NPH Design and Evaluation Philosophy

DOE-STD-1020-2002

hazards (i.e., performance goal) is a combined function of the annual probability of exceedance

of the event, factors of safety introduced by the design/evaluation procedures, and other sources

of conservatism. These criteria specify hazard annual probabilities of exceedance, response

evaluation methods, and permissible behavior criteria for each natural phenomena hazard and for

each performance category such that desired performance goals are achieved for either design or

evaluation. The ratio of the hazard annual probability of exceedance and the performance goal

annual probability of exceedance is called the risk reduction ratio, RR. This ratio establishes the

level of conservatism to be employed in the design or evaluation process. For example, if the

performance goal and hazard annual probabilities are the same (RR = 1), the design or evaluation

approach should introduce no conservatism. However, if conservative design or evaluation

approaches are employed, the hazard annual probability of exceedance can be larger (i.e., more

frequent) than the performance goal annual probability (RR > 1). In the criteria presented herein,

the hazard probability and the conservatism in the design/evaluation method are not the same for

earthquake, wind, and flood hazards. However, the accumulated effect of each step in the

design/evaluation process is to aim at the performance goal probability values which are

applicable to each natural phenomena hazard separately.

Design and evaluation criteria are presented in Chapters 2, 3, and 4 for earthquake,

wind, and flood hazards, respectively. These criteria are deterministic procedures that establish

SSC loadings from probabilistic natural phenomena hazard curves; specify acceptable methods

for evaluating SSC response to these loadings; provide acceptance criteria to judge whether

computed SSC response is acceptable; and to provide detailing requirements such that behavior

is as expected as illustrated in Figure B-1. These criteria are intended to apply for design of new

facilities and for evaluation of existing facilities. In addition, the criteria are intended to cover

buildings, equipment, distribution systems (piping, HVAC, electrical raceways, etc.), and other

structures.

DOE-STD-1020 primarily covers (1) methods of establishing load levels on SSCs from

natural phenomena hazards and (2) methods of evaluating the behavior of structures and

equipment to these load levels. These items are very important, and they are, typically,

emphasized in design and evaluation criteria. However, there are other aspects of facility design

that are equally important and that should be considered. These aspects include quality

assurance considerations and attention to design details. Quality assurance requires peer review

of design drawings and calculations; inspection of construction; and testing of material strengths,

weld quality, etc. The peer reviewers should be qualified personnel who were not involved in

the original design. Important design details include measures to assure ductile behavior and to

provide redundant load paths, as well as proper anchorage of equipment and nonstructural

building features. Although quality assurance and design details are not discussed in this report

to the same extent as NPH load levels and NPH response evaluation and acceptance criteria, the

importance of these parts of the design/evaluation process should not be underestimated. Quality

assurance and peer review are briefly addressed in Section 1.4, in addition to discussions in the

individual chapters on each natural phenomena hazard. Design detailing for earthquake and

wind hazards is covered by separate manuals. Reference B-5 describes earthquake design

B-2