value indicative of no significant health
effects (i.e., low risk of latent health effects and virtually no risk of prompt health effects).
There is no predetermined frequency cutoff value, such as 1 E-6 per year, for exclud ing
low frequency operational accidents (i.e., internally initiated). In fact, for operational
accidents there is no explicit need for a frequency component to the unmitigated release
calculations, since the determination of need is solely driven by the bounding consequence
potential. Per the body of this Standard, natural events are defined in terms of the
frequency of the initiating event, while external events (i.e., externally initiated man- made
events) are defined with a cutoff frequency of 10-6 per year, conservatively calculated, or