CHP Plant LH

Similar to the previous plant (5.3.2) the expected frequency of the accident scenarios considered for the LH2 storage at the CHP plant was synthesised from the component failure data associated with each of the identified failure modes, using the FTA approach. Three fault tree diagrams were fully reproduced, and are presented in Appendix G4. They include: (1) instantaneous release of hydrogen (Fig. G4.1); (2) continuous release of hydrogen in the liquid phase (Fig. G4.2); and (3) continuous release of hydrogen in the vapour phase (Fig. G4.3).

In general, the fault trees are similar to those for previous object (as described for the LH2 storage plants). They are different as to component types and their arrangements. In order to protect it against overpressure the tank is equipped with two set of safety valves and rupture disks are installed in parallel. Therefore, the fault tree introduces common cause failure (CCF) events. Besides, to protect the tank from underpressure rupture (implosion), the tank is equipped with pressure control PCV-2 that closes the outlet lines automatically (i.e. it is triggered by TSL) in case the tank pressure is low.

The expected frequencies for the LH2 at CHP plant were estimated by assigning to each basic event of the fault trees the associated equipment failure rates or probabilities given in Table G4 (Appendix G). Summary of the expected frequency results is presented in the Appendix G4, and is also shown in Table 5-14.

Table 5-14 Expected frequencies of the LH

Table 5-14 Expected frequencies of the LH

storage at CHP plant(/yr)

Failure modes

5%

50%

Mean

95%

K-95

Percentage

Instantaneous release

3,9E-07

3,1E-06

6,9E-06

2,5E-05

8,0

4,8%

Liquid continuous release

8,3E-07

1,6E-05

8,4E-05

3,2E-04

19,7

58,8%

vapour continuous release

2,3E-07

6,6E-06

5,2E-05

1,9E-04

28,3

36,4%

Overall frequency (/yr)

5,1E-06

5,2E-05

1,4E-04

5,4E-04

10,3

100,0%

storage at CHP plant(/yr)

Table 5-15 Accident outcome frequencies of the LH2 Storage at CHP plant (/yr)

Release Scenarios

Accident Outcomes

Conditional Probability on Release

5%

50%

Mean

95%

Instantaneous

Early explosion

0,0087

4,4E-08

4,5E-07

1,2E-06

4,7E-06

Fireball

0,0348

1,8E-07

1,8E-06

5,0E-06

1,9E-05

Pool fire

0,0007

3,8E-09

3,9E-08

1,1E-07

4,0E-07

Late explosion

0,0000

1,7E-11

1,7E-10

4,8E-10

1,8E-09

Flash fire

0,0000

6,8E-11

7,0E-10

1,9E-09

7,3E-09

Continuous

Jet Fire

0,4759

2,4E-06

2,5E-05

6,8E-05

2,6E-04

Pool fire

0,0761

3,9E-07

4,0E-06

1,1E-05

4,1E-05

Late Explosion

0,0017

8,7E-09

8,9E-08

2,4E-07

9,3E-07

Flash Fire

0,0069

3,5E-08

3,6E-07

9,8E-07

3,7E-06

No effect

0,3927

2,0E-06

2,0E-05

5,6E-05

2,1E-04

Overall frequency (/yr)

0,9975

2,4E-05

9,9E-05

1,4E-04

4,0E-04

Table 5-14 shows that the overall expected frequency of the loss of containment of hydrogen from the LH2 tank at the CHP plant is 4.3 x 10-5 /year (once per 23,148 years), with the instantaneous and continuous release are 6.2 x 10-6 /year (once per 161,290 years) and 5.7 x 10-5 /year (once per 27,027 years), respectively. The table also shows that only 14.4% of the release from the LH2 tank may occur instantaneously, and the rest (85.6%) may release continuously.

The accident outcome frequencies of the above hydrogen release incident were calculated by multiplying the expected frequencies (Table 5-14) with the outcome probabilities resulted from the event tree diagram for LH2 release (Fig. 4.5). The result is shown in Table 5-15. The table shows that the fire mostly dominate of the accidents outcomes with account for about 61%, and only about 3% of the accident may result in explosion. The rest (about 36%) of the accidents may result no effect to the population.

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