Appendix A: Terminology and Definitions

DOE-STD-1020-2002

Appendix A

Terminology and Definitions

Note: - Definitions common to DOE O 420.1, the accompanying Guide, this Standard,

and other NPH Standards are contained in the NPH Guide.

Annual Probability of Exceedance - The likelihood of natural phenomena hazards

must be evaluated on a probabilistic basis for this performance goal based NPH criteria. The

frequency of occurrence of parameters describing the external hazard severity (such as

earthquake ground acceleration, wind speed, or depth of inundation) is estimated by probabilistic

methods. Common frequency statistics employed for rare events such as natural phenomena

hazards include return period and annual probability of exceedance. Return period is the average

time between consecutive events of the same or greater severity (for example, earthquakes with

maximum ground acceleration of 0.2g or greater). It must be emphasized that the return period

is only an average duration between events and should not be construed as the actual time

between occurrences, which would be highly variable. A given event of return period, T, is

equally likely to occur any year, thus the probability of that event being exceeded in any one

year is 1/T. The annual probability of exceedance, p, of an event is the reciprocal of the return

period of that event (i.e., p = 1/T). As an example, consider a site at which the return period for

an earthquake of 0.2g or greater is 1000 years. In this case, the annual probability of exceedance

of 0.2g is 10-3 or 0.1 percent.

It is of interest in the design of facilities to define the probability that an event will be

exceeded during the design life of the facilities. For an event with return period, T, and annual

probability of exceedance, p, the exceedance probability, EP, over design life, n, is given by:

EP = 1 - (1 - p ) = 1 - (1 - 1 / T ) ≈ 1 - e

(A-1)

-n/T

Where EP and p vary from 0 to 1, and n and T are expressed in years. As an example,

the exceedance probabilities over a design life of 50 years of a given event with various annual

probabilities of exceedance are as follows:

EP over 50 years

10-3

0.05

10-4

0.005

10-5

0.0005

A-1