Production Plant GH

Expected frequencies of the accident scenarios for the GH2 tank at the production plant were synthesised from the component failure rate data associated with each of the identified failure mode, using the FTA approach (described in section 4.4.2). The accident scenarios of the GH2 storage can be classified into two Top events, i.e. instantaneous and continuous. Therefore two fault tree diagram were developed for the Top events.

Fig. G1.1 (Appendix G) shows the fault tree diagram for the instantaneous release (e.g. tank rupture) of GH2 storage at the production plant. The undesired events which may contribute to this Top events include: (1) tank excessive over pressure while all the pressure relief devices fail closed; (2) an external events (e.g. Earthquake, mechanical impacts, etc); and (3) spontaneous events (e.g. H2 embrittlement, fatigue failure, etc). The tank overpressure may be caused by tank overfilling or an external heat source (e.g. fire around the installation). Due to inadequate information on the basic events the last two undesired events are presented as undeveloped events. The reliability data for these events have been estimated from similar studies, and they require judgment from the experts. The expected frequency of the scenario is estimated by assigning the failure data (Table G1.1.1) to each basic events of the FTA (Fig. G1.1). Summary of the FTA result is shown in the Appendix G. It includes lists of minimal cut sets, expected frequencies of the initiating events considered, and the top event frequency and its uncertainties for 10,000 trial numbers.

The fault tree related to the second Top event was a continuous release of hydrogen from the tank and its piping system. The fault tree diagram was fully reproduced, and is shown in Fig. G1.2 (Appendix G). The events may contribute to this Top events includes: (1) hydrogen release at venting circuits, (2) release from piping system, (3) release from holes of the tank or vessel, or drain valve fails open. A continuous release through venting system may be occurred due to the PRDs (i.e. rupture disk or pressure safety valve) fail open prematurely. Pipe rupture was considered as the continuous release of hydrogen. It may result in great damage to environment. This may occur if the associated emergency (remotely operated) shutoff valves fail to close. Hydrogen release due to tank leak or drain valve failure was represented as an undeveloped event, because information about it is insufficient. By assigning the failure rate data (Table G1.2.1) to each basic events of the fault tree the Top event frequency is estimated with the same procedure as above. The summary of the FTA result is presented in the Appendix G.

Table 5-8 Expected frequencies of the GH2 storage at production plant(1/yr)

Release Type

Distribution parameters

K-95

Percentage

5%

50%

Mean

95%

Instantaneous

4.8E-09

1.7E-07

1.8E-06

6.2E-06

35.8

5.0%

Continuous

1.9E-06

1.5E-05

3.4E-05

1.2E-04

14.2

95.0%

Overall

1.8E-06

1.5E-05

3.6E-05

1.3E-04

8.6

100.0%

Table 5-9 Accident outcome frequencies of the GH2 Storage at production plant(1/yr)

Release Scenarios

Accident Outcomes

Conditional Probability

5%

50%

Mean

95%

Instantaneous

Early explosion

0.008

1.4E-08

1.1E-07

2.7E-07

9.8E-07

Fireball

0.030

5.4E-08

4.5E-07

1.1E-06

3.9E-06

Late Explosion

0.000

4.1E-10

3.4E-09

8.1E-09

2.9E-08

Flash Fire

0.001

1.6E-09

1.4E-08

3.2E-08

1.2E-07

Continuous

Jet Fire

0.475

8.5E-07

7.1E-06

1.7E-05

6.2E-05

Late Explosion

0.043

7.7E-08

6.4E-07

1.5E-06

5.6E-06

Flash Fire

0.171

3.1E-07

2.6E-06

6.1E-06

2.2E-05

No effect

0.273

4.9E-07

4.1E-06

9.8E-06

3.5E-05

Overall

1.000

8.6E-06

2.8E-05

3.6E-05

9.0E-05

The summary of the fault tree analyses results for the GH2 storage plant with the trial number of 10,000 is shown in Table 5-8. The table shows that the expected frequency of the overall system is 3.6 x 10-5/year (once per 27,777 years), with the contribution of instantaneous and continuous release of hydrogen from the GH2 tank is 1.8 x 10-6 /year (once per 555,556 years) and 3.4 x 10-5 /year (once per 29,412 years), respectively. The table also shows that only 5% of the hydrogen release from the GH2 storage occurs instantaneously.

The accident outcome frequencies of the two initial frequencies above were calculated by multiplying the expected frequencies with the outcome probabilities calculated from the event tree diagram for GH2 release (Fig. 4.6). The result is shown in Table 5-9. The table shows that fire outcomes (with account for about 67%) are more dominant than explosion. Only about 7% of the accident outcome may result in an explosion, and the rest (26%) may have no effect on the population.

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