Event Scenario Identification and Classification

DOE-STD-6003-96

h. initiating events in auxiliary systems [e.g., neutral beams, radio frequency (RF),

pumping, and fueling];

initiating events in balance of plant systems (e.g., loss of off-site power);

operator errors; and

k. external events.

The initiating events should consider all aspects of fusion facility operation, including

plasma operation, bakeout and conditioning, and maintenance. Because fusion facilities operate

in modes that are different from other facilities (e.g., bakeout and conditioning, in pulsed mode

for some machines), these potential plant states should be examined carefully. In the develop-

ment of these postulated initiating events, completeness is somewhat problematic. Practical

completeness can then best be achieved by collective review of the results by safety analysts

and designers who understand the facility.

From these postulated initiating events, event scenarios should be developed that exam-

ine the response of the fusion facility to these initiating events, accounting for potential failure of

other systems (e.g., confinement). The use of event trees or event sequence diagrams may be

useful here. The event scenarios should span a wide range of expected frequencies, including

those events expected to occur once or more during the operating life of the facility [i.e., an

anticipated operational occurrence (f > ~102/yr); those events not expected to occur, but may

occur, during the life of the plant (102/yr > f > 104/yr); those events that would not be expected

to occur during the life of the plant but which form the limiting events needed in the design basis

(104/yr > f > 106/yr); and events beyond the design basis (f < ~106/yr)].

Once the events have been developed, they should be categorized into three types based

on their estimated frequency: anticipated operational occurrence, off-normal conditions, and

beyond-design-basis events. The off-normal conditions type includes both the anticipated

operational occurrences and events expected to occur once or more during the lifetime of the

facility. Based on these events, bounding or limiting events of each kind (e.g., loss of flow, loss

of coolant, and loss of vacuum) should then be selected for detailed quantitative analysis.

Two types of analysis methodologies should be used for the safety assessment for the

fusion facilities: a deterministic, conservative approach and a best-estimate, realistic approach.

Each type of analysis methodology is required for a different portion of the required safety

assessment. The deterministic, conservative approach is to be used in the design-basis assess-

ment for the SAR to ensure that a bounding estimate of the facility safety is determined. The

best-estimate, realistic approach is to be used for analysis of beyond-design-basis events for

the SAR and in the determination of the emergency planning assessment. However, a conser-

vative risk-based approach can be used in place of the deterministic conservative approach

since either approach would satisfy the intent of performing a conservative safety assessment.

These are discussed in the next two sections.