study on the issues about hydrogen explosion at fukushima...
TRANSCRIPT
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Japan Nuclear Energy Safety Organization
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Study on the Issues about Hydrogen Explosion at Fukushima
Dai-ichi NPS
Masao Ogino Nuclear Energy System Safety Division
Japan Nuclear Energy Safety Organization
Technical Workshop on the Accident of TEPCO’s Fukushima Dai-ichi NPS
July 2012
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❏ Background
❏ Hydrogen Explosion at Unit 1 and Unit 3
❏ Cause of explosion at R/B of Unit 4
❏ Summary and Remarks
Outline
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Japan Nuclear Energy Safety Organization
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Background
❏ Hydrogen explosion occurred in the R/Bs of Unit 1, Unit 3 and Unit 4 during accident progression at Fukushima Dai-ich NPS in March 2011.
❏ R/B has safety functions of confinement to protect from FP release to the environment as well as installment of many safety equipments in it.
❏ Explosion in the R/B made obstacles against operators to mitigate the accident at the site.
❏ Investigate the cause of hydrogen explosion in the R/B to investigate Fukushima accident and to study countermeasures for such hydrogen explosion.
❏ In this presentation, describe about the hydrogen explosion at Unit 1 and Unit 3 at first, then the cause of explosion at Unit 4.
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Hydrogen Explosion at Unit 1 and Unit 3
❏ Investigate the cause of hydrogen explosion at Unit 1 and Unit 3 by explosion analyses comparing with the existing conditions of R/B after explosion.
❏ Total amount of hydrogen gas released into R/B from
PCV and its released rate were used referring the accident progression analysis results of Unit 1 and Unit 3 by severe accident analysis code MELCOR.
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Hydrogen Explosion Analysis
(4F floor)
(5F floor)
Mixing model and explosion model
❏ Purpose: Investigate what exactly has happened in the reactor
buildings
❏ Approach: - Mixing analysis with FLUENT code - Explosion (detonation) analysis with AUTODYN code
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[m/s]
PCV top flange leak
Hydrogen gas flow velocity
Released hydrogen gas accumulated evenly in the top floor (5F) with high concentration of about 20vol%
Released flow rate of hydrogen gas was 100kg/h for 4 hours
0%
5%
10%
15%
20%
25%
0 1 2 3 4 5 6
経過時間 hour
平均水素モル分率
5F
4F
3F
2F
1FMole
fra
ction
Time (h)
Hydrogen gas conc.
Hydrogen mixing in R/B (Unit 1)
【Assumptions】
Hydrogen gas of 400kg released to the top floor (5F) of R/B from the PCV top flange
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PCV leak to 1F floor
Hydrogen gas distribution
Mole fraction (-)
0%
5%
10%
15%
20%
25%
30%
35%
0 1 2 3 4 5 6 7 8
経過時間 hour
平均水素モル分率
5F
4F
3F
2F
1F16.5%(1F)
Time (h)
Released flow rate of hydrogen gas was 200kg/h for 5 hours
Mole
fra
ction
Hydrogen gas conc.
Released hydrogen gas accumulated evenly in the whole of R/B with high concentration of 16-17vol%
Hydrogen mixing in R/B (Unit 3)
【Assumptions】
Hydrogen gas of 1000kg released to the first floor(1F)of R/B from PCV penetration, S/C or etc.
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t = 100 (ms)
[m/s]
t = 100 (ms)
[m/s]
Propagation of detonation pressure in the reactor building of Unit 1
Walls and roofs of top floor (5F) were largely destroyed and debris scattered around the R/B
Good simulation of the actual explosion at Unit 1
【Assumption】 Hydrogen gas of 400kg released to the top floor (5F)of R/B
Hydrogen explosion in R/B (Unit 1)
(福島中央テレビホームページ) (東京電力ホームページ)
Detonation analysis with AUTODYN:
圧力コンター図(10~100ms) t = 20 (ms) t = 100 (ms)
[kPa]
t = 10 (ms) 圧力コンター図(10~100ms) t = 20 (ms) t = 100 (ms)
[kPa]
t = 10 (ms)
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Propagation of detonation pressure in the reactor building of Unit 3
[m/s]
(福島中央テレビホームページ)
(東京電力ホームページ)
Walls and roofs of top floor(5F) and 4F were largely destroyed and debris scattered around the R/B
Good simulation of the actual explosion at Unit 3
【Assumption】 Hydrogen gas of 1000kg released to the first floor (1F)of R/B
Hydrogen explosion in R/B (Unit 3)
Detonation analysis with AUTODYN:
t = 100 (ms) t = 35 (ms) t = 5 (ms)
[kPa]
t = 100 (ms) t = 35 (ms) t = 5 (ms)
[kPa]
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-200
-150
-100
-50
0
50
100
-200 -150 -100 -50 0 50 100 150 200
North-South (m)
West-
East
(m)
West Side Wall (Center)
West Side Roof (Center)
1F3
7sec Wind
Debris
Debris
120m
Web:福島中央テレビ
Scattered debris on the ground around Unit 3
❏ Distribution of scattered debris of walls and roofs of the west side of Unit 3 agreed with the actual distribution.
