Sequence Selection - s1027cn10033

DOE-STD-1027-92

the design of the plant systems, especially safety systems. Large power

reactors and very large chemical processing or petroleum refineries are

representative of this class of facilities.

For such facilities, the extensive use of event trees and fault trees is needed

to understand the potential release mechanisms. The specification of the

use of these techniques is due to the complex system interdependencies

found in such facilities. ET/FT is capable of clarifying these

interdependencies. The ET/FT technique involves defining initiating events

leading to process disturbance and constructing detailed ET and FT models

to represent plant response to various accident conditions resulting from

those disturbances. These techniques have been proven to be especially

useful in evaluating processes involving very complex systems with high

levels of integration and interdependency.

Connecting of the initiating event and ET and FT models in a structured

fashion is a proven technique capable of handling, in an efficient and

comprehensive fashion, the very complex nature of the system designs,

interactions, and dependencies prevalent in these processes. A large part of

the reason for selecting this technique is that the nature of the hazard is

straightforward, but its possible causes are numerous. For example,

insufficient cooling to the reactor core leads to the release of large quantities

of radionuclides from the core, but the causes of loss of coolant are many

and intricate. Thus, the emphasis on systems is a key benefit for evaluating

these processes; other techniques structured to consider the hazards

themselves (such as HAZOP) are not required.

Further, because the integrated nature of the processes results in a large

number of intricate combinations of failures which can lead to a release, the

probabilistic approach used is essential in determining which of these

combinations is necessary to consider in addressing consequences, for the

sheer number of them makes the use of engineering judgment more

complicated and less reliable.

Sequence Selection

As general guidance for Category 2 facilities, it is necessary to include a range of

accident conditions to adequately characterize the safety basis for the facility.

Accident sequences should be selected to provide insight into the hazards

associated with the facility. Because these accidents are used in establishing the

Technical Safety Requirements for the facility, their selection is very important.

The selection process should be based on the implementation of the analysis

techniques discussed in the previous section. The search should include higher-

probability unique events which pose hazards only to workers as well as unusual,

lower-probability events which include a reasonable maximum release from the

facility. Design basis natural phenomena should also be included in the range of