❏ Debris with initial velocity of 70m/s can reach to about 250m height from the top of R/B in 7 seconds.
❏ Agreed with the actual phenomena observed
at the Unit 3.
Debris with initial velocity of 20 m/s to 70 m/s
[m/s] ~70m/s
Hydrogen explosion and debris scattering (Unit 3) TV broadcast picture
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❏ Analyzed R/B hydrogen explosion phenomena during
Fukushima Dai-ichi accident with FLUENT code and
AUTODYN code getting insight of the R/B hydrogen
explosion at Unit 1and Unit 3.
❏ Estimation by the hydrogen explosion analysis:
- Hydrogen gas(~400kg) leaked from PCV upper flange
and exploded in the top floor of R/B (Unit 1)
- Hydrogen gas(~1,000kg) leaked to the1F of R/B
and exploded in the whole R/B (Unit 3)
(Observed high radiation around the equipment hatch (1F)
by inspection )
Summary of R/B hydrogen explosions
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Cause of explosion at R/B of Unit 4
❏To investigate the cause of explosion in the R/B of Unit 4 by evaluating analytically reverse transportation of the vented gas from Unit 3 to the R/B of Unit 4 through vent lines.
❏To survey the way of inspection to confirm the
analytical results
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❏ Containment venting flow rate of Unit 3
Time (h)
0
5
10
15
20
25
0 50 100 150 200
Gas
flow
rate
(kg
/s)
Mass flow rate of containment venting (Unit 3)
- MELCOR results(*)
- Vent gas flow rate:
0~ 20kg/s
(*)Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety - IV. Occurrence and Development of the Accident at the Fukushima Nuclear Power Stations, June 2011.
Containment venting at Unit 3
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- Analysis code: FLUENT
- Mesh Model: 237,600meshes
- Stuck height: 120m
❏ CFD model of containment venting line
Analytical Model of Containment Vent Line
Junction
Vent flow
Stuck
Unit 3 Unit 4
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・ All of vented gas from Unit 3 releases to the environment through the stuck with low venting flow rate.
・ Around one quarter of the
vented gas from Unit 3 reversely flows into the R/B of Unit 4 with high venting flow rate of 20kg/s.
❏ Reverse gas flow in to the R/B of Unit 4
- FLUENT results show that:
Velocity field in the containment venting line
[m/s]
Unit 4 Unit 3
Outlet of Stack
CFD analysis results
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Picture of exploded R/B of Unit 3 and Unit 4
Unit 3 Unit 4
(写真:東京電力ホームページより)
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❏ Expected cause of explosion
- Condition of the exploded R/B of Unit 4 was between that of Unit 1 and Unit 3.
- Indicating the order of total amount of hydrogen gas
exploded in the R/B of Unit 4 was between Unit 1(400kg) and Unit 3 (1,000kg).
- Reported that the water level of the spent fuel pool kept
covering the top of fuels at Unit 4 . - Supposed small amount of hydrogen gas generation by
radiolysis in a few days before the explosion
Cause of explosion at R/B of Unit 4
→Indicating the vented gas from Unit 3 reversely flowed into the R/B of Unit 4
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- Hydrogen gas generation in SFP of Unit 4: Small amount of hydrogen gas generation in the pool. - Reverse flow of vented gas from Unit 3 to Unit 4: How did large amount of hydrogen gas flow into the
R/B of Unit 4 from Unit 3?
❏ Validation
Cause of explosion at R/B of Unit 4
→Need of inspection of the junction area of vent lines of Unit 3 and Unit 4 (bottom of stuck) to evaluate the flow distribution at the junction area with radiation monitor or any other ways
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Summary and Remarks
❏ JNES analyzed hydrogen explosion phenomena in R/B of Fukushima Dai-ichi NPS getting insight of the cause of explosions.
❏ Estimation by the hydrogen explosion analysis:
- At Unit 1: hydrogen gas(~400kg) leaked from PCV upper flange
and exploded in the top floor of R/B (Unit 1)
- At Unit 3: Hydrogen gas(~1,000kg) leaked to 1F of R/B and
exploded in the whole R/B (Unit 3)
- At Unit 4: Total amount of hydrogen gas between that of Unit 1
and Unit 3 exploded in the R/B (Unit 4),
which caused by reverse flow through containment vent line
from Unit 3
❏ Need to continue further investigation to fix above estimation.